1. Chapter 39: Plant Communication

2. Plants and their Environments
Plants are living and respond to envir.
They detect light and gravity especially.
Plants respond by altering growth.
Receptors receive signals and initiate a cellular response.
Signal Transduction Pathway
Ex: Potatoes bud in dark not light
b/c they grow in absence of light (underground)

3. Hormones Chemical Signals
Sent by one cell to initiate responses in others
Can be introduced to soil or water to affect plant
Minimal concentrations needed for big effect
Triggers a cascade response
Relative, not absolute levels of hormones regulate plant responses

4. Tropisms Whole body response of plant to stimuli.
Phototropism – Plant grows towards light.
Coleoptile tip must be exposed to light. If removed or covered, no response.
Detection at coleoptile, but growth occurs further down shoot.
Chemical response sent from tip to shoot.
Gravi-. Photo- and thigmo-tropism
Gravity, light, touch

5. Phototropism
Plants detect the intensity, direction, and wavelength of light.
They will bend towards the area with the best light (intensity/quality)
Phytochromes = receptors

6. Phototropism is a Hormonal Response

7. Auxins – Plant Growth Hormone
Responsible for tropisms.
2 Theories:
Auxin concentrated on dark side of plant  traditional theory
An auxin inhibitor concentrated on light side of plant
 more recent research

10. Auxins
Main function is to promote shoot elongation at low concentrations
Loosen cell wall and allow increase H2O uptake (elongation)
Also induce division in cambium
Too high of a [auxin] will cause production of ethylene which inhibits elongation
Causes rapid translation of proteins
Gene regulation

11. Cytokinins
Induce cytokinesis (cell division)
Operate in balance with auxins
Equal: No differentiation, just division
More cytokinin: Shoot buds form
More auxin: roots form
Auxins inhibit auxiliary growth, cytokinins promote it
Slow aging

12. Effect of Gibberellins
Produced in roots and leaves
Stimulate growth of leaves and stems
Elongation and division
Works in cooperation with auxin to loosen cell walls (more H2O in = elongation)
Effect of Gibberellins

13. Seed Dormancy and Gibberellins
When conditions are appropriate, the release of gibberellins signals the seed to germinate
May stimulate digestive enzymes to mobilize stored nutrients

14. Ratio b/t ABA and others Seed dormancy
Abscisic Acid (ABA)
Slows growth
Works antagonistically against the other growth hormones (gibberellins, auxins, cytokinins)
Ratio b/t ABA and others
Seed dormancy
Allows plants to withstand droughts (close stomata)

15. Leaf abscission (fall off) Fruit ripening Programmed cell death
Ethylene
Leaf abscission (fall off)
Fruit ripening
Programmed cell death
High [auxins] stimulate production
Involved in thigmotropism (touch)
Apoptosis

16. Systematic, preprogrammed (DNA) cell death.
Apoptosis
Systematic, preprogrammed (DNA) cell death.
Enzymes break down organics (DNA, chlorophyll, RNA, proteins)
Bursts of ethylene during autumn cause plants to lose their leaves and prepare for winter. First, they break down the organic molecules.
Stored in stem parenchyma cells (reused by new spring leaves)

17. Seed Dispersal  Fruits
When a bear eats a piece of fruit, he eats the seeds.
He doesn’t digest the seeds.
He poops the seeds.
The poop is fertilizer for the seeds.
Thanks Fozzy.

18. Ethylene production causes ripening
Ripening of Fruit
Ethylene production causes ripening
Storing fruits in a paper bag accelerates ripening (traps ethylene)
Storing orange juice in the cabinet accelerates ripening

19. Plants go through processes in a 24-hour (approx.) cycle
Circadian Rhythms
Plants go through processes in a 24-hour (approx.) cycle
When isolated, they lose sync with the external ques and regain when replaced
Think Jetlag

20. Photoperiodism
Plant leaves monitor the relative length of day and night
Allows seasonal changes in plants (ie: flowering)
Short-day plants require light period shorter than a critical length to flower.
Long-day plants flower when the light period is longer than a critical number of hours
Day-neutral plants flower at maturity, regardless of day length

21. Short/Long Day Plants Short day = long night Long Day = short night
Many plants require uninterrupted darkness hours to flower

22. Water Deficit
Bright, sunny days  plants can lose more H2O than they take in
As H2O dec. the guard cells close stomata
Inc. ABA production (close stomata)
Inhibit growth of young leaves
Leaves roll to dec. surface area
All designed to decrease transpiration

23. Heat/Cold Stress
Denatures plant enzymes
Transpiration = evaporative cooling
Heat shock protein = produced when temp. is raised beyond threshold
Often bind to enzymes to prevent denaturization
Extreme cold also freezes plants
Many plants collect solutes (dec. fp)

24. Defense Some “call” other species to fight predators off
Plants release signal chemicals to alert other plants of an infestation of herbivores