1. Plant Response to Stimuli
Chapters 39

2. What you need to know! The 3 steps to a signal transduction pathway.
The role of auxins in plants.
The survival benefits of phototropism and gravitropism.
How photoperiodism determines when flowering occurs.

3. Signal Transduction Pathway
Organismal responses to stimuli involve a cascade of cellular interactions:
Reception: cells detect signals when ligands bind to specific receptors which change shape in response
Gprotein
Transduction: amplification of response allowing for small number of molecules (hormones) to trigger a large response
Response: Cell responds to specific stimuli by:
Increasing or decreasing mRNA production, or
Activating existing enzyme molecules.

4. Signal Transduction Pathway

5. Hormones
Hormones are chemical messengers that allow coordinated cellular responses; they are produced by one part of the organism and transported to another
Tropism is a hormonal plant growth response toward or away from a stimulus
Phototropism: positive or negative growth toward or away from light
Gravitropism (growth parallel to gravity)
Thigmotropism (detect support structure)

6. Phototropism

7. Gravitropism

8. Hormones in Plants Name Function
Auxins Early plant growth; elongation of root and shoot
Ethylene Apoptosis (leaves, flowers), fruit
(gas) ripening (positive feedback loop)
Cytokinin Stimulates germination and
plant growth
Gibberilin Works in concert with auxins in stem
elongation, germination
Abscisic Inhibits growth, maintains
Acid (ABA) dormancy, closes stomata
during water stress

9. Plant Responses to Light
Light is critical for plant success
Light receptors (phytochromes) detect specific wavelengths of light
Triggers plants development
Photoperiodism is a response to the length of day/night
Short-day plants (long-night) flower in early spring or fall
Long-day plants (short-night) flower in late spring or early summer
Day-neutral plants flower in any day length
Circadian rhythms are physiological cycles of about 24 hours (not dependent on light)

10. Plant Defensive Responses
Herbivore Defense:
Thorns
Distasteful and/or poisonous chemicals
Airborne attractants draw predators
Bacterial and Viral Immune Response
First line of defense = epidermal layer
Second line of defense =
recognition of pathogens
Develop resistance or isolate infected area
Deliver resistance throughout plant or apoptosis

12. Mimosa Plant Reacting to Herbivores