Chapter 39: Plant Communication

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)

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

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

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

Phototropism is a Hormonal Response

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

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

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

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

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

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)

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

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)

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.

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

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

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

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

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

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)

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