Plant Hormones and Tropisms: Responding to Internal and External Signals

1. Plant Responses to Internal & External Signals Chapter 39

2. Tropism • A plant growth response from hormones that results in the plant growing either toward or away from a stimulus • Hormones – chemical messengers that coordinate the different parts of a multicellular organism

3. Types of tropisms: • Phototropism – growth toward or away from light • Gravitropism – growth toward or away from a gravitational source • Thigmotropism – growth toward or away from a touch • Vines grow toward a support and then grow toward (around) that support

4. Positive vs. Negative Tropisms • Positive is growth toward a stimulus • Positive Phototropism – growth toward light • Positive Gravitropism – growth toward a gravitational source • Positive Thigmotropism – growth toward a tactile stimulus • Negative is growth away from a stimulus

5. Hormones • Auxins • Stimulate cell elongation • Cause proton pumps to activate  Lower pH  weakening of cell wall  turgor pressure expands & elongates cell wall • High concentrations of synthetic auxins KILL certain plants, usually weeds (eudicots) • Synthetically produced auxins in high concentration = Herbicides

6. How do you make a …? • Cytokinins • Stimulate cell division or cytokinesis • Proper ratio of auxins & cytokinins  cell division & differentiation • Gibberellins • Work with auxins to stimulate stem elongation • Loosen cell walls allowing cellular expansion  stem expansion • Signal the seed to cease dormancy and germinate • Many dwarf plant varieties have non-functional gibberellins

7. Hormones (Page 3) • Abscisic acid • SLOWS Growth • Antagonistic to previously mentioned hormones • Promotes seed dormancy, but gibberellins cease it • Causes stomata to close to conserve water

8. Ethylene • Gas • Plays crucial role in programmed cell death (apoptosis) • Promotes ripening of fruit • Ripening in one fruit  ripening in other fruits • Positive feedback mechanism = rapid ripening of fruit • One bad apple does spoil the lot.

9. Plants respond to Light • Plants can detect presence, direction, intensity, and wavelength of light • Red & Blue wavelengths are most important • Blue light is most important for phototropisms and light-induced opening of stomata • Sensed by photoreceptors

10. Phytochromes • Photoreceptors for red light (mainly) • Exist in 2 isomer forms which can switch forms depending on wavelength available • Responsible for Circadian Rhythms • 24 hour cycles

11. Photoperiodism • Physiological response to a photoperiod (relative lengths of day and night) • Night is always the more important of the 2!! • But we (humans) focus on the daylight. • Controls Flowering • Short-day plants • Require a period of continuous darkness exceeding a critical measure in order to flower • Early Spring or Fall flowering • Sometimes called “Long-Night” plants

12. Besides Short-Day plants, • Long-Day Plants • Flower only if a period of continuous darkness is less than a critical value • Flower in Late Spring or early Summer • Considered “short-night” plants • Day-Neutral Plants • No length of continual darkness is needed for flowering

13. Plants respond to things other than light • Gravitropism – growth toward the earth (gravity) • Auxin plays key role • If root is plased horizontally, then gravity causes an accumulation of auxins in root’s lower side • Remember HIGH auxin = inhibition of growth at high concentration, so • lower side = no growth • Upper side = growth • Allows the root to grow down into the ground

14. Auxin Accumulation

15. Plants respond to other things… • Drought • Stomata close • Leaves will cease growth • Leaves roll into shape that reduces transpiration • Deep roots (where H2O is) will speed their growth, but shallow roots will stop growing • Predators • Thorns, chemicals, distasteful compounds • Some plants even attract parasites • First layer of defense - Epidermis

16. Coordinated Plant-Parasite Defense