auxin • Auxins (indole-acetic acid, IAA) • low concentration • produced in apical meristem • transported in parenchyma cells • overview of auxin action: plantchapter 37 hormones • usually involved in growth • stimulates fruit development • inhibits abscission Shaded side of coleoptile Light

Illuminated side of coleoptile

cytokinins gibberellins • Cytokinins • Gibberellins / gibberillic acid (GA) • Zeatin • Japanese scientist isolated GA in the 1930s • direct inhibition hypothesis • Gibberella fujikuroi. • cytokinins from the roots counteract auxin • overview of gibberellin action • An overview of cytokinin action: • produced in young , roots, shoot apical meristem • travel upward in the xylem • effects: • effects • seed germination, division and elongation • promotes cell division. • stimulates flowering -> long days • promotes chloroplast development • breaks seed, winter dormancy • stimulates lateral buds • inhibits abscission brassinosteroids abscisic acid • brassinosteroids • Abscisic acid (ABA) • effects similar to auxins • isolated in the 1960s. • cell elongation/division • overview of ABA activity: • retard leaf abscission • travels in vascular tissue. • promote xylem differentiation. • effects: • responsible for overall growth in • closes stomata • trigger growth spurts • second messengers • Inhibits shoot growth • inhibits seed germination • promotes winter, seed dormancy

H ethylene H C ethylene C • Ethylene H • overview of ethylene activity: response to stress effects: • H • • probably diffuses out of tissues • ripening • Apoptosis • triple response in seedling • Abscission of leaves • Slowing of stem elongation • Enzymes hydrolyze cell walls. • Thickening of the stem • layer of cork cells formed on the stem side 0.5 mm • Curving

0.00 0.10 0.20 0.40 0.80

Protective layer Abscission layer ETHYLENE CONCENTRATION (PARTS PER MILLION) STEM behavior blue light • Behavior • blue-light photoreceptors: • what is a ? • Cryptochromes • taxis vs. tropism • Phototropin • • Zeaxanthin • can measure the passage of days and seasons. • Blue light • phototropism • light breaks down auxin • slowing of hypocotyl elongation • light-induced opening of the stomata.

red light • photoperiodism. • Short-day plants • phytochromes TWO IDENTICAL SUBUNITS • Minimum critical night length • Found in all vascular plants. • Pfr inhibits flowering. • photoreversibility Chromophore • Long-day plants • Pr, absorbs red light at 660 nm • Maximum critical night length • Pfr, absorbs far-red light at 730 • Pfr induces flowering. nm. Photoreceptor activity • Day-neutral plants • involved in the germination of seeds: • How? • Exposure to red light converts • “FT protein” Pr to Pfr Kinase activity • • Thigmomorphogenesis • auxin promotes cell • Mechanical stress elongation in shoot and • plants to grow shorter and inhibits it in roots. stockier • Statoliths -> accumulation of • /thigmonasty auxin

Statoliths 20 µm

(a) Root gravitropic bending (b) Statoliths settling

• Thigmotropism (thigmonasty) CONSIDER ALL THE EFFECTS OF THESE DIFFERENT • carnivorous plants - response to insect movement STIMULI ON AND ECOLOGY other stresses cold • Responses to drought • Response to cold stress • Stimulates the production of abscisic acid • altering lipids in plasma • Causes stomata to lose and close membranes • may cause leaves to drop. • e. g. more unsaturated • Inhibits the growth of young leaves. fatty acids -- maintain fluidity. • Inhibits the growth of shallow roots. water in the cell wall and Responses to flooding • • intercellular spaces freezes • low O2 causes release of ethylene --> apoptosis. • increase the concentration of • Response to heat stress sugars • evaporative cooling • heat-shock proteins.

defense allelopathy • defense against • Allelochemics • Proteinase inhibitors • constantly produced in plants. • Herbivores detect proteinase • secondary metabolites inhibitors by taste • wide range of organic chemicals: • attraction of parasitoids • Phenolics / Tannins • Caterpillar saliva --> volicitin • Glycosides • wasps lay their eggs in the • alkaloids - Nicotine, caffeine, cocaine, caterpillars morphine pathogens pathogen plant pathogens • R receptor protein Avr • virulent product • able to overcome the host's defenses • plant reacts by releasing phytoalexins • Avirulent • Gene-for-gene hypothesis • hypersensitive response or HR • Plant has dominant resistant allele R, • recognizes pathogens with complementary dominant avirulent (Avr) allele. • Infected cells respond by dying. • Non-resistant plants • can mount localized responses