1. Plant responses to internal and external signals
Chapter 39

2. You must know
The three components of a signal transduction pathway and how changes could alter cellular responses
The role of auxins in plants
How phototropism & photoperiodism use changes in the environment to modify plant growth & behavior
How plants respond to attacks by herbivores and pathogens

3. I. Review
Reception – cell signals bind to receptors which undergo changes in shape (G protein-coupled receptors and receptor tyrosine kinases)
Transduction – multistep pathway which amplifies the signal
Response – turning genes on/off (if response is in the nucleus) or activating enzymes (in cytoplasm)

4. II. Plant hormones
Hormones - signaling molecules produced in one part of an organism and transported to other parts – help coordinate different part of the organism

5. 1. Phototropism – growth in response to light
Tropism – plant growth response to hormone production in which plant grows towards or away from a stimulus
1. Phototropism – growth in response to light
Positive – towards the light
Negative – away from the light

6. Auxins – plant hormones that stimulate elongation of cells in developing shoots
1. Indoleacetic acid (IAA)
2. Produced in apical meristems
3. Activate proton pumps in cell membrane, lowers pH of cell wall, weakening it and allowing turgor pressure to expand the cell wall

7. D. Other Hormones
1. Cytokinins: stimulate cytokinesis or cell division (aide in growth). Work with auxins to stimulate cell division and differentiation 2. Gibberellins: work with auxins to stimulate cell elongation by loosening cell walls, allowing for expansion of cells and stems. Effects: fruit growth, stem elongation, seed germination

8. 3. Abscisic Acid: slows growth , promotes seed dormancy, drought tolerance (water stress=close stomata to save water) 4. Ethylene: gas, role in programmed cell death (apoptosis) such as shedding of leaves/death of annual after flowering. Promotes ripening of fruits which triggers more ethylene (positive feedback loop)- 1 rotten apple can spoil the bunch…

9. III. Plant responses to light
Plants detect lights’ presence, direction, intensity, and wavelength
1. Blue light receptors – initiate phototropism and opening of stomata
2. Phytochromes (red light receptors)

10. Pfr – triggers developmental responses to light
Pr – naturally produced by plant, but is converted to Pfr by light
Relative amounts of the two pigments can be used to measure the amount of sunlight in a day

11. Circadian Rhythms – 24 cycles
1. Pfr levels increase at dawn (even if plant put in the dark)
C. Photoperiodism – response of the plant to the length of the night (ex. why plants flower in spring)

12. Short-day plants – flower only if the night is longer than critical amount, flower in early spring or fall
Long-day plants – flower only if night is shorter than critical period, flower in summer
3. Day-neutral – length of night doesn’t matter

13. IV. Other stimuli
Gravitropism – growth toward (positive - roots) or away from (negative - shoots) source of gravity
1. Auxins involved again

14. Thigmotropism – growth in response to touch
1. Vines wrapping around supports
Response to drought
1. Guard cells lose turgor, stomata close, young leaves stop growing and roll inward to decrease transpiration
2. Deep roots continue to grow, but shallow do not

15. V. Response to attack Thorns Chemicals 1. Bitter taste
2. Poisonous compounds
3. Airborne attractants to recruit predators (attract wasps to lay eggs to kill caterpillars)

16. 2. Immune response – local and plant-wide
Defense from viruses
1. Epidermal layer
2. Immune response – local and plant-wide
a. Lesions – seal of pathogen and then kill that area
Spread of signal dependent on signal transduction pathways