1. Plant Responses to Stimuli

2. Signal Transduction
Environmental signals trigger specific responses in plants
Etiolation- adaptations for darkness
Potatoes sprouting in a dark pantry
De-etiolation- “greening”; occurs when a shoot reaches sunlight
Potato leaves expand and roots elongate when exposed to sunlight

3. From Reception to Response
Receptor proteins change shape in response to stimulus (hormonal, chemical, or environmental)
Transduction
Signal is amplified by second messengers
Specific protein kinases activated
Response
Regulation of cell activity
Regulation of transcription or translation
Modification of proteins (post-translation)

4. Tropisms
Tropism- growth response that orients plant toward or away from a stimuli
Growth of a shoot toward light = phototrophism
Coleoptile senses light and transmits signal (Darwin and Darwin/ Boyen-Jensen)

5. Plant Hormones
Growth caused by the concentration of horomones within plant cells
Phototropism caused by a higher concentration of auxin (growth-promoting horomone) on the dark side
Fritts Went

6. Auxin Indoleacetic Acid (IAA) Promotes elongation of coleoptiles
Synthesized in apical meristem; moves down the shoot
Activates expansins (break down cellulose microfibrils)
Alters gene expression, causing cells to produce new proteins
Used in vegetative propigation of roots and also as a herbicide

7. Cytokinins Promote cytokinesis (cell division)
Work with auxin to promote growth (not effective alone)
Used in the control of axillary bud growth; promotes apical dominance
Anti-aging effects
Inhibit protein breakdown, stimulate transcription and protain synthesis, and mobilize nutrients

8. Gibberellins Stimulate growth of roots and leaves
Present in seed embryos, roots, and young leaves
Little effect on roots (can cause plants to topple if the concentration is too high)
Stimulates cell-wall loosening enzymes
Causes grapes to grow larger because stems are elongated, giving fruit more room to grow

9. Brassinosteroids and Abscisic Acid
Induce cell elongation and division in stem segments
Retard loss of leaves (abscission)
Promote xylem differentation
Abscisic Acid (ABA)
Slows growth
Keeps seeds dormant/ prevents early sprouting
Increases drought tolerance

10. Ethylene Response to mechanical stress Apoptosis Leaf abscission
Occurs if an emerging shoot meets resistance (rock, etc.)
Triple response- slows elongation, thickens stem, and causes it to curve
Apoptosis
Programmed cell death
Shedding leaves/ death of xylem components, etc.
Leaf abscission
Separates leaf from tree at petiole
Fruit ripening
Triggers ripening of fruit; causes chain reaction that ripens all fruit on the plant

11. Photomorphogenesis
Plants can detect the direction, intensity, and wavelength of light
Action spectrum- different wavelengths are more effective at driving particular processes
Blue-light photoreceptors
Phototrophism, opening of stomata, slowing of hypocotyl elongation
Phytochromes (red light)
Seed germination, shade avoidance in trees

12. Circadian Rhythms Plants “cycle” their processes and responses daily
Length of day determines growth of plants
Must have darkness for a certain amount of time… critical dark period
Short-day plants- flower when darkness is longer than CDP (late summer/fall/winter)
Long-day plants -flower when darkness is shorter than CDP (late spring/summer)
Day-neutral plants- unaffected by photoperiods

13. Touchy, touchy… Gravitropism Adjust growth according to gravity
Detect gravity by settling of statoliths (dense starch grains)
Roots- positive gravitropism
Shoots- negative gravitropism
Thigmomorphogenesis
Change in form due to touch
Inhibition of growth/ change in leaf conformation

14. Environmental Stress Drought Flood Salt Heat Cold
Reduce transpiration, inhibit new growth
Flood
O2 deprivation stimulates ethylene production
Salt
Can cause desiccation
Heat
Denatures proteins
Cold
Alters solute transport
Freezing can cause cell death due to dehydration

15. Plant Defenses Herbivores Pathogens Physical (thorns, etc.)
Chemical (Distasteful compounds, "recruitment” of predatory animals)
Pathogens
Virulent (little specific defense) and avirulent (cause mild harm)
Gene-for-gene recognition- resistance (R) genes
Elicitors (oligosaccharins)
Stimulate antimicrobial compounds (phytoalexins) and produce pathogenesis-related (PR) proteins
Hypersensitive response- seals off infected region
Systemic Acquired Resistance- protects entire plant from infection due to phytoalexin and PR protein production