1. Plant responses to Internal and External Stimuli
Chapter 39
Plant responses to Internal and External Stimuli

2. I. Signal Transduction Reception
- reception proteins change shape due to stimulus
- phytochrome in greening
Transduction
- second messengers are produced
- many interact with G-proteins
- In greening GTP replaces GDP and cGMP is produced or Ca+ channels open and work with calmodulins
Response
- transcriptional regulation or post-translational modification of proteins

4. II. Hormones
Compound produced by one part of the body that when transported to another part triggers a response in target cells or tissue.

5. II. Hormones
Darwins
- Decided that a signal was transmitted down from the tip to elongation region of the coleoptile
- Boysen-Jensen proved this with gelatin and mica blocks
Went
- put block with “coleoptile tip juice” on and it stimulated growth

6. Darwins

7. Went

8. II. Hormones
6 Hormones
- alter expression of genes by effecting enzymes or changing membranes
- have multiple effects depending on site of action, concentration, and developmental stage of plant

9. 1. Auxin Apical meristem / shoot tips / only moves from tip to base
Cell elongation through weakening of cell walls (expansins)
Form adventitious roots / put on leaf cuttings
Herbicide
Promotes fruit growth
c. Works closely with cytokinins and gibberellins

10. 2. Cytokinins Actively growing tissue (roots, embryos, fruits)
Stimulate cytokinesis
Anti-aging effect / stimulate protein production and slows down protein breakdown
Works with auxin to determine differentiation
Controls apical dominance in plants along with auxin (C-shoots / A-roots)

11. 3. Gibberellins Roots and young leaves / seeds Stem Elongation
Seed Germination after water is imbibed
c. Works with auxin for fruit set

12. 4. Abscisic Acid (ABA) Stems, roots, leaves, green fruit
Slows down growth
Germination / washed out, cold
Closes stomata during drought
Antagonizes action of growth hormones
Ratio of Gibberellins to ABA determines germination

13. 5. Ethylene Ripening fruit, aging leaves and flowers, nodes / gas
Ripens fruit
Triple Response / slow stem, thicken, curvature
Apoptosis
Leaf abscission
c. Works with auxin to abscise leaves

15. 6. Brassinosteroids Seeds, fruits, shoots, leaves, floral buds
Cell elongation
Division of stem segments
Retards leaf abscission
Promotes xylem differentiation
c. Very similar to auxin

16. 2 types of photoreceptors Photoperiodism and Flowering
III. Responses to Light
2 types of photoreceptors
Photoperiodism and Flowering

17. A. 2 Types of Photoreceptors
Blue-Light Photoreceptors
Phytochromes

18. 1. Blue-Light Photoreceptors
Absorb blue light only
Cryptochromes – inhibit hypocotyl growth
Phototropin – phototropism
Zeaxanthin – stomatal opening

19. 2. Phytochromes Absorb red light
Chromophore (light absorbing part of protein) reverts back and forth between 2 isomers: Pr (red absorbing) and Pfr (far-red absorbing)
Photoreversible
Seeds produce almost all Pr and when exposed to light (red) Pr converts to Pfr and the seed germinates
If a tree is shaded Pr increases and resources are put towards vertical growth / if sunny Pfr is high and the tree grows out
Set biological clock to 24-hr periods / can’t make Pfr at night and when the sun rises Pfr levels spike / this resets the clock everyday

20. B. Photoperiodism and Flowering
Physiological response to day length / photoperiod
Night length determines flowering
Probably a hormone in the leaves that causes the flowering (removing leaves stops all flowering)
Short-Day (long night) – requires longer periods of darkness to flower
Long-Day (short night) – requires shorter period of darkness for flowering
What role does Pr and Pfr play in this?

22. IV. Other Responses
Gravitropism – response to gravity / roots positive and shoots negative
- statoliths (plastids) settle in cells and allow auxins to accumulate at the bottom and inhibit growth
- Movie
Thigmotropism – directional growth due to touch / action potentials

24. V. Responses to Stress

25. A. Drought Loses water through transpiration Close stomata
Inhibit growth of young leaves
Inhibit growth of surface roots

26. B. Flood Drowns roots Aerial roots
Ethylene kills cells in the cortex and creates “snorkels”

27. C. Salt
Lowers water potential in soil / hypertonic soil “pulls” water out of the cells.
Salt is toxic
Produce competing solutes
Salt glands

28. D. Heat Denatures enzymes Lowers evaporative cooling
2. Heat-shock proteins (help enzymes achieve their normal shape / scaffold protein)

29. E. Cold Change fluidity of membranes Freezes cytosol
Increase unsaturated fat production
Increase cytoplasmic levels

30. VI. Defense Physical and Chemical
- thorns, toxic compounds (canavanine), recruit predators of herbivores, warn nearby plants
Lines of Defense
- Epidermis
- Gene-for-Gene Recognition
- Hypersensitive Response – kills cells and seals off area and sends warning signal
- Systemic Acquired Resistance – after HR signals are sent through entire plant for protection / salicylic acid – hormone signal