Chapter 14 Physiology – Hormones, signals, growth and development

Topics What is a hormone? – method of action Roles in survival of a sessile organism in the environment Mostly studied five plant hormones Other hormones Specific growth and developmental effects Hormone interactions Environmental signaling thru hormones Agricultural uses

Hormones - Chemical Messengers Regulate growth, differentiation and development (among other roles e.g. stomatal activity, environmental signaling) Organic molecules produced in one part of the plant and transported to other parts where they initiate a response – 3 common characteristics Act in very low concentrations and in collaboration Released into general circulation - not carried specifically to the target At the site of response, receptor molecules, usually on plasma membrane, bind to them triggering a variety of gene expressions

Hormones

Hormones Other compounds show hormone-like activity - much less studied than the big five Brassinosteroids (first isolated from rapeseed (Brassica napus) = complex chemicals involved in leaf morphogenesis, root and stem growth, and vascular differentiation – auxin-like effect – works thru auxins? Jasmonic acid (JA) - involved in defense against animals and fungi Salicylic acid (SA - related to aspirin) - involved in resistance to pathogens, especially viruses

Auxin: Activation/Inhibition of Shoots Auxin - produced as shoot apical meristems (SAM) grow and initiate new cells Auxin flows mainly downward, surrounds all stem cells, causing 3 unique responses: Cell elongation Apical dominance Differentiation of vascular tissues

Auxin: Apical dominance Apically produced auxin induces dormancy in axillary buds Each shoot tip gets only one active apical meristem In lower buds where auxin level is below a threshold – buds grow branches, flowers etc. Spring - buds release auxins to stimulate vascular cambium in stem

Hormone Interactions Auxins and Cytokinins: In some species, apical dominance is established by interplay of 2 or 3 hormones Active roots synthesize cytokinins Cytokinins are transported to the shoot and stimulate axillary buds Ratio of cytokinin and auxin determines whether buds become active or remain dormant

Hormone Interactions Auxins and Gibberellins: Auxin alone activates vascular cambium and elicits differentiation of xylem Gibberellin is also present in a healthy stem and causes some new cells to differentiate as phloem

Hormones: Environmental signaling Hormones help communicate with various plant parts when one part has encountered an environmental change If a leaf is damaged by animal feeding or water stress, auxin production and transport drop - abscission zone does not stay quiescent Autumn conditions stimulate production of ethylene

Hormones: Environmental signaling Ethylene suppresses auxin production and transport in time for abscission before winter Premature abscission of fruits is prevented by the presence and export of auxin through the pedicel

Hormones: Environmental signaling Auxin causes the developing fruits to release ethylene At maturity, high concentration of ethylene stimulates the pedicel abscission zone, overriding auxin concentration

Hormones: Tropisms Blue - most effective wavelength for phototropism Small pigment protein called phototropin in stem tips or coleoptile (outermost protective leaf at the stem tip) absorbs blue light Darker side of the stem receives extra auxin That side grows more rapidly and bends toward the light

Hormones: Tropisms In gravitropism - root cap acts as the organ of perception of gravity by statocytes with their statoliths Lower side of the root is detected by sinking statoliths - extra auxin transported to that lower side of the root cap and then into the root Growth inhibition on lower side Root grows downward

Flowering: Vernalization and Photoperiodism Conversion from juvenile to adult is phase change – flowering (fruits, seeds – survival or agricultural significance) Cold temperatures stimulate phase change in some – vernalin production – flowering Photoperiod (day length – night length really) triggers flowering in some – phytochrome perceives length of night – next slide Some need both critical low T and night length Hormones mediate these responses too – e.g. GA can substitute vernalization and night length requirements in some

Photoperiodic Induction to flower more common in temperate spp. Pr - regulates circadian and seasonal phenomena Day neutral plants - no phytochrome regulation

So many agricultural uses – growth regulators Seed germination - GA Propagation by cuttings, rooting - Auxin Partheniocarpic fruits - Auxin, GA, Cytokinin Flower and fruit set or fall - Auxin induced Ethylene, Cytokinin Fruit size - Auxin Fruit ripening, abscision - Ethylene Synchronized flowering - friuting - Ethylene Cut flower, vegetable postharvest shelf life - Cytokinin Stem/stalk length - GA Weedicides - Auxin Hybrid plant production e.g. cucumber hybrids - GA Antitarnspirants, enhanced color in red grapes - ABA Tissue culture - Auxin, cytokinin