Plant hormones auxins gibberellins abscisic acid ethylene cytokinins brassicosteroids 2005-2006

Response to light: Phototropism Growth towards light Hormone: Auxin asymmetrical distribution of auxin cells on darker side elongate faster than cells on brighter side 2005-2006

Apical dominance Controls cell division & differentiation axillary buds do no grow while apical bud exerts control shoot root 2005-2006

Gibberellins Family of hormones Effects over 100 different gibberellins identified Effects stem elongation fruit growth seed germination plump grapes in grocery stores have been treated with gibberellin hormones while on the vine 2005-2006

Abscisic acid (ABA) Effects slows growth seed dormancy high concentrations of Abscisic acid germination only after ABA is inactivated down or leeched out survival value: seed will germinate only under optimal conditions light, temperature, moisture drought tolerance rapid stomate closing 2005-2006

Ethylene Ethylene is a hormone gas released by plant cells Multiple effects response to mechanical stress triple response slow stem elongation thickening of stem curvature to stem growth leaf drop (like in Fall) apoptosis fruit ripening 2005-2006

Apoptosis & Leaf drop Ethylene & auxin What is the evolutionary advantage of loss of leaves in autumn? Ethylene & auxin many events in plants involve apoptosis (pre-programmed cell death) death of annual plant after flowering differentiation of xylem vessels loss of cytosol shedding of autumn leaves The loss of leaves each autumn is an adaptation that keeps deciduous trees from desiccating during winter when the roots cannot absorb water from the frozen ground. Before leaves abscise, many essential elements are salvaged from the dying leaves and are stored in stem parenchyma cells. These nutrients are recycled back to developing leaves the following spring. Fall color is a combination of new red pigments made during autumn and yellow and orange carotenoids that were already present in the leaf but are rendered visible by the breakdown of the dark green chlorophyll in autumn. Photo: Abscission of a maple leaf. Abscission is controlled by a change in the balance of ethylene and auxin. The abscission layer can be seen here as a vertical band at the base of the petiole. After the leaf falls, a protective layer of cork becomes the leaf scar that helps prevent pathogens from invading the plant (LM). 2005-2006

Fruit ripening Adaptation Ethylene hard, tart fruit protects developing seed from herbivores ripe, sweet, soft fruit attracts animals to disperse seed Ethylene triggers ripening process breakdown of cell wall softening conversion of starch to sugar sweetening positive feedback system ethylene triggers ripening ripening stimulates more ethylene production 2005-2006

Applications Truth in folk wisdom! one bad apple spoils the whole bunch ripening apple releases ethylene to speed ripening of fruit nearby Ripen green bananas by bagging them with an apple Climate control storage of apples high CO2 storage = reduces ethylene production 2005-2006

Flowering Response Triggered by photoperiod relative lengths of day & night night length—“critical period”— is trigger Plant is sensitive to red light exposure What is the evolutionary advantage of photoperiodism? Synchronizes plant responses to season 2005-2006 Short-day plants Long-day plants

Circadian rhythms Internal (endogenous) 24-hour cycles 4 O’clock Noon Midnight Morning glory 2005-2006

Response to gravity How does a sprouting shoot “know” to grow towards the surface from underground? environmental cues? roots = positive gravitropism shoots = negative gravitropism settling of statoliths (dense starch grains) may detect gravity 2005-2006

Response to touch Thigmotropism Mimosa (Sensitive plant) closes leaves in response to touch Caused by changes in osmotic pressure = rapid loss of K+ = rapid loss of H2O = loss of turgor in cells 2005-2006

Plant defenses Defenses against herbivores 2005-2006

Plant defenses coevolution Defenses against herbivores Parasitoid wasp larvae emerging from a caterpillar 2005-2006