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Plant Development and Signals Plants use development to solve ecological problems – Repair, maintain, flourish – Resist competitors, herbivores, pathogens, abiotic factors – Seek out resources Plants respond to environmental and endogenous signals Signals alter gene reading programs and influence: – Cell division, expansion and differentiation at meristems – Tissue, organ and whole plant modification – Fitness Gene reading programs and flow of information gene DNA promoter m RNA transcript + - + - +Protein synthesis Signal(s) + enzymes protein cytoskeletal + regulatory structural transporter Classes of signals Internal (endogenous); generally chemical – Hormone – Non-hormone – Electrical gradients (but much less so than animals) External, generally non-chemical – Electrical, light, gravity, temperature, mechanical forces – Some chemical cues (e.g. MeJ) Internal chemical signals Hormones (a definition from animal biologists) – Naturally occurring organic molecules that act in small concentrations – Sole function is to serve as a developmental signal • Signal is context dependent – Typically move from source to target Plant biologist- naturally occurring organic molecule whose sole function is to serve as a development signal – 5 classes of plant hormones Auxin • • • • Indoleacetic acid (IAA), IBA Activity determined by charge separation Many synthetics Roles: + 0.55 nm - Gibberellins (GA) • Large family of compounds – Numbered according to discovery • Three linked rings with side groups • Roles: Cytokinins (CK) • Derivative of adenine • Different functional (R) groups attached • Roles: R group Ethylene (C2H4) • Gas at room temperature • Roles: H H C H C H Abscisic acid (ABA) • Named b/c it was discovered in abscised lvs • Not the direct cause of leaf loss though • Roles: Internal non-hormone chemical signals Phenolics (Phe) – – – – Coumarin Anthocyanin Salicylic acid Ferulic acid R O Methyl jasmonate (MeJ) • Related to stress response(s) • Root initiation, tendril coiling, pollen germination • Volatile interspecies ‘messages’ Oligosaccarides (Oligos) • Polymers of carbohydrates • Related to defense response – Phytoalexins, chitinase, and protease inhibitors External chemical signals • Nitrate (NO3-) – Controls nitrogen assimilation • Hydroquinones – Can signal parasitic plants to start ‘parasite gene reading program’ Principles of plant hormone action 1. Each hormone causes many responses - IAA Abscission + Root initiation + Stem elongation + Bud expansion Vascular repair Principles of plant hormone action 2. A hormone’s effect can differ between organs and species, and with age. Stem elongation + Wheat seedling elongation Young Old IAA Yes No GA No Yes 10-6 IAA - Root elongation Principles of plant hormone action 3. Many hormones can affect the same response CK IAA + + GA + Cell division Hormone sensors Since signals vary, the meaning of a signal depends on receptors and sensitivity (kinds of sensors a cell contains) Cells must have sensors/receptors; when hormone binds w/ following (blue) molecules response set off • Membrane channels or carriers receive and transport signal (A) • Signal activates enzyme (CD) • Receptor is regulatory protein (B) A +- Gene reading + C +D +B Setting off a response Responses may occur when the cell has a threshold number of receptors bound to hormones Equilibrium conditions predicts that response can occur when: – Hormone concentration increased – Receptor concentration increased (= sensitivity) Hormone Receptor Response + HR Response depends on concentration • Induction – on/off response once threshold hormone level is exceeded 100% Response 0% [Hormone] • Modulation – ongoing log linear dose response Physiologic range Response 10-8 10-6 [Hormone] Hormone concentration control • • • • • Synthesis Transport Destruction Sequestration Conjugation • Example - IAA Concentration control • Self-inhibition • Interactions between IAA and C2H4 • Interactions between CK and GA • De-stabilizing concentrations Measuring hormone concentrations • Bioassays – Grow specimens in a range of known concentrations – Generate dose response curve – Find value for unknown • Chromatography • Mass spectrometry • Antibody Control of phenolic synthesis Phenolics may act as 2o messengers gene + Injury Pathogens + gene Phenylalanine + + PAL Cinnamic acid Ferulic acid aa + Cinnamyl CoA Chalcone synthase + Chalcone Phenolics and flavonoids, including some phytoalexins Lignin
