How do plants respond to their environment?
- Regulate / control their developmental stage:
- Seed germination
- Fruit development
- Leaf abscission
- Apical dominance
Or move in a non-directional way – a “nastic” response
(plant defences against pathogens see p448)
Synthesise and secrete chemicals:
Tannins – defence against herbivory
Alkaloids – defence against herbivory
Pheromones – defence against insect attack
Callose – mechanical defence
Pine resin – insect repellant
Citronella – insect repellant
Pyrethrins – insecticide
Phenols - antibacterial
Introduction to plant hormones (phytohormones)
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 plant 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
Hormonal action is regulated by the synthesis of a plant hormone and its accumulation in active form. Transport also controls regional accumulation of the hormone, but in many cases we know little about how plant hormones are regulated. Plant 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 involves growth and elongation, leaf and branch initiation, and branch elongation, all of which are hormonally-regulated.
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 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.
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 Gainsensitive phenotype, which has proven useful in increasing grain yields. DELLA protein stability is a target for many environmental and hormonal inputs.
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 lar
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 and Gibberellic Acid
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).
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. ABAbinding 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.
“Other (not in A-Level syllabus) plant hormones.
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.
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.
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.
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.
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.