ORIGINAL PAPER

Rhizobacterial potential to alter auxin content and growthof Vigna radiata (L.)

Basharat Ali • Anjum Nasim Sabri •

Shahida Hasnain

Received: 1 August 2009 / Accepted: 4 January 2010 / Published online: 14 January 2010

� Springer Science+Business Media B.V. 2010

Abstract Potential of non-symbiotic plant growth pro-

moting rhizobacteria (PGPR) to influence the endogenous

indole-3-acetic acid (IAA) content and growth of Vigna

radiata (L.) was evaluated. The bacterial strains used

belonged to Pseudomonas, Escherichia, Micrococcus and

Staphylococcus genera. All strains were able to produce

IAA (1.16–8.22 lg ml-1) in the presence of

1,000 lg ml-1 of L-tryptophan as revealed by gas chro-

matography and mass spectrometric (GC–MS) analysis.

However, strains exhibited variable results for other growth

promoting traits such as phosphate solubilization and sid-

erophore or hydrogen cyanide production. Bacterial IAA

production showed significant positive correlation with

endogenous IAA content of roots (r = 0.969; P = 0.01)

and leaves (r = 0.905; P = 0.01) under axenic conditions.

Bacterization of V. radiata seeds significantly enhanced

shoot length (up to 48.10%) and shoot fresh biomass (up to

43.80%) under fully axenic conditions. Bacterial strains

applied under wire-house conditions also improved shoot

length, number of pods, and grain weight up to 58, 65, and

17.15% respectively, over control. Hence, free living (non-

symbiotic) PGPR have the ability to influence endogenous

IAA content and growth of leguminous plants.

Keywords Non-symbiotic PGPR � Endogenous IAA �Growth promoting traits � L-Tryptophan � Axenic

conditions

Introduction

Indole-3-acetic acid (IAA) is quantitatively the most

abundant type of auxins that play a crucial role in many

developmental processes in plants (Woodward and Bartel

2005). Although plants are able to synthesize IAA them-

selves, the microorganisms that are the inhabitants of rhi-

zosphere also contribute to the plant’s auxin pool

(Arkhipova et al. 2005). Due to this ability, IAA producing

bacteria have been used as inoculants to enhance the

growth and yield of crops (Asghar et al. 2004; Khalid et al.

2004). Bacterial strains from different genera have been

shown to produce IAA in the presence of L-tryptophan (Ali

et al. 2009a). The release of L-tryptophan in root exudates

may result in its conversion into IAA by rhizosphere

microbes (Kravchenko et al. 2004). However, IAA pro-

duced by rhizobacteria can influence plant growth only if it

is absorbed by plant roots and not utilized by other

microorganisms present in the vicinity (Martens and

Frankenberger 1994). The application of plant growth

promoting rhizobacteria (PGPR) as crop inoculants for

biofertilization, phytostimulation, and biocontrol would be

an attractive option to reduce the use of chemical fertilizers

which also cause environmental pollution (Bloemberg and

Lugtenberg 2001). Symbiotic associations between legu-

minous plants and rhizobia are well documented for posi-

tive effect on the growth and yield of legumes (Raza et al.

2004; Shaharoona et al. 2006). Auxin regulates the

expression of different genes in Rhizobium-legume inter-

action that involved in plant signal processing and attach-

ment to plant roots. Moreover, changes in auxin balance in

host plant are prerequisite for nodule organogenesis

(Mathesius et al. 1998; Spaepen et al. 2009). In the present

study, we demonstrated the effect of free living (non-

symbiotic) PGPR on the growth of a leguminous plant, i.e.,

B. Ali � A. N. Sabri � S. Hasnain (&)

Department of Microbiology and Molecular Genetics, University

of the Punjab, Quaid-e-Azam Campus, Lahore 54590, Pakistan

e-mail: shahida@mmg.pu.edu.pk

123

World J Microbiol Biotechnol (2010) 26:1379–1384

DOI 10.1007/s11274-010-0310-1

Vigna radiata (mungbean). The ability of PGPR to alter

endogenous IAA content of V. radiata was examined in

order to determine if growth promotion by PGPR is med-

iated by changes in the plant IAA levels. In addition to

auxin production, the bacterial strains were also screened

for other growth promoting attributes, i.e., phosphate sol-

ubilization and siderophore or hydrogen cyanide (HCN)

production.

Materials and methods

Rhizobacteria and determination of growth promoting

traits

Six different PGPR strains, Pseudomonas alcaliphila AvR-

2, Pseudomonas sp. AvH-4, P. aeruginosa As-17, Esche-

richia hermannii SnR-1, Micrococcus sp. AvR-5, and

Staphylococcus saprophyticus CdR-1 were previously

isolated from the rhizosphere of different plants (Ali et al.

2009a). Bacterial strains were evaluated for different

growth promoting traits such as phosphate solubilization,

siderophore, hydrogen cyanide (HCN) and IAA produc-

tion. Phosphate solubilization was determined qualitatively

by streaking strains on Pikovskaya agar plates (Pikovskaya

1948). Development of a clear zone around the colonies

was observed after incubation at 28�C for 7 days. Bacterial

strains were also assayed qualitatively for siderophore

production on CAS agar medium (Alexander and Zuberer

1991). CAS agar plates were streaked with test organisms

and incubated at 28�C for 72 h. Development of yellow to

orange color around a colony was considered as a positive

test for siderophore production. HCN production was

determined as described by Ahmad et al. (2008). For

quantification of bacterial IAA, strains were grown in

100 ml L-broth medium in the absence and presence

(1,000 lg ml-1) of L-tryptophan. IAA from bacterial cul-

ture supernatant was purified, derivatized, and quantified

by gas chromatography and mass spectrometry (GC–MS)

as described by Ali et al. (2009b).

Determination of endogenous IAA content and growth

of V. radiata

Effect of bacterial inoculation on endogenous IAA content

of V. radiata was determined under in vitro conditions.

Seeds were inoculated in pre-autoclaved glass test tubes

(25 9 160 mm) containing 25 g of moistened sand. Seeds

procured from Punjab Seed Cooperation, Lahore, Pakistan,

were surface sterilized with 0.1% solution of HgCl2 and

washed several times with sterile distilled water. Test tubes

with planted seeds were covered with plastic sheets to

avoid contamination. After seed germination, 2-day old

seedlings were treated with 500 ll of bacterial suspension

(107 CFU ml-1). About 500 ll of sterile solution of L-

tryptophan (1,000 lg ml-1) was also injected into rhizo-

sphere as IAA precursor. For control, water and L-trypto-

phan treated seedlings were kept for comparison.

Amendment of seedlings with bacterial suspension and L-

tryptophan was carried out in laminar air flow cabinet

under fully sterile conditions. Six seedlings of each variant

were grown in the growth chamber (MRL-350H; Sanyo,

Osaka, Japan) as described previously (Ali and Hasnain

2007). Leaf and root tissue was collected after 14 days of

seedling growth and immediately frozen in liquid nitrogen

for quantification of free IAA. The frozen samples con-

taining 15 mg of fresh tissue were homogenized in tripli-

cate in 0.5 ml of 0.05 M sodium phosphate buffer (pH 7.0),

containing 0.02% diethyldithiocarbamic acid as an anti-

oxidant and 5 ng 13C6IAA internal standard, using the

MixerMilll (Retsch) and a 3-mm tungsten carbide beads at

a frequency of 25 Hz for 3 min. The samples were then

incubated for 15 min at 4�C with continuous shaking. The

pH was adjusted to 2.7 with 1 M HCl, and the samples

were purified, derivatized, and analyzed by GC–MS as

described in Andersen et al. (2008). In addition to IAA

quantification, shoot and root length of seedlings was

recorded to observe the effect of microbial IAA on plant

growth.

Pot trials under axenic conditions

Pot experiments were conducted to evaluate the ability of

bacterial strains on the growth of V. radiata under axenic

conditions. Seed bacterization was carried out by incubat-

ing sterilized seeds in bacterial cell suspension adjusted

to 107 CFU ml-1. Seeds were inoculated in pots

(6.5 9 6.5 cm) containing autoclaved soil mixture and

vermiculite as mentioned earlier (Ali et al. 2009b). Ini-

tially, eight seeds were inoculated per pot in triplicate and

the experiment was repeated twice. After germination, 5

uniform seedlings were left in each pot. All pots, plastic

trays and covers used during the experiment were sterilized

with 5% solution of sodium hypochlorite for 20 min to

maintain sterility during the experiment. Growth parame-

ters in terms of shoot length, shoot fresh mass, root length

and root biomass were recorded after 2-weeks growth

period. The experiments were conducted at 22 ± 2�C

temperature, 50% humidity and 12 h photoperiod with

light intensity of 150–200 lmol m-2 s-1 in the green

house.

Pot trials under natural conditions

For wire house experiments sterilized seeds treated with

PGPR strains were sown in large pots (30 9 30 cm)

1380 World J Microbiol Biotechnol (2010) 26:1379–1384

123

containing 10 kg of unfertilized garden soil. Seeds were

treated with bacterial suspension as mentioned above.

Initially, 15 sterilized seeds were inoculated in each pot in

six replicates. After germination, seedlings were thinned to

10 per pot. After 6 weeks, further thinning was carried out

by keeping 5 seedlings per pot, which were grown till

maturity. All pots were arranged in a completely random-

ized design in the wire house of Department of Botany,

University of the Punjab, Lahore, Pakistan. Experiment

was conducted between March to June, 2007 under ambi-

ent light and temperature. Plants were irrigated with tap

water when required. At full maturity, growth parameters

including shoot length, number of leaves, number of pods,

and weight of 100 seeds were recorded.

Statistical analysis

For experiments, data were subjected to analysis of vari-

ance (ANOVA) using software SPSS 12 program and

means of different treatments were separated using Dun-

can’s multiple range test (P = 0.05). The correlation

coefficients between bacterial auxin production (in the

presence of L-tryptophan) and different growth parameters

were also calculated at P = 0.01 or P = 0.05.

Results

Growth prompting traits of rhizobacteria

Bacterial strains showed variable growth promoting activ-

ities (Table 1). Phosphate solubilization was detected only

with P. aeruginosa As-17 (Fig. 1). All strains except

E. hermannii SnR-1 were able to produce siderophores.

HCN production was exhibited by P. aeruginosa As-17 and

Micrococcus sp. AvR-5. GC–MS analysis of bacterial

extracts showed positive results for auxin production in the

presence and absence of L-tryptophan with all strains.

However, the strains expressed high IAA levels only when

medium (L-broth) was supplemented with L-tryptophan.

Auxin production ranged from 0.05 to 0.84 and 1.16 to

8.22 lg ml-1 in the absence and presence of L-tryptophan,

respectively (Fig. 2).

Endogenous IAA content and growth of V. radiata

Gas chromatography and mass spectrometric analysis of

plant samples showed elevated levels of IAA when seed-

lings were grown in vitro under bacterial inoculations in

the presence of L-tryptophan (Fig. 3). All bacterial

Table 1 Growth promoting traits of rhizobacteria

Strains Auxin Phosphate

solubilization

Siderophores HCN

P. alcaliphila AvR-2 ? - ? -

Pseudomonas sp. AvH-4 ? - ? -

P. aeruginosa As-17 ? ? ? ?

E. hermannii SnR-1 ? - - -

Micrococcus sp. AvR-5 ? - ? ?

S. saprophyticus CdR-1 ? - ? -

Fig. 1 Phosphate solubilization by P. aeruginosa As-17. Arrowsindicate the clearing zones around colonies

0

2

4

6

8

10

AvR-2 AvH-4 As-17 SnR-1 AvR-5 CdR-1

Bacterial strains

IAA

( g

ml-I

)

Without TRPWith TRP

b

A

a

A

a

A Ab

A

d

b

c

Fig. 2 GC–MS quantification of bacterial indole-3-acetic acid (IAA)

in the presence and absence of L-tryptophan. Each bar represents

mean ± SE of three replicates and bars with same letters are not

statistically different using Duncan’s multiple range test (P = 0.05)

0

100

200

300

400

500

Control TRP AvR-2 AvH-4 As-17 SnR-1 AvR-5 CdR-1

Bacterial strains

En

do

gen

ou

s IA

A (

pg

mg

-I)

Root Leaf

A A

DE

b

ab

Baa a

ab ab

F

C

B

ab

Fig. 3 Effect of bacterial strains on endogenous IAA content of

roots and leaves (pg mg-1 fresh weight) under in vitro conditions.

Each bar represents mean ± SE of three replicates and bars with

same letters are not statistically different using Duncan’s multiple

range test (P = 0.05)

World J Microbiol Biotechnol (2010) 26:1379–1384 1381

123

inoculations enhanced IAA content of roots. Significant

increases were observed with P. aeruginosa As-17 (303%),

Pseudomonas sp. AvH-4 (250%), P. alcaliphila AvR-2

(208%) and E. hermannii SnR-1 (172%), over water treated

seedlings. It was also observed that bacterial treatment

inhibited root elongation. Maximum reduction over water

treated control was observed with Pseudomonas sp. AvH-4

(60.85%) (Fig. 4). Bacterial strains (except Micrococcus

sp. AvR-5) also enhanced IAA content of leaves (Fig. 3).

Significant increases were observed with P. aeruginosa As-

17, Pseudomonas sp. AvH-4 and P. alcaliphila AvR-2 that

showed 294, 245.50 and 244.30% increases, over control.

Increases in IAA content were not accompanied with

concurrent improvement in shoot elongation in majority of

the treatments. However, Pseudomonas sp. AvH-4 and

S. saprophyticus CdR-1 recorded 9.90 and 6.30% increases

respectively, over water treated control plants (Fig. 4).

Bacterial IAA production recorded highly significant

positive correlation with endogenous IAA content of roots

(r = 0.969; P = 0.01) and leaves (r = 0.905; P = 0.01).

Similarly, significant positive correlation (r = 0.961;

P = 0.01) was also observed between IAA content of roots

and leaves. However, in vitro growth of seedlings in terms

of shoot and root length did not show a significant corre-

lation with bacterial auxin production.

Pot trials under axenic conditions

Bacterization of V. radiata seeds with IAA-producing

bacterial strains revealed highly significant increases in

shoot length and shoot fresh mass under axenic conditions

(Table 2). The most pronounced increases in shoot length

were observed with Micrococcus sp. AvR-5 (48.10%),

S. saprophyticus CdR-1 (37.30%), Pseudomonas sp. AvH-

4 (34.20%), E. hermannii SnR-1 (33.10%), P. alcaliphila

AvR-2 (30%) and P. aeruginosa As-17 (29.20%), over

control. Similarly, 43.80, 38.60, 26.60, and 25.75%

increases in shoot fresh weight were observed with Pseu-

domonas sp. AvH-4, P. aeruginosa As-17, P. alcaliphila

AvR-2 and E. hermannii SnR-1, respectively. Bacterial

IAA production positively correlated with shoot fresh mass

(r = 0.936; P = 0.01) but correlation with shoot length

was not significant. For root growth, majority of the bac-

terial strains showed inhibitory effect on root length

(Fig. 5). However, significant increase in root length was

recorded with Micrococcus sp. AvR-5 (35%), over control.

On the other hand, maximum stimulatory effect for root

biomass (32.55%) was observed with P. aeruginosa As-17.

Pot trials under wire house conditions

Bacterial strains exhibited variable growth response for

different growth parameters under natural wire house

conditions (Table 3). For instance, P. alcaliphila AvR-2,

Micrococcus sp. AvR-5, Pseudomonas sp. AvH-4 and

P. aeruginosa As-17 showed significant increases of 58,

57.70, 46.60 and 42.35% in shoot length, respectively, over

control. Maximum increase in number of leaves (39%) was

recorded with Micrococcus sp. AvR-5 and P. alcaliphila

AvR-2. Significant increases in number of pods were

observed with Pseudomonas sp. AvH-4 (65%) and

0

2

4

6

8

10

12

Control TRP AvR-2 AvH-4 As-17 SnR-1 AvR-5 CdR-1

Bacterial strains

Len

gth

(cm

)Shoot Root

bcde abcdab

ecde

a

de

A

B B

AA

A A

abc

A

Fig. 4 Effect of bacterial strains on shoot and root length of V.radiata under in vitro conditions. Each bar represents mean ± SE of

six replicates and bars with same letters are not statistically different

using Duncan’s multiple range test (P = 0.05)

Table 2 Effect of bacterial inoculations on growth of V. radiata (L.) under axenic conditions

Strains Shoot length (cm) Shoot fresh weight (g)a Root length (cm) Root biomass (g)

Control 9.00 ± 0.38 (a) 2.33 ± 0.11 (a) 10.3 ± 0.43 (bc) 0.172 ± 0.03 (a)

P. alcaliphila AvR-2 11.70 ± 0.62 (b) 2.95 ± 0.21 (bc) 8.38 ± 0.40 (ab) 0.180 ± 0.01 (ab)

Pseudomonas sp. AvH-4 12.08 ± 1.07 (b) 3.35 ± 0.17 (d) 8.96 ± 0.36 (ab) 0.178 ± 0.05 (ab)

P. aeruginosa As-17 11.63 ± 0.51 (b) 3.23 ± 0.26 (cd) 7.90 ± 0.50 (a) 0.228 ± 0.04 (b)

E. hermannii SnR-1 11.98 ± 0.77 (b) 2.93 ± 0.11 (bc) 10.0 ± 0.29 (bc) 0.165 ± 0.01 (a)

Micrococcus sp. AvR-5 13.33 ± 1.20 (c) 2.77 ± 0.18 (b) 13.9 ± 1.20 (d) 0.201 ± 0.05 (ab)

S. saprophyticus CdR-1 12.36 ± 0.44 (bc) 2.69 ± 0.10 (b) 11.8 ± 0.38 (c) 0.187 ± 0.02 (ab)

Mean ± SE of two repeated experiments (30 plants). Different letters within same column indicate significant difference between treatments

using Duncan’s multiple range test (P = 0.05)a Fresh weight of 5 plants

1382 World J Microbiol Biotechnol (2010) 26:1379–1384

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E. hermannii SnR-1 (65%) and P. aeruginosa sp. As-17

(57.50%). Similarly, Pseudomonas sp. AvH-4, E. her-

mannii SnR-1, P. aeruginosa As-17 and P. alcaliphila

AvR-2 showed 17.15, 15.80, 13.90 and 10.45% increases

in grain mass as compared to control. Significant positive

correlation between bacterial IAA production and number

of pods (r = 0.854; P = 0.05) and seed weight (r = 0.881;

P = 0.01) were observed under natural wire house condi-

tions. Highly significant positive correlation (r = 0.885;

P = 0.01) was also found between shoot length and

number of leaves as well as number of pods and seed

weight (r = 0.984; P = 0.01).

Discussion

In the present study, we demonstrated the ability of free

living non-symbiotic PGPR to alter endogenous IAA

content and growth of V. radiata (L.). Bacterial strains

were able to produce IAA in liquid culture medium but

showed variable results for different growth promoting

attributes such as phosphate solubilization, siderophore and

HCN production (Table 1). Phosphorus is very essential for

plant growth and inoculation with phosphate-solubilizing

bacteria has been shown to improve phosphate uptake by

plants (Hameeda et al. 2008). Bacterial strains may protect

plants from phytopathogenic fungi due to siderophore and

HCN production (Ahmad et al. 2008). However, growth

promotion of plants by PGPR is mainly attributed to their

phytohormone production ability especially IAA (Asghar

et al. 2004; Idris et al. 2007). We also previously demon-

strated that bacterial auxin in comparison with standard

IAA (as positive control) induced plant growth in vitro,

over control (Ali and Hasnain 2007). Treatment of V.

radiata seedlings with PGPR strains with addition of L-

tryptophan enhanced the endogenous IAA content of roots

(r = 0.969; P = 0.01) and leaves (r = 0.905; P = 0.01) as

indicated by significant positive correlation with bacterial

IAA production. Ability of bacterial strains to increase IAA

content of seedlings showed a close link to the bacterial

IAA production and plant growth promotion. It was

observed that IAA produced in root environment can be

transported to leaves as indicated by positive correlation

(r = 0.961; P = 0.01) between endogenous IAA content

of roots and leaves. However, despite increases in IAA

content, most of the treatments did not showed concurrent

increases in shoot length under in vitro conditions in test

tube experiments. In the case of root growth, all bacterial

treatments recorded reduction in root length that might be

due to the production of high levels of IAA as a result of

microbial transformation of L-tryptophan to IAA in rhizo-

sphere. This is evident from the high concentrations of IAA

in root samples that may be responsible for the inhibitory

effect on root length. The impact of exogenous IAA on

plant growth ranges from positive to negative that depends

on the amount and sensitivity of the plant tissue to IAA

concentrations (Spaepen et al. 2007). On the other hand,

bacterial strains stimulated growth of V. radiata under

axenic and natural wire house conditions in the absence of

L-tryptophan. In pot experiments, plants may have relieved

from high concentrations of IAA in the absence of

Fig. 5 Effect of bacterial strains on root growth of V. radiata (L.)

under axenic conditions. (a) Pseudomonas sp. AvH-4; (b) P. aerugin-osa As-17; (c) Micrococcus sp. AvR-5; (d) Control. Bar is 1 cm

Table 3 Effect of bacterial inoculations on growth of V. radiata (L.) at final harvest under wire house conditions

Strains Shoot length (cm) No. of leaves No. of pods Seed weight (g)

Control 16.46 ± 0.90 (a) 8.40 ± 0.41 (a) 13.33 ± 1.12 (a) 3.73 ± 0.10 (a)

P. alcaliphila AvR-2 26.00 ± 1.16 (c) 11.00 ± 0.46 (c) 19.00 ± 1.64 (ab) 4.12 ± 0.15 (bc)

Pseudomonas sp. AvH-4 24.13 ± 1.10 (bc) 10.00 ± 0.37 (bc) 22.00 ± 2.00 (b) 4.37 ± 0.26 (c)

P. aeruginosa As-17 23.43 ± 2.0 (bc) 10.00 ± 0.47 (bc) 21.00 ± 1.73 (b) 4.25 ± 0.40 (c)

E. hermannii SnR-1 20.36 ± 0.83 (ab) 9.00 ± 0.60 (ab) 22.00 ± 1.52 (b) 4.32 ± 0.13 (c)

Micrococcus sp. AvR-5 25.46 ± 1.48 (c) 11.00 ± 0.43 (c) 17.00 ± 1.50 (ab) 3.92 ± 0.11 (ab)

S. saprophyticus CdR-1 19.20 ± 0.98 (a) 10.00 ± 0.65 (bc) 18.00 ± 1.30 (ab) 3.96 ± 0.10 (ab)

Mean ± SE of six replicates (30 plants). Different letters within same column indicate significant difference between treatments using Duncan’s

multiple range test (P = 0.05)

World J Microbiol Biotechnol (2010) 26:1379–1384 1383

123

precursor. This growth response might be associated to root

exudates containing tryptophan (that may be in low con-

centrations) being released in rhizosphere and favoring the

adaptation of auxin synthesizing bacteria.

It is evident from significant increases in shoot length

(up to 48.10%), shoot fresh mass (up to 43.80%), root

length (35%) and root biomass (32.55%) under axenic

conditions. In wire house experiments, bacterization of

V. radiata seeds significantly increased shoot length

(16.65–58%), number of leaves (7.10–39%), number of

pods (27.50–65%) and seed weight (5–7.15%), over con-

trol. Growth promotion under axenic and natural conditions

reflected the stimulatory effect of low levels of bacterial

IAA in the absence of precursor L-tryptophan.

Conclusion

In conclusion, bacterial strains expressed high levels of IAA

when medium was supplemented with L-tryptophan. In

plant–microbe experiments, non-symbiotic PGPR showed

the potential to alter endogenous IAA content and growth of

V. radiata. It is reasonable to assume that PGPR can be

assayed as non-rhizobial inoculants for leguminous crops in

field trials.

Acknowledgments We thank Dr. Karin Ljung, Umea Plant Science

Centre (UPSC), Swedish University of Agricultural Sciences, Umea,

Sweden, for GC–MS facilities.

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