Transcript Control Systems in Plants

Plant Responses to Internal and
External Signals
Plant Hormones
Plant Movements
Control of Daily and Seasonal Responses
Phytochromes
Plant Hormones
Hormone-compound produced by one part of an organism
that is transported to other parts where it triggers a
response in target cells
 Sends signals between different plant parts
 Tracks time of day and year
 Sensing and responding to gravity, direction of light, etc
 Adjusts growth patterns and development
 Phototropism-responsible for many of the discoveries
about plant hormones
Experiments
 Darwin: experiment-removed coleoptiles; result-no
phototropism; conclusion- tip responsible for sensing light
 Peter Boysen-Jensen: experiment-separated tip w/ block of
gelatin; result-normal behavior; conclusion-signal was a
mobile substance
 F.W.Went: experiment-removed tip, placed it on agar
block, placed block back on plant; result-normal
behavior;conclusion-chemical in block from tip was
responsible. AUXIN
Functions of Plant Hormones
 Coordinate growth and development by
affecting division, elongation and
differentiation of cells
 Regulate responses to environmental stimuli
 Difficult to identify-varying concentrations
affect target cells differently; and different
target cells are affected differently
Classes of Plant Hormones
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Auxins (IAA)
Cytokinins
Gibberellins (GA)
Abscisic Acid (ABA)
Ethylene
Auxins
 Produced by apical meristem
 Stimulate cell growth
 Induces vascular cambium cell division and
differentiation of secondary xylem
 Promotes formation of adventitious root
 Promotes fruit growth
 2,4 D-selective herbicide-dicots
Cytokinins
 Modified adenine
 Stimulates cytokinesis
 Controls cell division and differentiation (in
conjunction with auxins) by stimulating
RNA and protein synthesis
 Controls apical dominance
 Anti-aging hormone
Gibberellins
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More than 80 gibberellins have been identified
Primarily produced in roots and young leaves
Stimulate growth of leave and stems, but not roots
Work with auxins to stimulate cell elongation
Control fruit development along with auxins
Causes seeds to break dormancy, stimulated by
imbibing water
Abscisic Acid
 Produced by terminal bud-prepares plant for
winter (suspends primary and secondary
growth)
 Stress hormone-closes stomata as a result of
excess transpiration
Ethylene
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Growth inhibitor
Produced by high auxin concentrations
Gas
Promotes sinescence (aging) examples:
xylem, leaf fall, withering of flowers, death
of annuals after flowering
 Fruit ripening
 Leaf abscission
Plant Movements
 Tropisms-growth responses (+ or -), one time
events, not repeatable
 Phototropism-light, differential distribution of
auxins, blue light
 Gravitropism-gravity, roots positive, stems
negative; process involve statoliths (starch grains),
calcium and auxin concentration changes
 Thigmotropism-touch, twining of a tendril
Turgor Movements (Nastic
Movements)
 Reversible movements caused by changes
in turgor pressure
 Rapid leaf movements (Mimosa)
 Sleep movements
 K+ movements changes osmotic conditions
and leads to turgor changes
Control of Daily and Seasonal
Responses
 Circadian rhythm-physiological cycle with
a frequency of about 24 hours
 Photoperiodism-physiological response to
day length—flowering,
 Phytochromes play a critical role in
seasonal cycles.
 Two photoreversible forms: Pfr and Pr