gain-of-function mutants of the cytokinin receptors ahk2 and … · 2017. 1. 17. · 5 102...

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Short title: Gain-of-Function Variants of Cytokinin Receptors 1

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Corresponding authors 3

Prof. Dr. Thomas Schmülling and Prof. Dr. Tomáš Werner 4

Institute of Biology/Applied Genetics 5

Dahlem Centre of Plant Sciences (DCPS) 6

Freie Universität Berlin 7

Albrecht-Thaer-Weg 6 8

D-14195 Berlin, Germany 9

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Email: [email protected], [email protected] 11

Phone: +49 30 838 55808 / 56796 12

Fax: +49 30 838 54345 13

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Gain-of-function mutants of the cytokinin receptors AHK2 and 15

AHK3 regulate plant organ size, flowering time and plant longevity 16

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Isabel Bartrina1$, Helen Jensen1$, Ondřej Novák2, Miroslav Strnad2, Tomáš Werner1,3* 18

and Thomas Schmülling1* 19 1Institute of Biology/Applied Genetics, Dahlem Centre of Plant Sciences (DCPS), Freie 20

Universität Berlin, Albrecht-Thaer-Weg 6, D-14195 Berlin, Germany 21 2Laboratory of Growth Regulators, Palacký University and Institute of Experimental Botany 22

ASCR, CZ-78371 Olomouc, Slechtitelu 11, Czech Republic 23 3Institute of Plant Sciences, Department of Plant Physiology, University of Graz, 24

Schubertstraße 51, 8010 Graz, Austria 25 26 $These authors have contributed equally to this work. 27

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One sentence summary: 29

Gain-of-function variants of two Arabidopsis cytokinin receptors show the impact of cytokinin 30

on regulating shoot organ size, flowering time, plant longevity and seed yield. 31

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Footnotes: 34

I.B., T.W. and T.S. designed experiments and coordinated the project; I.B. and H.J. 35

performed experiments; O.N. and M.S. carried out hormone analyses, I.B., T.W. and T.S. 36

wrote the article, all authors revised the article. 37

Plant Physiology Preview. Published on January 17, 2017, as DOI:10.1104/pp.16.01903

Copyright 2017 by the American Society of Plant Biologists

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Funding: This project was funded by grants of Deutsche Forschungsgemeinschaft in the 39

frame of CRC 429 to T.S. and T.W., in the frame of SPP 1530 to T.S. and by the Czech 40

Grant Agency (No. 15-22322S) to M.S. 41

Corresponding Author email: [email protected] and [email protected]

berlin.de 43



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ABSTRACT 44

The phytohormone cytokinin is a regulator of numerous processes in plants. In Arabidopsis 45

thaliana, the cytokinin signal is perceived by three membrane-located receptors named 46

ARABIDOPSIS HISTIDINE KINASE2 (AHK2), AHK3 and AHK4/CRE1. How the signal is 47

transmitted across the membrane is an entirely unknown process. The three receptors have 48

been shown to operate mostly in a redundant fashion and only very few specific roles have 49

been attributed to single receptors. Using a forward genetic approach, we isolated 50

constitutively active gain-of-function variants of the AHK2 and AHK3 genes, named repressor 51

of cytokinin deficiency2 (rock2) and rock3, respectively. It is hypothesized that the structural 52

changes caused by these mutations in the sensory and adjacent transmembrane domains 53

emulate the structural changes caused by cytokinin binding resulting in domain motion 54

propagating the signal across the membrane. Detailed analysis of lines carrying rock2 and 55

rock3 alleles revealed how plants respond to locally enhanced cytokinin signaling. Early 56

flowering time, a prolonged reproductive growth phase and, thereby, increased seed yield 57

suggest that cytokinin regulates various aspects of reproductive growth. In particular, it 58

counteracts the global proliferative arrest, a correlative inhibition of maternal growth by 59

seeds, an as yet unknown activity of the hormone. 60

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Keywords: Arabidopsis thaliana, cytokinin, cytokinin receptor, flowering time, global 63

proliferative arrest (GPA), organ size, plant development 64



4

INTRODUCTION 65

The phytohormone cytokinin regulates numerous developmental processes, including cell 66

proliferation and differentiation, shoot and root growth, seed germination, and leaf 67

senescence (Werner and Schmülling, 2009; Kieber and Schaller, 2014; Zürcher and Müller, 68

2016). Cytokinin signaling is mediated by a phosphorelay system that resembles bacterial 69

two-component signaling systems (Hwang and Sheen, 2001; Müller and Sheen, 2007). In 70

Arabidopsis, there are three membrane-spanning histidine protein kinases (AHKs) which 71

serve as cytokinin receptors, AHK2, AHK3, and AHK4/CRE1 (named AHK4 in the following) 72

(Inoue et al., 2001; Ueguchi et al., 2001; Yamada et al., 2001). The hormone is recognized 73

and bound by an about 270 amino acids long extra-cytoplasmic binding domain called 74

CHASE domain (cyclin histidine kinase associated sensory). This domain is flanked by two 75

transmembrane domains and followed towards the C-terminal end on the cytoplasmic side 76

by a histidine kinase and a receiver domain (Steklov et al., 2013). The 3D structure of the 77

CHASE domain of AHK4 has been resolved by X-ray crystallography (Hothorn et al., 2011). 78

This has revealed that the receptor acts as a dimer and that the hormone is bound by a Per-79

Arnt-Sin (PAS)-like domain. A simple size exclusion mechanism allows only for the binding of 80

the free cytokinin bases, which are isopentenyladenine (iP), trans-zeatin (tZ), cis-zeatin (cZ) 81

and dihydrozeatin (DHZ). Binding of bulkier metabolites such as ribosides and nucleotides is 82

prohibited (Hothorn et al., 2011). This result has been confirmed by an in planta cytokinin 83

receptor binding assay (Lomin et al., 2015). However, it is clear that the three cytokinin 84

receptors have different affinities for the different cytokinin bases (Suzuki et al., 2001; 85

Spíchal et al., 2004; Romanov et al., 2006; Stolz et al., 2011; Lomin et al., 2015). 86

Interestingly, the ligand affinity of AHK2 resembles that of AHK4, while AHK3 differs from 87

these in its lower affinity to iP (Romanov et al., 2006; Stolz et al., 2011). 88

The bulk of cytokinin receptors is located in the endoplasmatic reticulum (Caesar et al., 89

2011; Wulfetange et al., 2011; Lomin et al., 2015). Upon ligand binding, the signal is 90

transmitted via an as yet unknown mechanism across the membrane, the cytoplasmic kinase 91

domain is activated and the protein autophosphorylates at the His residue. The high energy 92

phosphoryl group is next transferred within the same molecule to the Asp residue of the 93

receiver domain and from there to histidine phosphotransfer proteins (HPTs). These 94

translocate to the nucleus where they transfer the phosphoryl group to B-type response 95

regulators, which act as transcription factors and orchestrate downstream responses (Müller 96

and Sheen, 2007). In Arabidopsis, the transcript abundance of numerous genes is altered 97

within minutes in response to a cytokinin stimulus (Brenner et al., 2012; Bhargava et al., 98

2013; Brenner and Schmülling, 2015). 99

Loss-of-function mutations of single cytokinin receptor genes have no or only weak 100

effects on plant growth indicating strong functional redundancy (Higuchi et al., 2004; 101



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Nishimura et al., 2004; Riefler et al., 2006). Similarly, simultaneous mutation of AHK2 or 102

AHK3 together with AHK4 has only mild effects on development suggesting that both AHK2 103

and AHK3 alone are sufficient to mediate central functions of cytokinin. In contrast, ahk2 104

ahk3 mutants are dwarfed plants showing stronger root growth demonstrating the importance 105

of AHK2 and AHK3 in regulating vegetative plant growth (Riefler et al., 2006). Additional 106

comprehensive examination of the cytokinin receptor genes revealed also some 107

specialization in cytokinin receptor function (Heyl et al., 2012). For example, it was shown 108

that AHK4 alone plays a role in embryonic root development, phosphate starvation response 109

and sulfate assimilation (Mähönen et al., 2000; Maruyama-Nakashita et al., 2004; Franco-110

Zorrilla et al., 2005). Furthermore, AHK3 plays a predominant role in regulating leaf 111

senescence and cell differentiation in the transition zone of the root meristem (Kim et al., 112

2006; Riefler et al., 2006; Dello Ioio et al., 2007). No specific function has been shown for 113

AHK2 so far. 114

The functional overlap is particularly high for AHK2 and AHK3, which both contribute to 115

mediate a large number of developmental cytokinin functions from seed germination to 116

regulation of shoot growth and also responses to abiotic stresses including drought (Tran et 117

al., 2007), cold (Jeon et al., 2010) and high light (Cortleven et al., 2014). The mostly 118

redundant action of AHK2 and AHK3 is intriguing and raises the question why both genes 119

with apparently similar functions have been conserved during evolution. Consistently, AHK2 120

and AHK3 have largely overlapping expression domains, both are predominantly expressed 121

in shoots (Higuchi et al., 2004). Stolz et al. (2011) showed that both AHK2 and AHK3 122

activate cytokinin response in leaf mesophyll cells (AHK4 does not) but only AHK3 mediates 123

a response in stomata cells. Interestingly, promoter-swap and domain-swap analyses have 124

shown that AHK4 can functionally replace AHK2 but not AHK3 (Stolz et al., 2011). 125

In view of the mostly redundant action of the AHK2 and AHK3 receptors it might be 126

informative to study gain-of-function mutants to compare receptor activities. In this work, we 127

report on novel gain-of-function mutants of AHK2 and AHK3 named repressor of cytokinin 128

deficiency2 (rock2) and rock3, respectively. These mutants were identified in a screen for 129

suppressor mutants of the cytokinin deficiency syndrome displayed by 35S:CKX1 130

overexpressing (CKX1ox) plants (Niemann et al., 2015). Reversion of the cytokinin 131

deficiency phenotype was due to a constitutive activity of the mutant receptors. The receptor 132

variants do have differential impact on individual phenotypic traits and are thus informative 133

about cytokinin signaling during plant development. The results yield new information on the 134

functions of these evolutionary closely related receptors and highlight their roles in regulating 135

flowering time and plant longevity. Given the promoting effect of the rock2 and rock3 alleles 136

on shoot organ growth and, in particular, seed yield, we propose their potential value for 137

biotechnological approaches. 138



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RESULTS 139

rock2 and rock3 Suppress the Cytokinin-Deficiency Phenotype 140

To identify molecular factors required for establishing the CK deficiency syndrome displayed 141

by CKX1ox plants (Werner et al., 2003), we searched for suppressor mutants reverting the 142

dwarf shoot phenotype of CKX1ox plants (Niemann et al., 2015). Amongst others, two 143

mutants named rock2 and rock3 which grew larger than CKX1ox (Fig. 1A, C) were identified 144

and selected for further study. Genetic analysis showed that rock2 and rock3 are two 145

dominant second site mutations (Supplemental Table S1). The reversion of the cytokinin-146

deficient phenotype was already obvious early after germination. In the CKX1ox background, 147

rock2 and rock3 developed strongly enlarged cotyledons with longer petioles, which even 148

exceeded the size of wild-type cotyledons (Fig. 1B). rock2 CKX1ox and rock3 CKX1ox plants 149

developed larger rosette leaves, grew taller inflorescence stems with more flowers (Fig. 1A, 150

C) and the flowers of both suppressor lines were enlarged (Fig. 1D). In addition, both 151

mutations suppressed the late flowering phenotype of CKX1ox plants under long-day 152

conditions (Werner et al., 2003). The rescue was partial in the case of rock3 CKX1ox 153

whereas rock2 CKX1ox flowered even earlier than wild-type plants (Fig. 1E). Under short-154

day conditions, the flowering transition defect of CKX1ox plants were more severe as these 155

plants remained in the vegetative stage. Interestingly, only the rock2 mutation was able to 156

suppress the non-flowering phenotype of CKX1ox under short-day conditions (Fig. 1F). In 157

contrast, rock3 CKX1ox still failed to flower under short-day conditions. This indicates that 158

rock2 particularly regulates processes associated with the change from vegetative to 159

reproductive development under different light periods. 160

Cytokinin is known to delay leaf senescence (Richmond and Lang, 1957; Gan and 161

Amasino, 1995; Riefler et al., 2006) and, consistently, leaves of CKX1ox plants showed a 162

reduced chlorophyll retention in a detached leaf assay (Fig. 1G). Both, rock2 and rock3, 163

retarded the breakdown of chlorophyll of CKX1ox plants during dark-induced senescence 164

compared to wild-type and CKX1ox plants. After seven days in the dark, the leaf chlorophyll 165

content was reduced by almost 80% in wild-type and by 90% in CKX1ox plants while it was 166

decreased only by 65 and 35% in rock2 CKX1ox and rock3 CKX1ox mutants, respectively. 167

This indicates that rock3 plays a more prevalent role than rock2 in delaying dark-induced leaf 168

senescence. 169

To study whether the rock2 and rock3 eventually exert a differential influence also on 170

root development we compared primary root elongation in the mutants and wild type. Figure 171

1H shows that the roots of rock2 CKX1ox seedlings displayed a complete and rock3 CKX1ox 172

seedlings a partial reversion of the enhanced primary root elongation caused by the reduced 173

cytokinin content of CKX1ox seedlings (Werner et al., 2003). 174

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rock2 and rock3 Increase Sensitivity Towards Exogenous Cytokinin 176

What could be the cause for the suppression of the cytokinin deficiency phenotype of 177

CKX1ox plants? Analysis of the expression of the CKX1 transgene in both the rock2 and 178

rock3 mutant showed that it was not affected by the mutation (Fig. 2A). In addition, 179

outcrossing the 35S:CKX1 transgene from the rock2 CKX1ox or rock3 CKX1ox background 180

revealed that the transgene was functionally intact (data not shown). Comparison of the 181

cytokinin content of wild type, CKX1ox, rock2 and rock3 seedlings showed that the mutations 182

did not increase the endogenous cytokinin content compared to CKX1ox (Fig. 2B and 183

Supplemental Table S2). This suggested that a mechanism different from interference with 184

35S:CKX1 gene expression or metabolic compensation must be the cause for the phenotypic 185

reversion. 186

Next, we analyzed whether rock2 and rock3 change the plants´ sensitivity to 187

exogenously applied cytokinin. Figure 2C shows that wild-type seedlings grew smaller on 188

media containing 25 nM BA, however, with the exception of few yellowing leaves, most 189

leaves stayed green. CKX1ox seedlings were less sensitive and grew on BA-containing 190

media similar as on medium without BA. In contrast, rock2 CKX1ox and rock3 CKX1ox 191

seedlings showed strong hypersensitive reaction to cytokinin. They stayed smaller than 192

control plants grown on standard media and formed yellow leaves, which is a typical reaction 193

to high exogenous cytokinin concentrations (Ainley et al., 1993). The altered growth 194

responses indicate that rock2 and rock3 enhance the plants´ cytokinin sensitivity and 195

suggest that the altered sensitivity may be causal for the suppression of the cytokinin-196

deficient phenotype. 197

198

rock2 and rock3 Encode Novel Dominant Gain-of-Function Alleles of AHK2 and AHK3 199

Intriguingly, genetic mapping of the rock2 and rock3 mutations revealed that both were 200

located in genes coding for different cytokinin receptors. The rock2 mutation turned out to be 201

a C to T transition in the 7th exon of the AHK2 (At5g35750) gene leading to a semi-202

conservative substitution of aliphatic leucine by aromatic phenylalanine at amino acid 203

position 552 (L552F) in the fourth predicted transmembrane domain (Fig. 3A, B). The rock3 204

mutation was identified as a single base change from C to T in the second exon of AHK3 205

(At1g27320). This results in a non-conservative amino acid change from polar threonine to 206

aliphatic isoleucine at position 179 (T179I) located closely to the predicted cytokinin-binding 207

domain of the AHK3 receptor (Fig. 3A, B). A second independent allele, named rock3-2, was 208

identified during the course of this work causing exchange of negatively charged glutamic 209

acid to positively charged lysine at position 182 (E182K) in close proximity to the T179I 210

mutation (Fig. 3A). The first identified allele rock3-1 was used for all analyses described in 211

this article and is called rock3 throughout. 212



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To prove that the identified mutant alleles of AHK2 and AHK3 are causal for the 213

suppression of the cytokinin-deficient phenotype, the mutant gene variants were cloned 214

under the control of their native promoters and transformed in the CKX1ox background. 215

Transforming CKX1ox was inherently difficult, nevertheless, two independent transgenic 216

AHK2:rock2 CKX1ox lines and three independent AHK3:rock3 CKX1ox lines were identified. 217

All five transgenic lines showed a suppression of the cytokinin deficiency phenotype similar 218

or even stronger than the rock2 CKX1ox and rock3 CKX1ox suppressor lines (Fig. 3, C-J). 219

This unequivocally confirmed that the phenotypes were caused by the mutations in the AHK2 220

and AHK3 genes. Thus, rock2 and rock3 encode two novel dominant gain-of-function 221

variants of the cytokinin receptors AHK2 and AHK3. 222

223

rock2 and rock3 Enhance Cytokinin Signaling 224

The revertant morphology and the enhanced cytokinin sensitivity indicated that the rock2 and 225

rock3 alleles might code for cytokinin receptors with increased signaling activity. To test 226

whether the mutant receptors act in a cytokinin-independent manner or could be activated by 227

lower cytokinin concentrations we used the yeast complementation assay described by Inoue 228

et al. (2001). In this assay, AHK4 rescues, in a cytokinin-dependent manner, the survival of a 229

yeast mutant that lacks the endogenous histidine kinase SLN1 (Inoue et al., 2001). It is 230

known that the yeast Δsln1 strain carrying one of the other two cytokinin receptors (AHK2 or 231

AHK3) can grow in the absence of cytokinin, but growth may be accelerated in the presence 232

of cytokinin (Mähönen et al., 2006; Tran et al., 2007). However, under our laboratory 233

conditions the cytokinin-dependent faster growth rate of strains containing AHK2 or AHK3 234

receptor genes was not observed and, therefore, we were not able to test the original rock2 235

or rock3 mutation in this system. Instead, we constructed rock2 and rock3 variants of AHK4 236

(named AHK4rock2 and AHK4rock3) as the positions affected by the rock2 and rock3 mutations 237

are conserved in all three cytokinin receptors of Arabidopsis (Fig. 3A). Both rock variants of 238

AHK4 suppressed the lethality of the Δsln1 mutation even without the addition of cytokinin to 239

the medium while the mutant carrying the wild-type AHK4 allele showed strictly cytokinin-240

dependent growth rescue (Fig. 4A). This shows that the His kinase activity of AHK4rock2 and 241

AHK4rock3 is independent of cytokinin and that rock2 and rock3 may be constitutively active 242

receptor proteins. 243

To test further the effects of rock2 and rock3 on cytokinin signaling output in planta we 244

compared the expression level of the well-established cytokinin reporter ARR5 in wild type 245

and both rock mutants. Fig. 4B shows increased steady state mRNA levels of ARR5 in rock2 246

and rock3 indicating enhanced cytokinin signaling in planta. Next we introgressed the mutant 247

alleles (without the 35S:CKX1 transgene) into a line harboring the ARR5:GUS reporter 248

(D'Agostino et al., 2000). Figures 4B-J show strongly increased GUS activity in all analyzed 249



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tissues of rock2 ARR5:GUS and rock3 ARR5:GUS seedlings. Seven days after germination 250

increased GUS activity was detected in the vascular tissue of cotyledons and leaves, in the 251

shoot apex, the root vasculature and the vascular procambium and root cap of the primary 252

roots. rock2 caused an overall stronger increase in GUS activity than rock3 in all analyzed 253

tissues. For example, rock2 mutants showed high GUS activity along the whole root, 254

whereas in rock3 roots the GUS signal was increased only weakly in the apical part of the 255

root. This correlates well with the observed stronger capacity of rock2 to suppress the 256

cytokinin deficiency phenotypes in shoot and root. The enhanced expression of the cytokinin 257

response gene ARR5:GUS as an output of the cytokinin signaling system, together with the 258

results of the yeast complementation assay, shows that the constitutively active rock2 and 259

rock3 activate more strongly the cytokinin signal transduction pathway. 260

Previous work has provided indications that cytokinin signaling is intimately linked to 261

metabolic homeostasis of the hormone (Riefler et al., 2006). The identified more active 262

receptors provide an opportunity to test this link further. We, therefore, analyzed whether the 263

endogenous cytokinin concentration responds to the enhanced cytokinin signaling and 264

determined the cytokinin levels of wild type, rock2, and rock3 seedlings and seedlings 265

expressing AHK2:rock2. Both rock mutants and the transgenic line showed a similar 266

reduction of 45-56% of the total cytokinin content (Table 1). Stronger differences between 267

rock2 and rock3 were found for the biologically active free bases. Both the rock2 mutation 268

and transgenic expression of AHK2:rock2 caused a decrease of about 40% for iP and cZ and 269

of about 55% for tZ in comparison to wild type (Table 1). rock3 seedlings contained only 24% 270

iP, 11% tZ and 14% cZ in comparison to wild-type seedlings. Taken together, the gain-of-271

function alleles of AHK2 and AHK3 have an impact on cytokinin homeostasis and lower the 272

cytokinin content supporting a feedback regulation of cytokinin metabolism by the cytokinin 273

signaling pathway. Moreover, the results corroborate the hypothesis that these receptor 274

variants display a high signaling activity independent of the steady state cytokinin 275

concentration. 276

277

rock2 and rock3 Increase Shoot Growth and Leaf Size 278

The functional redundancy within the cytokinin receptor family makes it difficult to dissect the 279

biological function of individual receptor proteins. In this situation, gain-of-function receptor 280

variants can be useful to obtain information on their individual functions. We have, therefore, 281

analyzed the consequences of the rock2 and rock3 mutations on growth and development in 282

more detail using lines containing the original rock mutations in the wild-type background as 283

well as transgenic lines expressing the rock2 and the rock3 coding sequence under their 284

native promoters. Results are shown for homozygous progeny of lines AHK2:rock2-10 and 285

AHK3:rock3-1. Two other independent lines, AHK2:rock2-7 and AHK3:rock3-5, showed 286



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similar results. qRT-PCR analysis showed that the levels of AHK2 transcripts were similar in 287

wild type, rock2 and the transgenic AHK2:rock2 line (Supplemental Fig. S1A). Likewise, wild 288

type, rock3 and the transgenic AHK3:rock3 line had comparable AHK3 transcript levels 289

(Supplemental Fig. S1B). 290

Figure 5 shows different aspects of the shoot development in these plants. rock2 291

mutants and transgenic plants grew significantly taller than wild type (Fig. 5A, B). The 292

inflorescence stems of 50-d-old wild-type and rock3 mutant plants measured 33.2 ± 1.3 cm 293

and 35.4 ± 2.3 cm in height (7% increase). rock2 mutant plants grew 42%, transgenic 294

AHK3:rock3 plants 60% and transgenic AHK2:rock2 plants even 85% taller than wild type. All 295

the mutant and transgenic lines had also an increased stem diameter (Figs. 5C to E). Also in 296

this case the transgenic lines showed the strongest increase, the stem diameter was 297

increased up to about 36% in comparison to wild type. Transverse sections of the primary 298

inflorescence stem showed that stem morphology was normal in all genotypes, but the 299

number of cells in stems of transgenic lines was higher than in wild-type stems. Figure 5E 300

shows as an example transverse sections of wild-type and AHK3:rock3 inflorescence stems. 301

The increased cell number could be due to a higher cambial activity, which has been 302

previously shown to be regulated by cytokinin (Matsumoto-Kitano et al., 2008; Nieminen et 303

al., 2008; Bartrina et al., 2011). This will, however, require further clarification. 304

Further inspection of the shoot phenotype revealed that rock2 and rock3 also positively 305

regulate shoot lateral organ size. rock2 and rock3 seedlings developed strongly enlarged 306

cotyledons (Fig. 6A), a phenotype which was more prominent in transgenic rock2 and rock3 307

lines. Later during development, rock2 and rock3 mutants formed larger rosette leaves (Fig. 308

6B) with the AHK2:rock2 and AHK3:rock3 transgenic lines forming the largest leaves. The 309

biomass of rosette leaves was increased in rock2 and rock3 mutants (Fig. 6C). As organ size 310

is influenced by cell number and cell expansion, we analyzed cell size of epidermal cells in 311

the sixth fully developed rosette leaf of wild-type and rock2 plants. The epidermal cell size of 312

rock2 plants was slightly but not significantly reduced (Fig. 6D). This together with the 313

increased leaf surface (Fig. 6E) revealed that rock2 rosette leaves formed about 40-50% 314

more epidermal cells compared to wild type. In conclusion, the rock2-dependent changes in 315

organ size are due to prolonged mitotic activity and/or faster cell proliferation during leaf 316

growth. 317

318

rock3 Prolongs the Life Span of Leaves 319

It is known that cytokinin delays leaf senescence (Gan and Amasino, 1995; Kim et al., 2006). 320

We compared the natural senescence as well as dark-induced senescence of detached 321

leaves of rock2 and rock3 with that of wild type. Visual examination of the sixth rosette 322

leaves throughout their life spans showed that the onset of natural leaf senescence was 323



11

delayed particularly in rock3 mutant plants (Fig. 7A). rock3 leaves had an about seven days 324

longer life span compared to wild-type leaves, whereas rock2 leaves showed only a slightly – 325

up to two days – delayed leaf senescence (Fig. 7A). These results were confirmed by 326

measuring the photosynthetic efficiency of photosystem II (Fv/Fm), which was maintained 327

about four days longer at a high level in leaves of rock2 and about 8 days longer in rock3 328

plants (Fig. 7B). Figure /C shows that also dark-induced leaf senescence was strongly 329

retarded in rock3 mutant plants. After seven days in the dark, the chlorophyll content was 330

reduced to 10% in wild-type and rock2 plants, whereas rock3 leaves were still green with a 331

remaining chlorophyll content of almost 60%. These results show that a gain-of-function 332

mutation in the AHK3 cytokinin receptor significantly delays different senescence-associated 333

symptoms in Arabidopsis. Enhanced activity of AHK2 on the other hand has only a minor but 334

still significant impact on delaying leaf senescence. 335

The strong delay of leaf senescence in rock3 mutants prompted us to compare rock3 336

with another AHK3 allele, ore12, reported to delay leaf senescence (Kim et al., 2006). Both 337

the visual inspection and the Fv/Fm values showed a significantly later onset of senescence 338

in rock3 than in ore12 mutants (Supplemental Figure S2), indicating quantitatively different 339

effects of these mutations on AHK3 receptor activity. 340

341

rock2 and rock3 Alter Flowering Time, and Increase Flower Size and Seed Yield 342

Under long-day conditions, rock2 mutants and AHK2:rock2 transgenic lines flowered 343

significantly earlier than wild type, as indicated by their lower number of rosette leaves at 344

flowering time (Fig. 8A). Likewise, the AHK3:rock3 transgenic plants, but not the rock3 345

mutant, flowered earlier. Interestingly, all mutant and transgenic lines showed also a 346

markedly increased duration of flowering and increased total life span. Seven-week-old wild-347

type plants ceased to form new flowers, while rock genotypes continued to flower for at least 348

one more week and up to two weeks in case of AHK2:rock2 transgenic plants (Fig. 8B). 349

The size of flowers was significantly enhanced in rock2 and rock3 mutants (Fig. 8C). To 350

determine whether the increase in petal size was a result of an increased cell proliferation, 351

cell expansion or both, abaxial petal epidermal cell size was analyzed exemplarily in 352

AHK2:rock2 flowers. Fig. 8D shows that the cell size was not significantly altered, indicating 353

that an enhanced cell proliferation was the cause for the larger petals, similar as was found 354

for rosette leaf size regulation (Fig. 6D and E). 355

rock2, AHK2:rock2, and AHK3:rock3 plants formed more siliques than wild-type plants 356

(Fig. 8E). The transgenic AHK2:rock2 line showed the largest increase, producing almost 357

twice as many siliques as wild type. Interestingly, despite activity of cytokinin in regulating 358

shoot meristem activity and size (Bartrina et al., 2011), microscopic analysis did not reveal 359

an increased size of rock2 or rock3 inflorescence meristems (Supplemental Fig. S3). 360



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Consistently, analysis of cytokinin signaling output in the inflorescence meristem of rock2 361

plants using the TCSn:GFP marker gene (Zürcher et al., 2013) showed no significant change 362

in comparison to wild type (Supplemental Fig. S3) suggesting dampening of the increased 363

receptor signaling in the inflorescence meristem. This indicates that the formation of more 364

flowers and, consequently, of more siliques was due to a longer flowering phase. Analysis of 365

seed yield did not show an increase in plants harboring a rock2 allele, which was probably 366

accountable to heterostylous flowers displaying disproportionally elongated gynoecia in 367

comparison to the stamen, which caused reduced self-fertilization (Supplemental Fig. S4). 368

However, AHK3:rock3 plants lacking this morphological defect produced about 40% more 369

seeds than wild-type plants. A similar result was found for a second transgenic rock3 line 370

(AHK3:rock3-5), confirming that seed yield is positively influenced by the rock3 mutation. 371

372

rock2 and rock3 Reduce Root Growth 373

In contrast to its promotional role in shoot organs, cytokinin is a negative regulator of root 374

development (Werner and Schmülling, 2009). As expected from the observed suppressor 375

activity (Fig. 1H), root growth was inhibited in all tested rock seedlings grown under in vitro 376

conditions (Fig. 9A). Primary root elongation was reduced by 37 and 28% in rock2 and rock3 377

seedlings, respectively. The transgenic rock lines AHK2:rock2-10 and AHK3:rock3-1 showed 378

slightly milder root phenotypes, with a reduction in root elongation by 23% and 18%, 379

respectively (Fig. 9A). Consistently, root meristem size was decreased in both the rock2 and 380

rock3 mutants (Fig. 9B, C). The formation of lateral roots was strongly inhibited as well. On 381

average, the number of lateral roots in wild-type plants was around 1.8 to 2.4 times greater 382

than that of the rock mutants, with rock2 showing the strongest reduction of lateral root 383

formation (Fig. 9D). The similar or even weaker consequences of transgenic rock gene 384

expression as compared to their original alleles contrasts with their generally stronger effects 385

on the shoot phenotype. 386

387

388

DISCUSSION 389

From the analysis of the rock2 and rock3 mutant alleles and transgenic lines carrying these 390

alleles we obtained valuable information about the cytokinin signaling mechanism and the 391

roles of these redundantly acting receptors in regulating various facets of the plant´s 392

phenotype. Both aspects are discussed in the following sections. 393

394

rock2 and rock3 Mutations Provide Insight into Transmembrane Signaling by 395

Cytokinin Receptors 396



13

How the cytokinin signal is transmitted across the membrane is an entirely unknown process. 397

The rock2 and rock3 mutations identified several amino acid residues that are relevant for 398

transmembrane signaling. Presumably, they induce changes in the receptor structure 399

resembling those induced by cytokinin binding to the CHASE domain and thus cause 400

constitutive cytokinin signaling. The fact that the AHK4 receptors harboring rock2 and rock3 401

mutations conferred growth rescue of Δsln1 yeast to a great extent similar to that caused by 402

cytokinin-induced signaling of wild-type AHK4 suggests that these mutations cause strong, if 403

not maximal, activation of the receptors. 404

We isolated two independent rock3 mutations which are located in close proximity to 405

each other in a region of the CHASE domain linking the long α-helical stalk domain with the 406

membrane-distal (ligand-binding) PAS domain (Hothorn et al., 2011). More precisely, they 407

are located in a slightly bent region comprising a 310-helix which connects helix α2 with the 408

first β-strand of the membrane-distal PAS domain (Hothorn et al., 2011)(see Figure 9). This 409

region, which is not involved in ligand binding, could participate in the receptor domain 410

movement expected to be triggered by cytokinin binding. The rock3 mutations, which alter 411

evolutionary highly conserved residues (Heyl et al., 2007; Steklov et al., 2013), may provoke 412

a similar intramolecular movement. 413

There is an intriguing structural similarity between the CHASE domain and the sensing 414

domain of a bacterial methyl-accepting chemotaxis protein (MCP), which is a membrane-415

bound His kinase recognizing isoleucine. The sensory domains of both receptor types have a 416

long α-helical stalk important to keep the receptor dimers together and to hold two PAS 417

domains of which the membrane-distal one binds the ligand (Hothorn et al., 2011; Liu et al., 418

2015). Analysis of the ligand-induced conformational changes of the PAS sensing domain of 419

MCP revealed that it likely signals by a piston displacement mechanism (Liu et al., 2015). 420

The signal-binding PAS domain fluctuates between a closed (ligand bound) and open form 421

resulting in a conformational change of the proximal PAS domain and movement towards 422

and away from the membrane. This movement is propagated causing a displacement of the 423

TM helix towards the cytoplasm thus generating a transmembrane signal (Liu et al., 2015). 424

Considering the close structural similarity between cytokinin receptors and MCP as well as 425

the fact that piston-type signaling is a common theme among other type of His kinases 426

(Chervitz and Falke, 1996; Cheung and Hendrickson, 2009; Moore and Hendrickson, 2009; 427

Bhate et al., 2015), we hypothesize that also the CHASE domain undergoes upon ligand 428

binding structural changes emulating a piston-like domain motion resulting in transmembrane 429

signaling (Fig. 10). Interestingly, the last β-strand of the membrane-proximal PAS domain is 430

linked to the stalk helix by a disulfide bridge (Hothorn et al., 2011), which might limit the 431

degree of structural rearrangements. Hence, the domain displacement will probably be of a 432

subtle nature, as known for other His kinases (Chervitz and Falke, 1996). 433



14

The rock2 mutation is located in the TM domain connecting the CHASE domain and the 434

cytosolic His-kinase domain. In the proposed signaling mechanism (Figure 9), this TM 435

domain is essential for transmitting the signal from the lumen of the endoplasmic reticulum to 436

the cytoplasm. Noteworthy, this TM domain shows a high degree of sequence conservation 437

among different cytokinin receptors, in contrast to the TM domain delimiting the CHASE 438

domain N-terminally (Steklov et al., 2013). The L552F substitution in the rock2 receptor 439

variant affects the highly conserved Leu residue of the helix core motif 440

Axxx(S/A)x(G/L)x(L/F)VIx(L/F)LxG(Y/H)I (Leu552 underlined). Furthermore, this residue is 441

located in immediate vicinity of two gain-of-function mutations of the AHK4 receptor that have 442

been reported to cause constitutive cytokinin signaling in a bacterial assay (Miwa et al., 443

2007) and that are partially conserved in AHK2. Presumably, the described mutations cause 444

constitutively conformational alterations of the TM domain which normally occur during 445

signaling. Although no crystal structure for any TM domain of any His kinase has been 446

resolved to date, there is a growing body of knowledge (Bhate et al., 2015) allowing to 447

predict that the structural changes may involve a lateral or vertical displacement of the 448

neighboring TM helices, and that such a displaced conformation is being locked by the rock2 449

and related mutations. 450

451

Constitutive Active Receptors Yield Novel Information About Cytokinin-Regulated 452

Processes 453

The analysis of mutants and transgenic lines expressing constitutive active variants of two 454

cytokinin receptors has revealed how plants with locally strongly enhanced cytokinin 455

signaling look like. Only a single gain-of-function cytokinin receptor mutant, ore12, has been 456

described previously in Arabidopsis (Kim et al., 2006) but the description has been limited to 457

the impact on leaf senescence. In addition, the ore12 mutation achieved a lower activation of 458

the AHK3 receptor as compared to the rock3 mutations described here, suggesting that this 459

receptor signals in a gradual rather than in an all-or-nothing fashion. Others reported that the 460

production of transgenic plants expressing constitutively active receptors was difficult if not 461

impossible (Miwa et al., 2007). Studying the influence of enhanced cytokinin signaling on the 462

plant phenotype is relevant as, up to now, our knowledge on the consequences of an 463

enhanced cytokinin status has been derived largely from plants ectopically expressing a 464

cytokinin-synthesizing IPT (Rupp et al., 1999; Sun et al., 2003) or LOG gene (Kuroha et al., 465

2009) or carrying mutations in cytokinin-degrading CKX genes (Bartrina et al., 2011). 466

However, the impact of an enhanced production of the hormone may differ from the 467

consequences of increased signaling. The former involves a mobile signal that can induce 468

local but also systemic effects through all cytokinin receptors that are present in a given cell 469

or tissue, while the latter acts in a cell-autonomous fashion involving only a single activated 470



15

receptor. In accord with this notion, the expression of the ARR5:GUS gene confirmed 471

enhanced cytokinin signaling by rock2 and rock3 and showed that signaling stays generally 472

limited to those tissues previously known to activate the cytokinin reporter by the 473

corresponding wild-type receptor (D'Agostino et al., 2000; Stolz et al., 2011). Interestingly, 474

rock2 and rock3 mutants have reduced levels of all analyzed cytokinin metabolites. This is in 475

agreement with the result that ahk loss-of-function mutants have an increased cytokinin 476

content (Riefler et al., 2006) and thus underpins the existence of homeostatic control 477

mechanisms. Part of these control mechanisms is an influence of cytokinin signaling on the 478

transcript level of cytokinin metabolism genes (Miyawaki et al., 2004; Werner et al., 2006; 479

Brenner et al., 2012). 480

Both rock2 and rock3 mutations affect most morphological aspects of the cytokinin 481

deficiency syndrome but revert them to a different degree. The similar although not identical 482

effects of these mutations on the plant phenotype reflect their high degree of functional 483

redundancy revealed by loss-of-function mutants (Higuchi et al., 2004; Nishimura et al., 484

2004; Riefler et al., 2006). There are a number of notable quantitative differences between 485

the rock2 and rock3 effects, which are also seen in the differential activation of the 486

ARR5:GUS reporter in different tissues. For example, the rock2 mutation has a stronger 487

impact on primary root elongation and root meristem size, which is surprising in view of the 488

proposed central role that AHK3 (but not AHK2) plays in the root apical meristem (Dello Ioio 489

et al., 2007). Similarly, the late flowering phenotype of CKX1ox plants was only partially 490

reverted by rock3 whereas rock2 CKX1ox flowered even earlier than wild type. In contrast, 491

rock3 had, for example, a stronger effect in retarding leaf senescence. These mostly gradual 492

differences may partly be due to differences in signaling strength caused by the mutations 493

but may also reflect differences in coupling to downstream signaling processes. Indeed, 494

ectopic expression of the rock2 and rock3 alleles under the transcriptional control of the 495

same promoters causes in Arabidopsis partly different responses (A. Stolz and T.S., 496

unpublished results). This indicates that, although the cytoplasmic domains of both receptors 497

interact with the same AHP proteins in a yeast two-hybrid assay (Dortay et al., 2006), the 498

affinities between signaling proteins may differ in planta. Their interaction can also be 499

modulated by accessory proteins in a similar fashion as known from bacterial two component 500

systems (Jung et al., 2012). 501

One important result derived from the phenotypic analysis is that cytokinin signaling is 502

limiting for the growth of different shoot organs and that rock2/rock3-dependent changes in 503

organ size are due to prolonged cell proliferation. This supports the hypothesis that cytokinin 504

primarily controls the duration of the cell proliferation phase in shoot organ primordia by 505

delaying the onset of cell differentiation (Holst et al., 2011). Consistently, a lower cytokinin 506

activity causes a reduced leaf size, and genetic analysis has revealed that this is redundantly 507



16

controlled by AHK2 and AHK3 (Werner et al., 2003; Higuchi et al., 2004; Nishimura et al., 508

2004; Riefler et al., 2006). It has been proposed that plant organ growth displays bell‐shaped 509

dose-responses to cytokinin (Ferreira and Kieber, 2005) and that cytokinin limits leaf size in 510

wild type (Efroni et al., 2013). The enhanced growth of different shoot organs in rock2 and 511

rock3 mutants underpins this view. However, the range of cytokinin activity promoting growth 512

appears to be limited since cytokinin overproduction may result in the formation of smaller 513

leaves (Hewelt et al., 1994; van der Graaff et al., 2001; Sun et al., 2003). This is likely due to 514

strongly increased cell proliferation and the concomitant inability of proper cell differentiation 515

and expansion. Cell number is apparently sensed in plant leaves (Hisanaga et al., 2015) and 516

an increase in cell number above a certain threshold may interfere with cell expansion 517

resulting in smaller leaves (Dewitte et al., 2003). The increased cell proliferation in rock2 and 518

rock3 mutants was linked to normal cell expansion suggesting that this threshold was not 519

reached. The present work further revealed that flower organ size is very sensitive to 520

enhanced cytokinin signaling too. The particularly strongly enlarged petals and gynoecia 521

suggest that cytokinin is especially involved in regulating development of these organs. 522

Since long time it is known that exogenously applied cytokinin can promote flowering in 523

Arabidopsis (Michniewicz and Kamienska, 1967; Besnard-Wibaut, 1981; Dennis et al., 1996; 524

D'Aloia et al., 2011). However, it has remained unclear whether also endogenous cytokinin 525

would have the same promotive activity. rock2 suppresses the late-flowering phenotype of 526

CKX1ox plants stronger than rock3. This effect is most obvious under short-day conditions 527

where CKX1ox remain in the vegetative state and rock2, but not rock3, reverts this non-528

flowering phenotype. It seems clear that in Arabidopsis a certain cytokinin threshold signal is 529

indispensable for flower induction in short days and that cytokinin has a promotive effect on 530

flowering also in long days. The fact that rock2 CKX1ox mutants start to flower even earlier 531

than wild type underpins the importance of cytokinin signaling for flowering time control. The 532

mechanistic basis of flowering time control by cytokinin is poorly understood. It has been 533

shown that cytokinin promotes flowering independently of FLOWERING LOCUS T, but 534

through transcriptional activation of its paralogue TWIN SISTER OF FT (TSF; D'Aloia et al., 535

2011). However, the question whether the flowering response to cytokinin is mediated 536

entirely through transcriptional activation of TSF in leaves or whether cytokinin might also 537

promote flowering by direct action in the shoot apical meristem (Corbesier et al., 2003; 538

D'Aloia et al., 2011) needs to be further scrutinized. 539

An unexpected phenotype was the increased plant longevity and prolonged reproductive 540

growth phase, which was particularly marked in AHK3:rock3 transgenic plants. The 541

prolonged reproductive growth resulted in a strongly increased number of flowers and 542

siliques. This is a very interesting observation, because the monocarpic plant Arabidopsis 543

generates dependent on the growth conditions a specific number of flowers which is followed 544



17

by cessation of reproductive meristematic activity. This correlative inhibition of maternal 545

growth is caused by the offspring (seeds) and is referred to as global proliferative arrest 546

(GPA; Hensel et al., 1994). The molecular mechanism underlying this phenomenon is largely 547

unknown. A recent study has suggested that the low mitotic activity in meristems linked to 548

GPA represents a form of bud dormancy (Wuest et al., 2016). Given that cytokinin is a major 549

factor determining the proliferative activity in axillary buds (Shimizu-Sato et al., 2009), it is 550

tempting to hypothesize that cytokinin counteracts GPA by maintaining cell division activity. 551

This scenario would predict that a drop in cytokinin activity is required for meristematic arrest 552

during GPA and that the constitutive signaling in rock3 transgenic plants delays it. How 553

cytokinin could affect GPA is currently unclear. One possibility is that rock3, and thus 554

cytokinin, acts locally in the reproductive meristem by sustaining its activity. This idea is 555

consistent with the previous hypothesis that meristematic sink activity, rather than the leaf 556

source capacity, is decisive for the activity of the meristem, and that sink strength is 557

particularly triggered by cytokinin (Werner et al., 2008). A second possibility is that the strong 558

capacity of rock3 to delay leaf senescence prolongs the activity of the reproductive 559

meristems and in addition provides sufficient source capacity to support the development of 560

supernumerary seeds. However, it has been also discussed that the activity of the 561

reproductive meristem is uncoupled from rosette leaf senescence in Arabidopsis (Hensel et 562

al., 1993; Noodén and Penney, 2001; Wuest et al., 2016). The two possibilities are not 563

mutually exclusive and further research is needed to understand the mechanism underlying 564

the action of cytokinin during GPA. 565

In the above context, it is noteworthy that the size of the inflorescence meristems of 566

rock2 and rock3 mutants was not increased although these receptors are expressed and 567

active in the meristem (Riefler et al., 2006; Stolz et al., 2011; Gruel et al., 2016). This is 568

surprising considering that plants producing more or less cytokinin develop a larger or 569

smaller inflorescence meristem, respectively (Bartrina et al., 2011). One plausible 570

explanation for these apparently incongruous observations might be that in tissues where 571

AHK2 and AHK3 expression overlaps with that of AHK4, as it is the case in the inflorescence 572

meristem (Gruel et al., 2016), the enhanced signaling activity might be dampened by the 573

phosphatase activity of AHK4 (Mähönen et al., 2006). In the presence of low cytokinin (as in 574

the rock2 and rock3 plants), the phosphatase activity of the AHK4 receptor may prevail and 575

alleviate downstream signaling thus counteracting the enhanced kinase activity of rock2 and 576

rock3 receptors. Consistent with this idea the TCSn:GFP cytokinin reporter indicated similar 577

signaling output activity in wild-type and rock2 inflorescence meristems. It this respect, it 578

would be interesting to analyze the rock2 and rock3 mutations in the ahk4 knockout 579

background (Inoue et al., 2001). In contrast to rock2/rock3 plants, ckx3 ckx5 mutation 580

(Bartrina et al., 2011) presumably increases the signaling output of all three cytokinin 581



18

receptors and lowers the phosphatase activity of AHK4; together this might lead to a 582

quantitatively and qualitatively different cytokinin output signal. 583

Last but not least, transgenic expression of the rock3-1 allele caused an about 50% 584

higher seed yield, which is similar to that brought about by ckx3 ckx5 mutation (Bartrina et 585

al., 2011). However, the mechanisms leading to increased seed yield appear to be at least 586

partly different. ckx3 ckx5 mutants have a larger inflorescence meristem forming an 587

increased number of flowers and siliques. In addition, siliques contain more seeds due to an 588

increased placenta activity producing more ovules (Bartrina et al., 2011). In the case of rock3 589

transgenic plants enhanced yield was mainly due to delayed GPA (see above) leading to 590

taller plants with more siliques (rock2 transgenic plants formed even more flowers but 591

suffered from reduced fertilization). In sum, our results corroborate the role of cytokinin as a 592

regulator of seed yield (Ashikari et al., 2005; Bartrina et al., 2011) and demonstrated that 593

different developmental mechanisms can be involved. We propose the rock2 and rock3 594

genes as a novel biotechnological tool to achieve yield enhancement. Because coupling of 595

the receptors to downstream signaling components is promiscuous, they could be used 596

directly for a gain-of-function approach in crop plants to increase cytokinin signaling in a 597

targeted and cell-autonomous fashion. 598

599

MATERIALS AND METHODS 600

Plant Material and Growth Conditions 601

The Columbia (Col-0) ecotype of Arabidopsis thaliana was used as the wild type. The 602

following lines were described previously: 35S:CKX1-11 (Werner et al., 2003), ahk2-5 603

(Riefler et al., 2006) ARR5:GUS (D'Agostino et al., 2000) and ore12-1 (Kim et al., 2006). All 604

plants were grown on soil or in vitro on half-strength Murashige and Skoog (MS) medium 605

under long-day (16 h of light/8 h of darkness) or short-day conditions (8 h of light/16 h of 606

darkness) at 22°C. For root growth assays, seedlings were grown on vertical plates, and the 607

length of the primary root was measured between day 4 and 12 after germination from digital 608

images using Scion Image software (http://www.scioncorp.com). For the cytokinin sensitivity 609

assay, benzyladenine (BA) dissolved in DMSO or DMSO as a solvent control were added to 610

the medium. 611

612

Mutagenesis and Gene Mapping 613

The rock2 and rock3 mutants were identified in a screen of an M2 population of 35S:CKX1 614

plants mutagenized with ethyl methanesulfonate (Niemann et al., 2015). Mapping 615

populations for rock2 and rock3 were generated by crossing the rock2 35S:CKX1 and rock3 616

35S:CKX1 plants with the Landsberg erecta ecotype. F2 progenies resistant to hygromycin 617

(co-segregating with 35S:CKX1) and showing the cytokinin deficiency syndrome were used 618



19

to map the recessive (wild-type) alleles of rock2 and rock3. By analyzing 535 F2 619

recombinants, rock2 was mapped to a 1.45 Mbp region (~21.1 cM). To map the rock3 locus 620

927 F2 recombinants were analyzed and a 350 kb interval (~0.4 cM) was identified. The 621

rock2 and the rock3 mutation were identified by sequencing candidate genes in these 622

intervals and subsequent complementation of 35S:CKX1 transgenic plants by the mutant 623

alleles of the respective gene. 624

625

DNA Cloning 626

The rock2 and rock3 point mutations were introduced into the AHK2:AHK2 and AHK3:AHK3 627

genes (Stolz et al., 2011) using the QuickChange site-directed mutagenesis kit (Stratagene, 628

La Jolla, USA). Both constructs were introduced into Agrobacterium tumefaciens strain 629

GV3101, and 35S:CKX1 and wild-type plants were transformed using the floral-dip method 630

(Clough and Bent, 1998). Transgenic lines were selected on medium containing 50 mg/L 631

kanamycin. 632

633

Analysis of Transcript Levels by Quantitative Reverse Transcription-PCR 634

Total RNA was extracted from seedlings with the TRIzol method (Thermo Fisher Scientific). 635

Equal amounts of starting material (1 µg of RNA) were used in a 10 µL SuperScript III 636

Reverse Transcriptase reaction (Thermo Fisher Scientific). First strand complementary DNA 637

synthesis was primed with a combination of oligo(dT) primers and random hexamers. Real 638

time PCR using FAST SYBR Green I technology was performed on a CFX96 Touch Real-639

Time PCR Detection System (Bio Rad). The qPCR temperature program consisted of the 640

following steps: 95°C for 15 min; 40 cycles of 95°C 15 s, 55°C 15 s, 72°C 15 s; followed by 641

melting curve analysis. Relative transcript abundance of each gene was calculated based on 642

the ΔΔCt method (Livak and Schmittgen, 2001). PP2AA2 (At3g25800) was used for 643

normalization. Primers used for reference genes and genes of interest are listed in 644

Supplemental Table S3. 645

646

Yeast Complementation Assay 647

The yeast complementation assay was performed as described before (Inoue et al., 2001; 648

Mähönen et al., 2006). The rock2 and rock3 mutations were introduced into the AHK4 gene 649

(named AHK4rock2 and AHK4rock3) of plasmid p423TEF-CRE1 (Mähönen et al., 2006) using 650

the QuickChange site-directed mutagenesis kit. After sequence confirmation, plasmids were 651

introduced into yeast strain (TM182) ∆sln1 (Inoue et al., 2001) and the yeast 652

complementation assay was performed with either 2% galactose or 2% glucose with 0.1 or 653

10 µM tZ added to the medium. OD600 was measured after 20 h. 654

655



20

GUS Staining, Microscopy and Scanning Electron Microscopy 656

GUS staining was performed as described in Köllmer et al. (2014). For microscopic analysis, 657

tissues were cleared according to Malamy and Benfey (1997). Hand-cut cross sections of 658

stems from 5-week-old plants were stained for 5 min in 0.02% aqueous toluidine blue O, 659

rinsed, and mounted in water. All samples were viewed with an Axioskop 2 plus microscope 660

(Zeiss, Jena, Germany). The inflorescence meristem of the main stem from 4-week-old soil 661

grown plants was dissected and analyzed by scanning electron microscopy as described 662

before (Bartrina et al., 2011). TCSn:GFP fluorescence was analyzed according to Zürcher et 663

al. (2013) using a Leica SP5 confocal microscope. Root meristem size was determined as 664

described by Dello Ioio et al. (2007). 665

666

Determination of the Cytokinin Content 667

Plants were grown in vitro for 14 days. For each sample, 100 mg of seedlings were pooled, 668

and five independent samples were analyzed for each genotype. The cytokinin content was 669

determined by ultra-performance liquid chromatography-electrospray tandem mass 670

spectrometry (Novak et al., 2008). 671

672

Analysis of Leaf Senescence and Photosynthetic Parameters 673

For the analysis of dark-induced leaf senescence, seedlings were grown in vitro for 18 days. 674

The sixth rosette leave was detached and floated on distilled water. After 7 days in the dark 675

at room temperature, chlorophyll was extracted as described before (Köllmer et al., 2011). 676

The maximum efficiency of PSII photochemistry (Fv/Fm ratio) of dark adapted plants was 677

measured with FluorCam (Photon Systems Instruments, Brno, Czech Republic). 678

679

Determination of Flowering Time, Stem Diameter, Plant Height and Yield Parameters 680

For flowering time analysis, seeds were stratified for three days at 4°C and sown on soil. 681

Onset of flowering was defined as the plant age when the first flower was visible. Termination 682

of flowering was defined as the time point when no new flowers were formed at the main 683

inflorescence. The number of rosette leaves was scored at the onset of flowering. The 684

diameter of the main inflorescence stem was determined when individual stems reached the 685

height of 15 cm. The hand-made transverse sections were taken 1 cm above the rosette. 686

The final plant height and the number of siliques were determined after termination of 687

flowering. For analysis of seed yield, the weight of all fully ripened and desiccated seeds was 688

determined. 689

690

Petal Surface Area and Cell Size Measurement 691



21

The petal surface area was measured from digital images of fully expanded organs with the 692

Scion Image. Petals were cleared (Malamy and Benfey, 1997), and average cell sizes were 693

calculated from the number of cells per unit area of digital micrographs. 694

695

696

ACKNOWLEDGEMENTS 697

We thank Ildoo Hwang for seeds of the ore12 mutant and Bruno Müller for the TCSn:GFP 698

line. We acknowledge funding by Deutsche Forschungsgemeinschaft (CRC 429, SPP 1530) 699

and the Czech Grant Agency (grant no. 15-22322S). I.B. and H.J. were supported by Elsa-700

Neumann-Scholarships of the state of Berlin. 701

702

SUPPLEMENTAL DATA 703

Supplemental Figure 1. Expression of AHK2 and AHK3 in different genotypes. 704

Supplemental Figure 2. Comparison of leaf senescence in wild type (WT), rock2, rock3 705

and ore12. 706

Supplemental Figure 3. Size and cytokinin activity in inflorescence meristems. 707

Supplemental Figure 4. Flower size and morphology of rock2 and rock3 mutants 708

compared to wild type (WT). 709

Supplemental Table 1. Genetic analysis of the rock2 and rock3 mutation. 710

Supplemental Table 2. Cytokinin content of rock2 CKX1ox and rock3 CKX1ox. 711

Supplemental Table 3. Oligonucleosis used in this study. 712



22

TABLES 713 714

Table 1: Cytokinin content of rock2 and rock3 715

716 Arabidopsis seedlings (14 DAG) were analyzed. iP, N6-(Δ2-isopentenyl)adenine; iPR, N6-(Δ2-717

isopentenyl)adenosine; iPRMP, N6-(Δ2-isopentenyl)adenosine 5’-monophospate; iP9G, N6-718

(Δ2-isopentenyl)adenine 9-glucoside; tZ, trans-zeatin; tZR, trans-zeatin riboside; tZRMP, 719

trans-zeatin riboside 5’-monophosphate; tZ9G, trans-zeatin 9-glucoside; tZOG, trans-zeatin 720

O-glucoside; tZROG, trans-zeatin riboside O-glucoside; cZ, cis-zeatin; cZR, cis-zeatin 721

riboside; cZRMP, cis-zeatin riboside 5’-monophosphate; cZOG, cis-zeatin O-glucoside; 722

cZROG, cis-zeatin riboside O-glucoside. Data shown are pmol/g fresh weight ± SD; n = 3. 723

724

725

genotype iP iPR iPRMP iP9G tZ tZR tZRMP Z9G

WT 1.91 ± 0.33 1.28 ± 0.18 2.18 ± 0.55 1.38 ± 0.09 0.71 ± 0.17 0.67 ± 0.19 1.29 ± 0.38 5.74 ± 0.90

rock2 1.13 ± 0.26 1.01 ± 0.18 2.23 ± 0.03 0.72 ± 0.06 0.32 ± 0.04 0.24 ± 0.05 0.40 ± 0.12 1.56 ± 0.38

AHK2:rock2 1.06 ± 0.29 1.25 ± 0.21 1.99 ± 0.31 0.93 ± 0.11 0.31 ± 0.01 0.23 ± 0.02 0.44 ± 0.09 1.88 ± 0.48

rock3 1.45 ± 0.41 1.08 ± 0.21 1.31 ± 0.07 0.84 ± 0.10 0.63 ± 0.11 0.20 ± 0.02 0.23 ± 0.06 1.30 ± 0.10

genotype tZOG tZROG cZ cZR cZRMP cZOG cZROG

WT 5.66 ± 1.00 0.46 ± 0.10 0.23 ± 0.04 0.89 ± 0.20 4.40 ± 0.76 12.26 ± 1.82 3.95 ± 1.27

rock2 1.83 ± 0.36 0.20 ± 0.03 0.14 ± 0.02 0.75 ± 0.23 3.15 ± 0.12 3.65 ± 0.18 4.46 ± 0.13

AHK2:rock2 1.67 ± 0.35 0.26 ± 0.03 0.13 ± 0.01 0.93 ± 0.18 3.84 ± 0.73 6.11 ± 1.78 4.33 ± 0.81

rock3 0.98 ± 0.04 0.21 ± 0.01 0.20 ± 0.05 1.02 ± 0.20 3.63 ± 1.03 3.92 ± 0.82 3.13 ± 0.45



23

FIGURE LEGENDS 726

Figure 1. The rock2 and rock3 mutations suppress the CKX1ox phenotype. A, Morphology of 727

wild-type (WT), CKX1ox, rock2 CKX1ox and rock3 CKX1ox plants at the rosette stage. 728

Plants were grown for 25 days under long-day conditions. B, rock2 CKX1ox and rock3 729

CKX1ox seedlings have larger cotyledons than WT and CKX1ox seedlings. The photo was 730

taken 10 days after germination. C, Adult phenotype of 46-d-old WT, CKX1ox, rock2 CKX1ox 731

and rock3 CKX1ox plants. D, Flowers of WT, CKX1ox, rock2 CKX1ox and rock3 CKX1ox 732

plants (from left to right). E and F, Flowering time of WT and mutant plants grown under long-733

day (E) or short-day (F) conditions. Crosses indicate that no transition to flowering occurred 734

(n = 20). G, Relative leaf chlorophyll content of the 4th and 5th leaf of 3-week-old soil-grown 735

plants after 7 days in the dark. Chlorophyll content before the start of dark incubation was set 736

100% (n = 3). H, Root elongation of seedlings between day 3 and 9 after germination (n ≥ 737

25). Error bars represent SD. Statistical significance of differences was calculated by two-738

way ANOVA. *, P < 0.01 compared to WT; °, P < 0.05 compared to CKX1ox plants. DAG, 739

days after germination. 740

741

Figure 2. rock2 and rock3 mutations alter cytokinin sensitivity. A, qRT-PCR analysis of CKX1 742

transcript levels in seedlings grown in vitro for 10 days. Expression values were normalized 743

to PP2AA2 and expression in WT was set to 1. Values are averages of three biological 744

replicates ± SE. B, Total cytokinin (CK) content of two-week-old seedlings. Contents of 745

individual cytokinin metabolites are shown in Supplemental Table S1. Error bars represent 746

SD. *, P < 0.05 compared to wild type as calculated by two-way ANOVA. C, Phenotype of 747

14-d-old seedlings grown on medium without (-CK) or with 25 nM BA (+CK). rock2 CKX1ox 748

and rock3 CKX1ox develop pale yellow leaves and show reduced shoot growth. 749

750

Figure 3. rock2 and rock3 are novel gain-of-function alleles of the AHK2 and AHK3 gene. A, 751

Two segments of the sequence alignment between the cytokinin receptor proteins AHK2, 752

AHK3 and AHK4. The amino acid residues which are mutated (shown in orange) in rock2 753

and rock3 are conserved in all three receptors. Full-length AHK protein sequences were 754

aligned using ClustalW. B, Schematic representation of the AHK2 and AHK3 protein domains 755

and the position of amino acid substitutions (red arrows) corresponding to the rock2 and 756

rock3 mutations. C-J, Genetic complementation of the CKX1ox phenotype by AHK2:rock2 757

and AHK3:rock3. Two independent transgenic AHK2:rock2 (G and H) and AHK3:rock3 (I and 758

J) lines in CKX1ox background in comparison to wild type (C), CKX1ox (D), rock2 CKX1ox 759

(E) and rock3 CKX1ox (F) at 18 DAG. 760

761



24

Figure 4. The rock2 and rock3 genes code for constitutively active receptors. A, Suppression 762

of the lethal sln1∆ mutation by histidine kinase activity of AHK4rock2 and AHK4rock3. Growth of 763

sln1∆ yeast strain expressing AHK4rock2 or AHK4rock3 was independent of the presence of 764

cytokinin. Error bars represent SD (n = 3). B, Relative expression levels of ARR5 in the rock 765

mutants compared with wild type. Quantitative RT-PCR analysis was performed using 5-d-766

old seedlings grown in vitro. Values are averages of three biological replicates ± SE. C-K, 767

Histochemical analysis of ARR5:GUS activity in wild type (C-E), rock2 (F-H) and rock3 (I-K) 768

background. Pictures show whole seedlings and shoot apices of 5-d-old plants stained 769

overnight and primary root tips of 7-d-old plants after 30 min of staining (from left to right). 770

Statistical significance of differences compared to wild type was calculated by ANOVA. *, P < 771

0.01. 772

773

Figure 5. Enhanced shoot growth of rock2 and rock3 mutants and transgenic plants. A and 774

B, Height of main inflorescence stem after termination of flowering (50 days after 775

germination). C and D, Primary inflorescence stems 3 cm above the rosette (C) and their 776

diameter (D). E, Stem sections of wild-type (WT) and AHK3:rock3 plants at the base of 777

primary inflorescence stems. Sections were stained with toluidine blue. Scale bar = 2 mm 778

(C). Error bars represent SD. Statistical significance of differences compared to wild type 779

was calculated by ANOVA. *, P < 0.001. 780

781

Figure 6. Leaf phenotype of rock2 and rock3 mutants and transgenic plants. A, Cotyledon 782

size of 5-d-old seedlings. Scale bar = 1 mm. B, Cotyledons and rosette leaves in the order of 783

appearance (from left to right) at 24 DAG. Scale bar = 1 cm. C, Fresh weight of rosette 784

leaves 32 DAG. D, Average size of abaxial epidermal cells of the sixth rosette leaf of wild-785

type (WT) and rock2 plants (n = 14). E, Leaf area of the sixth fully grown rosette leaf. 786

Statistical significance of differences compared to wild type was calculated by ANOVA. *, P < 787

0.001. 788

789

Figure 7. Natural and dark-induced leaf senescence. A, Age-dependent senescence 790

phenotype of the sixth leaf of WT and rock2/rock3 mutants grown under long-day conditions 791

starting at 16 days after leaf emergence (DAE). B, The photochemical efficiency of PSII 792

(Fv/Fm) of the sixth leaf at time points shown in A. C, Dark-induced senescence in a 793

detached leaf assay. The chlorophyll content of the sixth leaf was examined after 7 days in 794

the dark. The leaf chlorophyll content before the start of dark incubation was set at 100% for 795

each genotype tested. (n = 10). Error bars represent SD. Statistical significance of 796

differences compared to WT was calculated by ANOVA. *, P < 0.001. 797

798



25

Figure 8. rock2 and rock3 positively regulate flowering time and flower size. A, Number of 799

rosette leaves at the start of flowering of plants grown und long-day conditions. B, rock2 and 800

rock3 mutants and transgenic plants flower longer than wild type (WT). Shown are days until 801

termination of flowering. (n = 10). C, Flowers of rock2 and rock3 mutants and transgenic lines 802

compared to WT. D, Average size of abaxial epidermal cells of petals at stage 13 (Smyth et 803

al., 1990) (n = 5). E, Number of siliques on the main stem. Siliques, including unfilled and 804

partially filled siliques, were counted after the end of flowering (n = 15). F, Seed yield of rock2 805

and rock3 mutants and transgenic lines compared to WT. Seed yield of WT was set to 100%. 806

Error bars represent SD. Statistical significance of differences compared to WT was 807

calculated by Student’s t test. *, P < 0.005. 808

809

Figure 9. The rock2 and rock3 mutants have a reduced root system. A, Elongation of 810

primary roots between day 4 and 12 after germination (n = 20). B, Root meristems of wild 811

type, rock2 and rock3. Roots were analyzed 5 days after germination. White and black 812

arrowheads indicate, respectively, the QC and the start of the transition zone. Scale bar 50 813

µm. C, Number of cortex cells between the QC and the start of the transition zone (n = 10). 814

D, Number of lateral roots at 12 DAG (n = 20). Error bars represent SD. Statistical 815

significance of differences compared to WT was calculated by Student’s t test *, P < 0.005. 816

817

Figure 10. Model of conformational changes associated with transmembrane signaling 818

following cytokinin perception. Schematic topology of the sensory module of cytokinin 819

receptors based on the crystal structure of AHK4 (Hothorn et al., 2011). The N-terminal 820

helices (α1, α2 and its neighboring 310-helix) of the CHASE domain are shown in orange, the 821

two PAS domains are depicted schematically. The model predicts that cytokinin binding 822

causes reversible conformational changes (double-headed arrows) causing a piston-type 823

displacement of the different subdomains and ultimately resulting in transmission of the 824

signal across the membrane. rock2 and rock3 mutations (arrows) are predicted to mimic 825

those changes locking the receptor in a constitutively active conformation. 826

827



26

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01234567

Root elongation (cm

) **

◦

◦

0

20

40

60

80

Chlorophyll content (%

)

*

◦

◦

E

WT CKX1ox rock2 CKX1ox rock3 CKX1ox

B

G

FWT CKX1ox rock2 CKX1ox rock3 CKX1ox

A

WT CKX1ox rock2 CKX1ox rock3 CKX1ox

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H

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40

Flowering time (DAG)

**

* ◦◦

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Flowering time (DAG)

Figure 1. The rock2 and rock3mutations suppress the CKX1ox phenotype. A, Morphology of wild‐type(WT), CKX1ox, rock2 CKX1ox and rock3 CKX1ox plants at the rosette stage. Plants were grown for 25days under long‐day conditions. B, rock2 CKX1ox and rock3 CKX1ox seedlings have larger cotyledonsthan WT and CKX1ox seedlings. The photo was taken 10 days after germination. C, Adult phenotypeof 46‐d‐old WT, CKX1ox, rock2 CKX1ox and rock3 CKX1ox plants. D, Flowers of WT, CKX1ox, rock2CKX1ox and rock3 CKX1ox plants (from left to right). E and F, Flowering time of WT and mutant plantsgrown under long‐day (E) or short‐day (F) conditions. Crosses indicate that no transition to floweringoccurred (n = 20). G, Relative leaf chlorophyll content of the 4th and 5th leaf of 3‐week‐old soil‐grownplants after 7 days in the dark. Chlorophyll content before the start of dark incubation was set 100%(n = 3). H, Root elongation of seedlings between day 3 and 9 after germination (n ≥ 25). Error barsrepresent SD. Statistical significance of differences was calculated by two‐way ANOVA. *, P < 0.01compared to WT; °, P < 0.05 compared to CKX1ox plants. DAG, days after germination.



BA

C CKX1ox rock2 CKX1ox rock3 CKX1oxWT

‐CK

+CK

0

10

20

30

40

CK content (pmol/g FW

)

* * *

Figure 2. rock2 and rock3 mutations alter cytokinin sensitivity. A, qRT‐PCR analysis of CKX1 transcriptlevels in seedlings grown in vitro for 10 days. Expression values were normalized to PP2AA2 andexpression in WT was set to 1. Values are averages of three biological replicates ± SE. B, Totalcytokinin (CK) content of two‐week‐old seedlings. Contents of individual cytokinin metabolites areshown in Supplemental Table S1. Error bars represent SD. *, P < 0.05 compared to wild type ascalculated by two‐way ANOVA. C, Phenotype of 14‐d‐old seedlings grown on medium without (‐CK)or with 25 nM BA (+CK). rock2 CKX1ox and rock3 CKX1ox develop pale yellow leaves and showreduced shoot growth.

1

10

100Relative

CKX1exp

ression

(log1

0)



Figure 3. rock2 and rock3 are novel gain‐of‐function alleles of the AHK2 and AHK3 gene. A, Twosegments of the sequence alignment between the cytokinin receptor proteins AHK2, AHK3 and AHK4.The amino acid residues which are mutated (shown in orange) in rock2 and rock3 are conserved in allthree receptors. Full‐length AHK protein sequences were aligned using ClustalW. B, Schematicrepresentation of the AHK2 and AHK3 protein domains and the position of amino acid substitutions(red arrows) corresponding to the rock2 and rock3 mutations. C‐J, Genetic complementation of theCKX1ox phenotype by AHK2:rock2 and AHK3:rock3. Two independent transgenic AHK2:rock2 (G andH) and AHK3:rock3 (I and J) lines in CKX1ox background in comparison to wild type (C), CKX1ox (D),rock2 CKX1ox (E) and rock3 CKX1ox (F) at 18 DAG.

C E

G H

D F

I J

A

B

AHK2 AHK3

CHASE domain

Histidin kinase domain

Receiver domain

T179IL552F

TM domain

547 LVITFLVGYILYEAINRIATVEEDCQKMRELK410 LVIALLVAHIIHATVSRIHKVEEDCDKMKQLK409 FAIGFLVGYILYGAAMHIVKVEDDFHEMQELK

301 KIPSAIDQRTFEEYTERTNFERPLTSGVAYAL162 KIPSAIDQRTFSEYTDRTSFERPLTSGVAYAM174 KNPSAIDQETFAEYTARTAFERPLLSGVAYAE

rock3-1 (T179I)

rock2 (L552F)

AHK2AHK3 AHK4

rock3-2 (E182K)

AHK2AHK3AHK4

E182K



Figure 4. The rock2 and rock3 genes code for constitutively active receptors. A, Suppression of thelethal sln1∆ mutation by histidine kinase activity of AHK4rock2 and AHK4rock3. Growth of sln1∆ yeaststrain expressing AHK4rock2 or AHK4rock3 was independent of the presence of cytokinin. Error barsrepresent SD (n = 3). B, Relative expression levels of ARR5 in the rock mutants compared with wildtype. Quantitative RT‐PCR analysis was performed using 5‐d‐old seedlings grown in vitro. Values areaverages of three biological replicates ± SE. C‐K, Histochemical analysis of ARR5:GUS activity in wildtype (C‐E), rock2 (F‐H) and rock3 (I‐K) background. Pictures show whole seedlings and shoot apices of5‐d‐old plants stained overnight and primary root tips of 7‐d‐old plants after 30 min of staining (fromleft to right). Statistical significance of differences compared to wild type was calculated by ANOVA. *,P < 0.01.

A

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F G H

I J K

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2Optical D

ensity (600 nm) 0 µM tZ 1 µM tZ 10 µM tZ

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ARR5 exp

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* *



Figure 5. Enhanced shoot growth of rock2 and rock3 mutants and transgenic plants. A and B, Heightof main inflorescence stem after termination of flowering (50 days after germination). C and D,Primary inflorescence stems 3 cm above the rosette (C) and their diameter (D). E, Stem sections ofwild‐type (WT) and AHK3:rock3 plants at the base of primary inflorescence stems. Sections werestained with toluidine blue. Scale bar = 2 mm (C). Error bars represent SD. Statistical significance ofdifferences compared to wild type was calculated by ANOVA. *, P < 0.001.

0,0

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m)

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*

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)

WT AHK3:rock3

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**



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5

Leaf area (cm

2)

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eight (g)

Figure 6. Leaf phenotype of rock2 and rock3 mutants and transgenic plants. A, Cotyledon size of 5‐d‐old seedlings. Scale bar = 1 mm. B, Cotyledons and rosette leaves in the order of appearance (fromleft to right) at 24 DAG. Scale bar = 1 cm. C, Fresh weight of rosette leaves 32 DAG. D, Average size ofabaxial epidermal cells of the sixth rosette leaf of wild‐type (WT) and rock2 plants (n = 14). E, Leafarea of the sixth fully grown rosette leaf. Statistical significance of differences compared to wild typewas calculated by ANOVA. *, P < 0.001.

0

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2500

Cell size (µm

2)

A

C D

B

WT

rock2

AHK2:rock2

rock3

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rock2

AHK2:rock2

rock3

*E

*



0

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16 20 24 28 32 36

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DAE

WT

rock2

rock3

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70

Chlorophyll content (%

)

B C

16 18 20 22 25 28 30 32 35 DAE

WT

rock2

rock3

**

*

**** *

*

**

A

Figure 7. Natural and dark‐induced leaf senescence. A, Age‐dependent senescence phenotype of the sixth leaf of WT and rock2/rock3 mutants grown under long‐day conditions starting at 16 days after leaf emergence (DAE). B, The photochemical efficiency of PSII (Fv/Fm) of the sixth leaf at time points shown in A. C, Dark‐induced senescence in a detached leaf assay. The chlorophyll content of the sixth leaf was examined after 7 days in the dark. The leaf chlorophyll content before the start of dark incubation was set at 100% for each genotype tested. (n = 10). Error bars represent SD. Statistical significance of differences compared to WT was calculated by ANOVA. *, P < 0.001.



Figure 7:

0

20

40

60

80

Cell size (µm

2)

010203040506070

Days until term

ination

of flowering

020406080

100120140160180

Yield (%)

0

20

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60

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100

No. of siliq

ues

0

2

4

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12

No. of leaves

A B

E F

D

**

** *

** * **

* *

**

C

Figure 8. rock2 and rock3 positively regulate flowering time and flower size. A, Number of rosetteleaves at the start of flowering of plants grown und long‐day conditions. B, rock2 and rock3 mutantsand transgenic plants flower longer than wild type (WT). Shown are days until termination offlowering. (n = 10). C, Flowers of rock2 and rock3 mutants and transgenic lines compared to WT. D,Average size of abaxial epidermal cells of petals at stage 13 (Smyth et al., 1990) (n = 5). E, Number ofsiliques on the main stem. Siliques, including unfilled and partially filled siliques, were counted afterthe end of flowering (n = 15). F, Seed yield of rock2 and rock3 mutants and transgenic lines comparedto WT. Seed yield of WT was set to 100%. Error bars represent SD. Statistical significance ofdifferences compared to WT was calculated by Student’s t test. *, P < 0.005.



0

10

20

30

40

50

Meristem size(cellnumber)

Figure 9. The rock2 and rock3 mutants have a reduced root system. A, Elongation of primary roots between day 4 and 12 after germination (n = 20). B, Root meristems of wild type, rock2 and rock3. Roots were analyzed 5 days after germination. White and black arrowheads indicate, respectively, the QC and the start of the transition zone. Scale bar 50 µm. C, Number of cortex cells between the QC and the start of the transition zone (n = 10). D, Number of lateral roots at 12 DAG (n = 20). Error bars represent SD. Statistical significance of differences compared to WT was calculated by Student’s t test *, P < 0.005.

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)

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* * * *

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* * * *

Numberoflateral roots

WT rock2 rock3



CK

L552F

T179I

E182K

N C

PASproximal

PASdistal

Figure 10. Model of conformational changes associated with transmembrane signaling followingcytokinin perception. Schematic topology of the sensory module of cytokinin receptors based on thecrystal structure of AHK4 (Hothorn et al., 2011). The N‐terminal helices (α1, α2 and its neighboring310‐helix) of the CHASE domain are shown in orange, the two PAS domains are depictedschematically. The model predicts that cytokinin binding causes reversible conformational changes(double‐headed arrows) causing a piston‐type displacement of the different subdomains andultimately resulting in transmission of the signal across the membrane. rock2 and rock3 mutations(arrows) are predicted to mimic those changes locking the receptor in a constitutively activeconformation.



Parsed CitationsAinley WM, McNeil K, Hill J, Lingle W, Simpson R, Brenner M, Nagao R, Key J (1993) Regulatable endogenous production ofcytokinins up to 'toxic' levels in transgenic plants and plant tissues. Plant Mol Biol 22: 13-23

Pubmed: Author and TitleCrossRef: Author and TitleGoogle Scholar: Author Only Title Only Author and Title

Ashikari M, Sakakibara H, Lin SY, Yamamoto T, Takashi T, Nishimura A, Angeles ER, Qian Q, Kitano H, Matsuoka M (2005) Cytokininoxidase regulates rice grain production. Science 309: 741-745


Bartrina I, Otto E, Strnad M, Werner T, Schmülling T (2011) Cytokinin regulates the activity of reproductive meristems, flowerorgan size, ovule formation, and thus seed yield in Arabidopsis thaliana. Plant Cell 23: 69-80


Besnard-Wibaut C (1981) Effectiveness of gibberellins and 6-benzyladenine on flowering of Arabidopsis thaliana. Physiol Plant 53:205-212


Bhargava A, Clabaugh I, To JP, Maxwell BB, Chiang YH, Schaller GE, Loraine A, Kieber JJ (2013) Identification of cytokinin-responsive genes using microarray meta-analysis and RNA-seq in Arabidopsis. Plant Physiol 162: 272-294


Bhate MP, Molnar KS, Goulian M, DeGrado WF (2015) Signal transduction in histidine kinases: Insights from new structures.Structure 23: 981-994


Brenner WG, Ramireddy E, Heyl A, Schmülling T (2012) Gene regulation by cytokinin in Arabidopsis. Front Plant Sci 3: 8Pubmed: Author and TitleCrossRef: Author and TitleGoogle Scholar: Author Only Title Only Author and Title

Brenner WG, Schmülling T (2015) Summarizing and exploring data of a decade of cytokinin-related transcriptomics. Front Plant Sci6: 29


Caesar K, Thamm AMK, Witthöft J, Elgass K, Huppenberger P, Grefen C, Horak J, Harter K (2011) Evidence for the localization ofthe Arabidopsis cytokinin receptors AHK3 and AHK4 in the endoplasmic reticulum. J Exp Bot 62: 5571-5580


Chervitz SA, Falke JJ (1996) Molecular mechanism of transmembrane signaling by the aspartate receptor: a model. Proc Natl AcadSci USA 93: 2545-2550


Cheung J, Hendrickson WA (2009) Structural analysis of ligand stimulation of the histidine kinase NarX. Structure 17: 190-201Pubmed: Author and TitleCrossRef: Author and TitleGoogle Scholar: Author Only Title Only Author and Title

Clough SJ, Bent AF (1998) Floral dip: a simplified method forAgrobacterium-mediated transformation of Arabidopsis thaliana. PlantJ. 16: 735-743


Corbesier L, Prinsen E, Jacqmard A, Lejeune P, Van Onckelen H, Perilleux C, Bernier G (2003) Cytokinin levels in leaves, leafexudate and shoot apical meristem of Arabidopsis thaliana during floral transition. J Exp Bot 54: 2511-2517


Cortleven A, Nitschke S, Klaumünzer M, Abdelgawad H, Asard H, Grimm B, Riefler M, Schmülling T (2014) A novel protectivefunction for cytokinin in the light stress response is mediated by the Arabidopsis histidine kinase2 and Arabidopsis histidinehttps://plantphysiol.orgDownloaded on January 3, 2021. - Published by

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http://www.ncbi.nlm.nih.gov/pubmed?term=Ainley WM%2C McNeil K%2C Hill J%2C Lingle W%2C Simpson R%2C Brenner M%2C Nagao R%2C Key J %281993%29 Regulatable endogenous production of cytokinins up to %27toxic%27 levels in transgenic plants and plant tissues%2E Plant Mol Biol 22%3A 13%2D23&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Ainley WM, McNeil K, Hill J, Lingle W, Simpson R, Brenner M, Nagao R, Key J (1993) Regulatable endogenous production of cytokinins up to 'toxic' levels in transgenic plants and plant tissues. Plant Mol Biol 22: 13-23

http://scholar.google.com/scholar?as_q=&num=10&as_occt=any&as_sauthors=Ainley&hl=en&c2coff=1

http://scholar.google.com/scholar?as_q=Regulatable endogenous production of cytokinins up to 'toxic' levels in transgenic plants and plant tissues&num=10&btnG=Search+Scholar&as_epq=&as_oq=&as_eq=&as_occt=any&as_publication=&as_yhi=&as_allsubj=all&hl=en&lr=&c2coff=1

http://scholar.google.com/scholar?as_q=Regulatable endogenous production of cytokinins up to 'toxic' levels in transgenic plants and plant tissues&num=10&btnG=Search+Scholar&as_occt=any&as_sauthors=Ainley&as_ylo=1993&as_allsubj=all&hl=en&c2coff=1

http://www.ncbi.nlm.nih.gov/pubmed?term=Ashikari M%2C Sakakibara H%2C Lin SY%2C Yamamoto T%2C Takashi T%2C Nishimura A%2C Angeles ER%2C Qian Q%2C Kitano H%2C Matsuoka M %282005%29 Cytokinin oxidase regulates rice grain production%2E Science 309%3A 741%2D745&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Ashikari M, Sakakibara H, Lin SY, Yamamoto T, Takashi T, Nishimura A, Angeles ER, Qian Q, Kitano H, Matsuoka M (2005) Cytokinin oxidase regulates rice grain production. Science 309: 741-745

http://scholar.google.com/scholar?as_q=&num=10&as_occt=any&as_sauthors=Ashikari&hl=en&c2coff=1

http://scholar.google.com/scholar?as_q=Cytokinin oxidase regulates rice grain production&num=10&btnG=Search+Scholar&as_epq=&as_oq=&as_eq=&as_occt=any&as_publication=&as_yhi=&as_allsubj=all&hl=en&lr=&c2coff=1

http://scholar.google.com/scholar?as_q=Cytokinin oxidase regulates rice grain production&num=10&btnG=Search+Scholar&as_occt=any&as_sauthors=Ashikari&as_ylo=2005&as_allsubj=all&hl=en&c2coff=1

http://www.ncbi.nlm.nih.gov/pubmed?term=Bartrina I%2C Otto E%2C Strnad M%2C Werner T%2C Schm�lling T %282011%29 Cytokinin regulates the activity of reproductive meristems%2C flower organ size%2C ovule formation%2C and thus seed yield in Arabidopsis thaliana%2E Plant Cell 23%3A 69%2D80&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Bartrina I, Otto E, Strnad M, Werner T, Schm�lling T (2011) Cytokinin regulates the activity of reproductive meristems, flower organ size, ovule formation, and thus seed yield in Arabidopsis thaliana. Plant Cell 23: 69-80

http://scholar.google.com/scholar?as_q=&num=10&as_occt=any&as_sauthors=Bartrina&hl=en&c2coff=1

http://scholar.google.com/scholar?as_q=Cytokinin regulates the activity of reproductive meristems, flower organ size, ovule formation, and thus seed yield in Arabidopsis thaliana&num=10&btnG=Search+Scholar&as_epq=&as_oq=&as_eq=&as_occt=any&as_publication=&as_yhi=&as_allsubj=all&hl=en&lr=&c2coff=1

http://scholar.google.com/scholar?as_q=Cytokinin regulates the activity of reproductive meristems, flower organ size, ovule formation, and thus seed yield in Arabidopsis thaliana&num=10&btnG=Search+Scholar&as_occt=any&as_sauthors=Bartrina&as_ylo=2011&as_allsubj=all&hl=en&c2coff=1

http://www.ncbi.nlm.nih.gov/pubmed?term=Besnard%2DWibaut C %281981%29 Effectiveness of gibberellins and 6%2Dbenzyladenine on flowering of Arabidopsis thaliana%2E Physiol Plant 53%3A 205%2D212&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Besnard-Wibaut C (1981) Effectiveness of gibberellins and 6-benzyladenine on flowering of Arabidopsis thaliana. Physiol Plant 53: 205-212

http://scholar.google.com/scholar?as_q=&num=10&as_occt=any&as_sauthors=Besnard-Wibaut&hl=en&c2coff=1

http://scholar.google.com/scholar?as_q=Effectiveness of gibberellins and 6-benzyladenine on flowering of Arabidopsis thaliana&num=10&btnG=Search+Scholar&as_epq=&as_oq=&as_eq=&as_occt=any&as_publication=&as_yhi=&as_allsubj=all&hl=en&lr=&c2coff=1

http://scholar.google.com/scholar?as_q=Effectiveness of gibberellins and 6-benzyladenine on flowering of Arabidopsis thaliana&num=10&btnG=Search+Scholar&as_occt=any&as_sauthors=Besnard-Wibaut&as_ylo=1981&as_allsubj=all&hl=en&c2coff=1

http://www.ncbi.nlm.nih.gov/pubmed?term=Bhargava A%2C Clabaugh I%2C To JP%2C Maxwell BB%2C Chiang YH%2C Schaller GE%2C Loraine A%2C Kieber JJ %282013%29 Identification of cytokinin%2Dresponsive genes using microarray meta%2Danalysis and RNA%2Dseq in Arabidopsis%2E Plant Physiol 162%3A 272%2D294&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Bhargava A, Clabaugh I, To JP, Maxwell BB, Chiang YH, Schaller GE, Loraine A, Kieber JJ (2013) Identification of cytokinin-responsive genes using microarray meta-analysis and RNA-seq in Arabidopsis. Plant Physiol 162: 272-294

http://scholar.google.com/scholar?as_q=&num=10&as_occt=any&as_sauthors=Bhargava&hl=en&c2coff=1

http://scholar.google.com/scholar?as_q=Identification of cytokinin-responsive genes using microarray meta-analysis and RNA-seq in Arabidopsis&num=10&btnG=Search+Scholar&as_epq=&as_oq=&as_eq=&as_occt=any&as_publication=&as_yhi=&as_allsubj=all&hl=en&lr=&c2coff=1

http://scholar.google.com/scholar?as_q=Identification of cytokinin-responsive genes using microarray meta-analysis and RNA-seq in Arabidopsis&num=10&btnG=Search+Scholar&as_occt=any&as_sauthors=Bhargava&as_ylo=2013&as_allsubj=all&hl=en&c2coff=1

http://www.ncbi.nlm.nih.gov/pubmed?term=Bhate MP%2C Molnar KS%2C Goulian M%2C DeGrado WF %282015%29 Signal transduction in histidine kinases%3A Insights from new structures%2E Structure 23%3A 981%2D994&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Bhate MP, Molnar KS, Goulian M, DeGrado WF (2015) Signal transduction in histidine kinases: Insights from new structures. Structure 23: 981-994

http://scholar.google.com/scholar?as_q=&num=10&as_occt=any&as_sauthors=Bhate&hl=en&c2coff=1

http://scholar.google.com/scholar?as_q=Signal transduction in histidine kinases: Insights from new structures&num=10&btnG=Search+Scholar&as_epq=&as_oq=&as_eq=&as_occt=any&as_publication=&as_yhi=&as_allsubj=all&hl=en&lr=&c2coff=1

http://scholar.google.com/scholar?as_q=Signal transduction in histidine kinases: Insights from new structures&num=10&btnG=Search+Scholar&as_occt=any&as_sauthors=Bhate&as_ylo=2015&as_allsubj=all&hl=en&c2coff=1

http://www.ncbi.nlm.nih.gov/pubmed?term=Brenner WG%2C Ramireddy E%2C Heyl A%2C Schm�lling T %282012%29 Gene regulation by cytokinin in Arabidopsis%2E Front Plant Sci 3%3A 8&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Brenner WG, Ramireddy E, Heyl A, Schm�lling T (2012) Gene regulation by cytokinin in Arabidopsis. Front Plant Sci 3: 8

http://scholar.google.com/scholar?as_q=&num=10&as_occt=any&as_sauthors=Brenner&hl=en&c2coff=1

http://scholar.google.com/scholar?as_q=Gene regulation by cytokinin in Arabidopsis.&num=10&btnG=Search+Scholar&as_epq=&as_oq=&as_eq=&as_occt=any&as_publication=&as_yhi=&as_allsubj=all&hl=en&lr=&c2coff=1

http://scholar.google.com/scholar?as_q=Gene regulation by cytokinin in Arabidopsis.&num=10&btnG=Search+Scholar&as_occt=any&as_sauthors=Brenner&as_ylo=2012&as_allsubj=all&hl=en&c2coff=1

http://www.ncbi.nlm.nih.gov/pubmed?term=Brenner WG%2C Schm�lling T %282015%29 Summarizing and exploring data of a decade of cytokinin%2Drelated transcriptomics%2E Front Plant Sci 6%3A 29&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Brenner WG, Schm�lling T (2015) Summarizing and exploring data of a decade of cytokinin-related transcriptomics. Front Plant Sci 6: 29

http://scholar.google.com/scholar?as_q=&num=10&as_occt=any&as_sauthors=Brenner&hl=en&c2coff=1

http://scholar.google.com/scholar?as_q=Summarizing and exploring data of a decade of cytokinin-related transcriptomics.&num=10&btnG=Search+Scholar&as_epq=&as_oq=&as_eq=&as_occt=any&as_publication=&as_yhi=&as_allsubj=all&hl=en&lr=&c2coff=1

http://scholar.google.com/scholar?as_q=Summarizing and exploring data of a decade of cytokinin-related transcriptomics.&num=10&btnG=Search+Scholar&as_occt=any&as_sauthors=Brenner&as_ylo=2015&as_allsubj=all&hl=en&c2coff=1

http://www.ncbi.nlm.nih.gov/pubmed?term=Caesar K%2C Thamm AMK%2C Witth�ft J%2C Elgass K%2C Huppenberger P%2C Grefen C%2C Horak J%2C Harter K %282011%29 Evidence for the localization of the Arabidopsis cytokinin receptors AHK3 and AHK4 in the endoplasmic reticulum%2E J Exp Bot 62%3A 5571%2D5580&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Caesar K, Thamm AMK, Witth�ft J, Elgass K, Huppenberger P, Grefen C, Horak J, Harter K (2011) Evidence for the localization of the Arabidopsis cytokinin receptors AHK3 and AHK4 in the endoplasmic reticulum. J Exp Bot 62: 5571-5580

http://scholar.google.com/scholar?as_q=&num=10&as_occt=any&as_sauthors=Caesar&hl=en&c2coff=1

http://scholar.google.com/scholar?as_q=Evidence for the localization of the Arabidopsis cytokinin receptors AHK3 and AHK4 in the endoplasmic reticulum&num=10&btnG=Search+Scholar&as_epq=&as_oq=&as_eq=&as_occt=any&as_publication=&as_yhi=&as_allsubj=all&hl=en&lr=&c2coff=1

http://scholar.google.com/scholar?as_q=Evidence for the localization of the Arabidopsis cytokinin receptors AHK3 and AHK4 in the endoplasmic reticulum&num=10&btnG=Search+Scholar&as_occt=any&as_sauthors=Caesar&as_ylo=2011&as_allsubj=all&hl=en&c2coff=1

http://www.ncbi.nlm.nih.gov/pubmed?term=Chervitz SA%2C Falke JJ %281996%29 Molecular mechanism of transmembrane signaling by the aspartate receptor%3A a model%2E Proc Natl Acad Sci USA 93%3A 2545%2D2550&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Chervitz SA, Falke JJ (1996) Molecular mechanism of transmembrane signaling by the aspartate receptor: a model. Proc Natl Acad Sci USA 93: 2545-2550

http://scholar.google.com/scholar?as_q=&num=10&as_occt=any&as_sauthors=Chervitz&hl=en&c2coff=1

http://scholar.google.com/scholar?as_q=Molecular mechanism of transmembrane signaling by the aspartate receptor: a model&num=10&btnG=Search+Scholar&as_epq=&as_oq=&as_eq=&as_occt=any&as_publication=&as_yhi=&as_allsubj=all&hl=en&lr=&c2coff=1

http://scholar.google.com/scholar?as_q=Molecular mechanism of transmembrane signaling by the aspartate receptor: a model&num=10&btnG=Search+Scholar&as_occt=any&as_sauthors=Chervitz&as_ylo=1996&as_allsubj=all&hl=en&c2coff=1

http://www.ncbi.nlm.nih.gov/pubmed?term=Cheung J%2C Hendrickson WA %282009%29 Structural analysis of ligand stimulation of the histidine kinase NarX%2E Structure 17%3A 190%2D201&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Cheung J, Hendrickson WA (2009) Structural analysis of ligand stimulation of the histidine kinase NarX. Structure 17: 190-201

http://scholar.google.com/scholar?as_q=&num=10&as_occt=any&as_sauthors=Cheung&hl=en&c2coff=1

http://scholar.google.com/scholar?as_q=Structural analysis of ligand stimulation of the histidine kinase NarX&num=10&btnG=Search+Scholar&as_epq=&as_oq=&as_eq=&as_occt=any&as_publication=&as_yhi=&as_allsubj=all&hl=en&lr=&c2coff=1

http://scholar.google.com/scholar?as_q=Structural analysis of ligand stimulation of the histidine kinase NarX&num=10&btnG=Search+Scholar&as_occt=any&as_sauthors=Cheung&as_ylo=2009&as_allsubj=all&hl=en&c2coff=1

http://www.ncbi.nlm.nih.gov/pubmed?term=Clough SJ%2C Bent AF %281998%29 Floral dip%3A a simplified method forAgrobacterium%2Dmediated transformation of Arabidopsis thaliana%2E Plant J%2E 16%3A 735%2D743&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Clough SJ, Bent AF (1998) Floral dip: a simplified method forAgrobacterium-mediated transformation of Arabidopsis thaliana. Plant J. 16: 735-743

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http://www.ncbi.nlm.nih.gov/pubmed?term=D%27Aloia M%2C Bonhomme D%2C Bouche F%2C Tamseddak K%2C Ormenese S%2C Torti S%2C Coupland G%2C Perilleux C %282011%29 Cytokinin promotes flowering of Arabidopsis via transcriptional activation of the FT paralogue TSF%2E Plant J 65%3A 972%2D979&dopt=abstract

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http://www.ncbi.nlm.nih.gov/pubmed?term=Dello Ioio R%2C Linhares FS%2C Scacchi E%2C Casamitjana%2DMartinez E%2C Heidstra R%2C Costantino P%2C Sabatini S %282007%29 Cytokinins determine Arabidopsis root%2Dmeristem size by controlling cell differentiation%2E Curr Biol 17%3A 678%2D682&dopt=abstract

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http://www.ncbi.nlm.nih.gov/pubmed?term=Dennis ES%2C Finnegan EJ%2C Bilodeau P%2C Chaudhury A%2C Genger R%2C Helliwell CA%2C Sheldon CC%2C Bagnall DJ%2C Peacock WJ %281996%29 Vernalization and the initiation of flowering%2E Semin Cell Dev Biol 7%3A 441%2D448&dopt=abstract

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http://www.ncbi.nlm.nih.gov/pubmed?term=Dewitte W%2C Riou%2DKhamlichi C%2C Scofield S%2C Healy JMS%2C Jacqmard A%2C Kilby NJ%2C Murray JAH %282003%29 Altered cell cycle distribution%2C hyperplasia%2C and inhibited differentiation in Arabidopsis caused by the D%2Dtype cyclin CYCD3%2E Plant Cell 15%3A 79%2D92&dopt=abstract

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Moore JO, Hendrickson WA (2009) Structural analysis of sensor domains from the TMAO-responsive histidine kinase receptorTorS. Structure 17: 1195-1204


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Nieminen K, Immanen J, Laxell M, Kauppinen L, Tarkowski P, Dolezal K, Tahtiharju S, Elo A, Decourteix M, Ljung K, Bhalerao R,Keinonen K, Albert VA, Helariutta Y (2008) Cytokinin signaling regulates cambial development in poplar. Proc Natl Acad Sci USA105: 20032-20037


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Pubmed: Author and Titlehttps://plantphysiol.orgDownloaded on January 3, 2021. - Published by


http://www.ncbi.nlm.nih.gov/pubmed?term=Moore JO%2C Hendrickson WA %282009%29 Structural analysis of sensor domains from the TMAO%2Dresponsive histidine kinase receptor TorS%2E Structure 17%3A 1195%2D1204&dopt=abstract

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http://www.ncbi.nlm.nih.gov/pubmed?term=M�ller B%2C Sheen J %282007%29 Advances in cytokinin signaling%2E Science 318%3A 68%2D69&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=M�ller B, Sheen J (2007) Advances in cytokinin signaling. Science 318: 68-69

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http://search.crossref.org/?page=1&rows=2&q=Niemann MCE, Bartrina I, Ashikov A, Weber H, Nov�k O, Sp�chal L, Strnad M, Strasser R, Bakker H, Schm�lling T, Werner T (2015) Arabidopsis ROCK1 transports UDP-GlcNAc/UDP-GalNAc and regulates ER protein quality control and cytokinin activity. Proc Natl Acad Sci USA 112: 291-296

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http://www.ncbi.nlm.nih.gov/pubmed?term=Nieminen K%2C Immanen J%2C Laxell M%2C Kauppinen L%2C Tarkowski P%2C Dolezal K%2C Tahtiharju S%2C Elo A%2C Decourteix M%2C Ljung K%2C Bhalerao R%2C Keinonen K%2C Albert VA%2C Helariutta Y %282008%29 Cytokinin signaling regulates cambial development in poplar%2E Proc Natl Acad Sci USA 105%3A 20032%2D20037&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Nieminen K, Immanen J, Laxell M, Kauppinen L, Tarkowski P, Dolezal K, Tahtiharju S, Elo A, Decourteix M, Ljung K, Bhalerao R, Keinonen K, Albert VA, Helariutta Y (2008) Cytokinin signaling regulates cambial development in poplar. Proc Natl Acad Sci USA 105: 20032-20037

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http://search.crossref.org/?page=1&rows=2&q=Nishimura C, Ohashi Y, Sato S, Kato T, Tabata S, Ueguchi C (2004) Histidine kinase homologs that act as cytokinin receptors possess overlapping functions in the regulation of shoot and root growth in Arabidopsis. Plant Cell 16: 1365-1377

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http://www.ncbi.nlm.nih.gov/pubmed?term=Nood�n LD%2C Penney JP %282001%29 Correlative controls of senescence and plant death in Arabidopsis thaliana %28Brassicaceae%29%2E J Exp Bot 52%3A 2151%2D2159&dopt=abstract

http://search.crossref.org/?page=1&rows=2&q=Nood�n LD, Penney JP (2001) Correlative controls of senescence and plant death in Arabidopsis thaliana (Brassicaceae). J Exp Bot 52: 2151-2159

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http://search.crossref.org/?page=1&rows=2&q=Novak O, Hauserova E, Amakorova P, Dolezal K, Strnad M (2008) Cytokinin profiling in plant tissues using ultra-performance liquid chromatography-electrospray tandem mass spectrometry. Phytochemistry 69: 2214-2224

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http://www.ncbi.nlm.nih.gov/pubmed?term=Richmond AE%2C Lang A %281957%29 Effect of kinetin on protein content and survival of detached Xanthium leaves%2E Science 125%3A 650%2D651&dopt=abstract

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CrossRef: Author and TitleGoogle Scholar: Author Only Title Only Author and Title

Steklov MY, Lomin SN, Osolodkin DI, Romanov GA (2013) Structural basis for cytokinin receptor signaling: an evolutionaryapproach. Plant Cell Rep 32: 781-793


Stolz A, Riefler M, Lomin SN, Achazi K, Romanov GA, Schmülling T (2011) The specificity of cytokinin signalling in Arabidopsisthaliana is mediated by differing ligand affinities and expression profiles of the receptors. Plant J 67: 157-168


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Suzuki T, Miwa K, Ishikawa K, Yamada H, Aiba H, Mizuno T (2001) The Arabidopsis sensor His-kinase, AHK4, can respond tocytokinins. Plant Cell Physiol 42: 107-113


Tran L-SP, Urao T, Qin F, Maruyama K, Kakimoto T, Shinozaki K, Yamaguchi-Shinozaki K (2007) Functional analysis ofAHK1/ATHK1 and cytokinin receptor histidine kinases in response to abscisic acid, drought, and salt stress in Arabidopsis. ProcNatl Acad Sci USA 104: 20623-20628


Ueguchi C, Sato S, Kato T, Tabata S (2001) The AHK4 gene involved in the cytokinin-signaling pathway as a direct receptormolecule in Arabidopsis thaliana. Plant Cell Physiol 42: 751-755


van der Graaff EE, Hooykaas PJJ, Auer CA (2001) Altered development of Arabidopsis thaliana carrying the Agrobacteriumtumefaciens ipt gene is partially due to ethylene effects. Plant Growth Regul 34: 305-315


Werner T, Holst K, Pors Y, Guivarc'h A, Mustroph A, Chriqui D, Grimm B, Schmülling T (2008) Cytokinin deficiency causes distinctchanges of sink and source parameters in tobacco shoots and roots. J Exp Bot 59: 2659-2672


Werner T, Köllmer I, Bartrina I, Holst K, Schmülling T (2006) New insights into the biology of cytokinin degradation. Plant Biol 8: 1-12


Werner T, Motyka V, Laucou V, Smets R, Van Onckelen H, Schmülling T (2003) Cytokinin-deficient transgenic Arabidopsis plantsshow multiple developmental alterations indicating opposite functions of cytokinins in the regulation of shoot and root meristemactivity. Plant Cell 15: 2532-2550


Werner T, Schmülling T (2009) Cytokinin action in plant development. Curr Opin Plant Biol 12: 527-538Pubmed: Author and TitleCrossRef: Author and TitleGoogle Scholar: Author Only Title Only Author and Title

Wuest SE, Philipp MA, Guthörl D, Schmid B, Grossniklaus U (2016) Seed production affects maternal growth and senescence inArabidopsis. Plant Physiol 171: 392-404


Wulfetange K, Lomin SN, Romanov GA, Stolz A, Heyl A, Schmülling T (2011) The cytokinin receptors of Arabidopsis are locatedmainly to the endoplasmic reticulum. Plant Physiol 156: 1808-1818

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gain-of-function mutants of the cytokinin receptors ahk2 and … · 2017. 1. 17. · 5 102...

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