1. Plant Responses to Signals

2. Plant Reactions Stimuli and Stationary Life
Animals respond to stimuli by changing behavior
move toward positive stimuli
move away from negative stimuli
Plants respond to stimuli by adjusting growth & development

3. Mechanisms that Control Responses
Signal Transduction Pathway model
plants have cellular receptors
signal triggers receptor
receptor triggers internal cellular messengers &
then cellular response

4. A potato left growing in the darkness: - produces shoots that do not appear healthy, and will lack elongated roots - adaptations for growing in darkness After potato is exposed to light: - undergoes profound changes - shoots grow normally

5. Plant Hormones Chemical signals that coordinate different
parts of an multicellular organism
only minute amounts are required
produced by one part of plant and transported to another part
binds to specific receptor
triggers response in target cells & tissues

6. Important Plant Hormones
Auxin – stimulate cell elongation  phototropism & gravitropism (high concentrations = herbicide)
Cytokinins – stimulate cell division (cytokinesis) & differentiation
Gibberellins – stem elongation, leaf growth, germination, flowering, fruit development
Abscisic Acid – slows growth; closes stomata during H2O stress; promote dormancy
Ethylene – promote fruit ripening (positive feedback)
involved in apoptosis (shed leaves, death of annuals)

7. Auxin Chemical messenger that stimulates cell elongation near
apical meristems
Asymmetrical
distribution of auxin
Cause of phototropism
Cells on darker side
elongate faster than
cells on brighter side
Conclusion
Tip of coleoptile senses light  some signal was sent from tip to elongating region of coleoptile

8. Went 1926 Excised tip placed on agar block Growth-promoting
chemical diffuses
into agar block
Agar block
with chemical
stimulates growth
Offset blocks
cause curvature
Control
(agar block
lacking
chemical)
has no
effect

9. Cytokinins Family of hormones produced in roots, fruits & embryos
Effects
Controls cell division (cytokinesis) & differentiation
Causes apical domanance

10. Gibberellins Over 100 different gibberellins identified Effects
stem elongation
fruit growth
seed germination

11. Abscisic acid (ABA) Effects
Promotes seed dormancy and inhibits early germination
survival value: seed will germinate only under optimal conditions
drought tolerance
rapid stomate closing

12. Ethylene Gas released by plant cells Effects apoptosis
death of annual plant after flowering
abscission
shedding of various parts of plant
dropping a leaf, fruit, flower, or seed
shedding of autumn leaves
fruit ripening
burst of ethylene triggers ripening process
breakdown of cell wall softening
conversion of starch to sugar  sweetening
positive feedback system
ethylene triggers ripening
ripening stimulates more
ethylene production
untreated tomatoes vs. Ethylene treatment
abscission of the hypanthium during development of nectarine

13. Plant Movement
Tropisms: growth responses  SLOW Gravitropism – gravity (auxin) Thigmotropism – touch Phototropism – light (auxin) Turgor movement: allows plant to make relatively rapid & reversible responses Venus fly trap, mimosa leaves, “sleep” movement

14. Response to Gravity Gravitropism Positive: roots
downward growth in relation to gravity
starch-statolith hypothesis: gravity sensing statoliths (dense in starch) settle to bottom of root cells
Negative: shoots, stems
upward growth opposite gravity

15. Response to Touch Thigmotropism Mimosa (touch sensitive plant)
closes leaves in response to touch
Caused by changes in osmotic pressure
Rapid loss of K+  rapid loss
of H2O
causes loss of turgor in cells

16. Response to Light Photomorphogenesis effect of light on plant growth
Plants detect light
intensity
direction
wavelength
blue-light receptors
phytochromes (red-light receptors)

17. Biological Clocks Circadian rhythm: Internal biological clocks
Persist w/o environmental cues
24 hour cycles
Phytochrome system + Biological clock
plant can determine time of year based
on amount of light/darkness

18. Flowering Response
Short-day plants: flower when nights are long (mums, poinsettia)
Long-day plant: flower when nights are short (spinach, iris, veggies)
Day-neutral plant: unaffected by photoperiod (tomatoes, rice, dandelions)
Photoperiodism
physiological response to the relative length of night & day
*night length is the critical factor and trigger*

19. How does interrupting the dark period with a brief exposure to light affect flowering? critical factor is length of darkness (night) synchronizes plant responses to season

20. Responses to Stress Herbivores physical – thorns, spines
chemicals – garlic, mint,
poison ivy
recruitment of predatory animals- parisitoid wasps

21. Responses to Stress Drought Flooding (O2 Deprevation) close stoma
release abscisic acid to keep stoma closed
inhibit growth
roll leaves  reduce SA & transpiration
deeper roots
Flooding (O2 Deprevation)
release ethylene  root cell death  air tubes formed to
provide O2 to submerged roots

22. Responses to Stress Excess Salt Heat Cold
cell membrane – impede salt uptake
produce solutes to ↓ψ - retain H2O
Heat
evap. cooling via transpiration
heat shock proteins – prevent denaturation
Cold
alter lipid composition of membrane (↑unsat. fatty acids, ↑fluidity)
increase cytoplasmic solutes
antifreeze proteins