The Plant Cell, July 2011, www.plantcell.org © 2011 American Society of Plant Biologists. All rights reserved. 

www.plantcell.org/cgi/doi/10.1105/tpc.110.tt0310

Introduction to Phytohormones (TTPB6) – Teaching Guide Overview – Phytohormones are chemical messengers that coordinate cellular activities. This lecture introduces the plant hormones (auxin, cytokinin, gibberellins, brassinosteroids, ethylene, abscisic acid, jasmonates, and salicylates) through their roles, during the plants life, from seed-to-seed. The biosynthesis, transport, perception, signal transduction and downstream effects of each are introduced, as well as a few ways that hormonal signaling pathways intersect. Learning Objectives By the end of this lecture the student should be able to:

Identify how plant hormones contribute to their growth, development, reproduction and stress responses

Identify the major plant hormones Describe the factors contributing to hormone accumulation and response Describe what is meant by cross-talk in hormone signaling

Study / exam questions (understanding and comprehension)

What is a hormone? In what general ways are plant and animal hormones similar and different? What are the five “classical” plant hormones? What processes contribute to plant hormone accumulation? Describe two forms of plant hormone receptor. How does auxin regulate phototropism? True or False: Auxin and ethylene act antagonistically in most processes. What are the commercial applications of controlling ethylene synthesis and

response? The jasmonate receptor is very similar to the receptor of which other hormone? True or False: Jasmonates and Salicylates are expressed constitutively in most

tissues. Discussion Questions (engagement and connections)

Pheromones are chemicals that communicate between individuals of the same species. Well-studied examples include yeast mating factors and bacterial quorum sensing pheromones. What does the presence of pheromone signalling pathways in single-celled organisms suggest about the evolutionary origins of hormones in multicellular organisms?

What might be an advantage of a signal transduction pathway that involves proteolysis of a repressive protein as a way to activate a response?

How does the phosphorelay system that operates downstream of cytokinin receptors differ from the protein kinase / phosphatase system that operates downstream of the ethylene and brassinosteroid receptors?

Plants with defects in ABA or gibberellin synthesis and signalling pose particular problems for geneticists – what phenotypes associated with these defects are particularly problematic? What options are available to geneticists when faced with these problems?

 

 

The receptors for salicylates and strigolactones are not yet known. Propose a biochemical and a genetic method to identify these elusive receptors.

Plants whose guard cells have enhanced sensitivity to ABA show a reduced sensitivity to drought, but guard cells with extreme hypersensitivity to ABA are detrimental to the plant. Why?

Lecture synopsis Introduction (1 – 8) Phytohormones are plant hormones. Like hormones in animals, they are chemical messengers that coordinate the activity of organs with each other. Phytohormones (referred to from now on simply as hormones) regulate cellular activities, pattern formation, vegetative and reproductive development, and stress responses. The five classic plant hormones, discovered by the mid-20th century, are auxin, cytokinins, gibberellins, ethylene and abscisic acid (ABA). More recently characterized hormones include brassinosteroids, strigolactones, jasmonates and salicylates. Overview of hormone action (9 – 22) Hormonal action is regulated by the synthesis of a hormone and its accumulation in active form, which is also affected by conjugation and degradation. Transport also controls regional accumulation of the hormone, but in many cases we know little about how hormones are regulated. Hormones initiate signalling by binding to receptors. Some hormone receptors are transmembrane proteins that undergo a conformational change upon hormone binding, whereas for others the binding of hormone serves as “molecular glue” to facilitate the interaction between two proteins. Downstream signalling often includes protein phosphorylation and / or proteolysis, and culminates in changes in transcription, ion channel activities and other effects. Vegetative Growth Vegetative growth involves growth and elongation, leaf and branch initiation, and branch elongation, all of which are hormonally-regulated. Auxin (23 – 34) Charles Darwin observed auxin action in phototropic responses. Chemiosmotic movement of auxin is important in its action. Auxin receptors include F-box proteins, which when bound to auxin stimulate the proteolysis of Aux/ IAA repressor proteins. Auxin contributes to developmental patterning, cell elongation and organogenesis as well as tropic responses. Cytokinins (35 – 45) Cytokinins are a family of compounds derived from adenine that act antagonistically to auxins, and are involved in maintaining a stem cell population at the shoot apical meristem and promoting differentiation at the root apex. Cytokinins are also involved in nutrient uptake and allocation and leaf senescence. Cytokinin signaling is mediated by a two-component phosphorelay system. Manipulations of cytokinin levels or functions can contribute to drought resistance and lead to increased yields. Strigolactones (46 – 47) Strigolactones were first identified as root-exuded compounds that stimulate germination of parasitic plant seeds of the genus Striga. Subsequently strigolactones have been found to

 

 

stimulate branching of mycorrhizal fungi, and to function as endogenous plant hormones that affect branching in the shoot and root. Gibberellins (48 – 55) Gibberellins are a family of compounds with a generally growth-promoting function. Their name derives from the pathogenic fungus Gibberella fujikuroi, from which they were first isolated. Through gibberellin production, the fungus stimulates elongation growth in its host, perhaps to facilitate the dispersal of fungal spores. The biochemical pathway of gibberellin synthesis was deduced from analysis of GA-deficient dwarf plants. Gibberellin perception stimulates proteolytic degradation of growth-inhibitory DELLA-proteins (named for a conserved amino acid sequence). Mutations that stabilize DELLA proteins lead to a GA-insensitive phenotype, which has proven useful in increasing grain yields. DELLA protein stability is a target for many environmental and hormonal inputs. Brassinosteroids (56 – 60) Brassinosteroids are a family of steroid compounds of which the most active is brassinolide. Brassinosteroids promote cell elongation, and are involved in pollen development. Brassinosteroids are perceived by a plasma-membrane localized BRI1 receptor, promoting a protein-kinase cascade leading to transcriptional changes. Reproductive Growth (62 – 70) In plants reproductive growth involves a developmental transition called floral evocation, that is subject to environmental and hormonal controls. Flower development, sex determination, pollen maturation, embryogenesis, seed maturation and germination are all hormonally regulated processes. Ethylene (71 – 76) Ethylene promotes senescence and fruit ripening, stress responses and also vegetative development. Ethylene production and response are manipulated genetically and biochemically to promote or delay fruit ripening and so stabilize food as it is transported to consumers. Ethylene biosynthesis was initially characterized biochemically, but its signalling pathway was largely determined through early Arabidopsis molecular genetics approaches, In 1993 the ETR1ethylene receptor was the first protein unambiguously identified as a phytohormone receptor. Seed maturation and germination: ABA (77 – 80) and Gibberellic Acid (81 – 82) Abscisic acid (ABA) accumulates during seed maturation, leading to the production of proteins and compounds that protect the embryo during seed desiccation, and repressing germination. Gibberellins promote seed germination, and, in some seeds, the mobilization of storage reserves. Hormonal responses to stress Plants hormones are involved in responses to abiotic stress (e.g. heat and cold, drought and flooding), and biotic stress (e.g. herbivory and pathogenicity). Abscisic Acid (85 – 92) ABA synthesis is induced by drought or other abiotic stresses, leading to the expression of stress-responsive genes. Identification of ABA receptors was a significant challenge that has

 

 

recently been illuminated with the identification of the PYR/ PYL/ RCAR protein family. ABA-binding to these proteins facilitates their interaction with protein phosphatases, leading the the activation of protein kinases. Targets of ABA-regulated protein kinases include transcription factors and ion channels. ABA also controls stomatal aperture through modifications of ion channel permeability in guard cell membranes. St Jasmonates (94 – 101) Jasmonates are membrane-derived hormones that are rapidly induced by wounding or herbivory. JA accumulation induces expression of anti-herbivory chemicals and proteins, contributes to systemic defense responses and production of volatile compounds that attract natural enemies of herbivores. The active hormone is jasmonic acid conjugated to isoleucine (JA-Ile). The receptor and signal transduction pathway of JA-Ile are very similar to those of auxin. Salicylates (102 – 108) Salicylate synthesis is induced by pathogen attack, and salicylates contribute to systemic acquired resistance to pathogens. Events upstream and downstream of salicylate production are well understood, but the salicylate receptor is not yet known. Cross-talk and Summary (109 - 118) It is convenient to describe each phytohormone as if it functioned alone, but it is increasingly clear that synthesis and response to hormones involves extensive cross-talk and regulatory interactions. In some cases a single gene requires elevated levels of two hormones to be activated. In other cases, the presence of one hormone negates the effects of another. Still another form of cross-talk occurs when one hormone induces or represses the synthesis or action of another. Unravelling these interactions is one of the hottest topics in plant hormone biology. Our understanding of phytohormones is developing rapidly.

 

 

Slide Concepts: Slides Table of contents / concepts 1 Title 2 – 7 What are Phytohormones? 8 Lecture Outline 9 - 20 Overview of hormone signaling 21 - 59 Hormones and vegetative development 23 - 34 Auxin 24 - 26 Darwin’s study of phototropism 27 - 28 Auxin transport 29 Auxin synthesis 30 - 31 Auxin functions 32 - 34 Auxin signaling 35 - 45 Cytokinins36 – 37 Cytokinin synthesis 38 – 40 Cytokinin functions 41 – 44 Cytokinin signaling 45 Cytokinins and crops 46 - 47 Strigolactones 48 - 55 Gibberellins 50 - 51 Synthesis 52 - 53 Gibberellins and green revolution 54 - 55 Gibberellin signaling 56 - 60 Brassinosteroids 57 - 58 BR functions 59 - 60 BR signaling 61 Summary: hormones and vegetative development 62 - Hormonal control of reproductive development 63 - 65 Transition to flowering 66 - 70 Flower and fruit development and ripening 71 - 76 Ethylene 72 - 73 Ethylene promotes senescence 74 Ethylene synthesis 75 - 76 Ethylene signaling 77 - 80 Abscisic Acid 78 - 80 ABA and seed maturation and dormancy 81 - 82 Gibberellins are required for germination and used in brewing 83 Summary – hormonal regulation of reproductive development 84 - 92 Hormonal responses to abiotic stress 85 - Abscisic Acid 85 - 86 ABA synthesis regulated by stress 87 ABA induces stress responsive genes 88 - 89 ABA receptor and signaling 90 - 92 ABA and stomatal aperture 93 - 109 Hormonal responses to biotic stress 94 – 101 Jasmonates 95 Jasmonate synthesis 96 - 100 Jasmonates contribute to defense against herbivory 101 Jasmonate signaling 102 - 108 Salicylates103 Salicylates are induced upon pathogen attack 104 - 108 Perception and response to pathogens 109 Cross-talk in defense signaling 110 Summary: Stress responses 111 - 116 Hormonal crosstalk 117 – 118 Ongoing research