Control systems in plants
I. Plant Hormones
Hormones: Chemicals produced in minute amounts in specific plant tissues and are
transported to other locations where they exert their effect
 Tropism: Growth responses of whole plant organs toward or away from stimuli
 Phototropism: Growth toward or away from light
 Positive: growth toward light
 Negative: growth away from light
 Light causes auxin ( later discovered) to move from illuminated side of shoot or in
the case of a grass seedling, coleoptile (the emerging shoot of a monocot) to
shaded side of stem
 Cells on shaded side elongate more than cells on sunny side and plant bends
toward light
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Experiments on phototropism led to the discovery of a plant hormone
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In 1926, F.W. Went removed the coleoptile tip, placed it on an agar block, and
then put the agar (without the tip) on decapitated coleoptiles kept in the dark
A block centered on the coleoptile caused the stem to grow straight up
If the block was placed off-center, the plant curved away from the side with the
block.
Went concluded the agar block contained a chemical that diffused into it from the
coleoptile tip, and that this chemical stimulated growth.
Went called this chemical an auxin.
Kenneth Thimann later purified and characterized auxin.
II. Major classes of plant hormones
1. Auxin
 Auxin: A hormone that promotes elongation of young developing shoots
 The natural auxin found in plants is a compound named indoleacetic acid (IAA).
 The apical meristem is a major site of auxin production.
 Auxins is transported unidirectionally from apex to base of plant "polar transport"
Effects of auxin
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a. Cell elongation
 The acid-growth hypothesis states that cell elongation is due to stimulation
of a proton pump
 H+ pumped out lowers pH, activates cellulase enzyme
 Cellulase enzyme degrades cellulose and loosens the wall, allowing water
uptake, which results in elongation of the cell.
b.
c.
d.
e.
Secondary growth by inducing vascular cambium cell division
Promotes formation of adventitious roots
Promotes fruit growth in many plants
Auxins are used as herbicides. 2,4-D is a synthetic auxin which affects dicots
selectively, allowing removal of broadleaf weeds
2. Cytokinin
 Cytokinins: Derived from the nucleotide
 Called "cytokinins" because they stimulate cell division (i.e., cytokinesis)
 Produced mainly in roots; travels through the xylem
Effects of cytokinin
a. Control morphogenesis (cell division and differentiation)
 Cytokinins, in conjunction with auxin, control cell division and
differentiation.
 High cytokinin/auxin ratios favor the formation of shoots
 Low cytokinin/auxin ratios favor the formation of roots.
b. Control of apical dominance
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Cytokinins and auxin contribute to apical dominance through an
antagonistic mechanism.
Auxin promotes apical dominance by maintaining growth and elongation
of stem from the apical meristem and inhibiting axillary bud growth
Cytokinins stimulate axillary bud growth
c. Cytokinins as anti-aging hormones
 Cytokinins can retard aging of some plant organs, perhaps by inhibiting
protein breakdown and stimulating protein synthesis
d. Cytokinins delay the breakdown of chlorophyll in detached leaves by preventing
the genes that produce chlorophyll from being turned off
3. Gibberellin
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In 1930's, Ewiti Kurosawa and colleagues were studying "foolish seedling"
disease in rice. Disease caused by fungus called, Gibberella fujikuroi, which was
stimulating cell elongation and division. Compound secreted by fungus could
cause foolish seedling disease in uninfected plants. Kurosawa named this
compound gibberellin.
Effects of gibberellin
a. Stem elongation
 Gibberellins are produced primarily in roots and young leaves.
 Stimulate cell division and elongation in stems, possibly in conjunction
with auxin
b. Fruit growth
 Fruit development is controlled by both gibberellins and auxin.
 The most important commercial application of gibberellins is in the
spraying of Thompson seedless grapes. The hormones increase distance
between grapes in a cluster to minimize fungi/disease
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c. Germination
 The release of gibberellins signals seeds to break dormancy and
germinate.
 In cereal grains, gibberellins stimulate germination and support growth by
stimulating synthesis of mRNA coding for α-amylase. The α-amylase then
digests the stored nutrients, making them available to the embryo and
seedling.
4. Abscisic acid (ABA)
Effects of abscisic acid
a. Abscisic acid is produced in the terminal bud and helps prepare plants for winter
by suspending both primary and secondary growth.
b. ABA induces bud and seed dormancy in many plant species and may work in
conjunction with other plant hormones (gibberellins)
c. ABA also acts as a stress hormone, closing stomata in times of water-stress thus
reducing transpirational water loss.
5. Ethylene
 Ethylene: A gaseous hormone that diffuses through air spaces between plant cells
Effects of ethylene
a. Stimulates ripening of fruit (used commercially for this purpose)
 Ethylene triggers the breakdown of cell walls, which soften fruits
 The signal to ripen spreads from fruit to fruit since ethylene is a gas.
b. Abscission
 Promotes "abscission" dropping of leaves, fruit and flowers from plants at
appropriate times of year
 Mechanics of abscission are controlled by a change in the balance of
ethylene and auxin.
 When auxin levels in the leaf decline, the tissues become sensitive to
ethylene that promotes abscission by inducing synthesis of enzymes that
digest the polysaccharides in the cell walls, weakening the abscission layer
III. Signal-transduction pathways
 Plant cell responses to hormones and environmental stimuli are mediated by
signal-transduction pathways
 Three steps are involved in each pathway
1. Reception
 The binding of a hormone to a specific protein receptor in the cell or on its
membrane
 Reception of a hormone only occurs in target cells for that hormone.
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 Target cells possess the specific protein receptor to which the hormone must bind;
2. Transduction
 Amplification of the stimulus and its conversion into a second messenger
 Calcium ions appear to be important second messengers in many plant responses.
Calcium ions bind to the protein calmodulin, producing calmodulin-calcium
complex
3. Induction
 The pathway step in which the amplified signal induces the cell's specific
response to the stimulus.
 For example, in the signal-transduction pathway of auxin, the second messenger
activates a chain of events
• Proton pump to acidify cell walls during cell elongation
• The golgi apparatus is stimulated to discharge materials to increase
thickness of the cell wall
• Induction of gene expression that lead to production of proteins required
for growth
IV . Plant Movements
 Tropisms: Growth responses of whole plant organs toward or away from stimuli.
 Three primary stimuli that result in tropisms are light (phototropism ), gravity
(gravitropism), and touch (thigmotropism).
Gravitropism
 Gravitropism = Orientation of a plant in response to gravity
 Roots display positive gravitropism (curve downward).
 Shoots display negative gravitropism (bend upward).
Mechanism of gravitropism in roots:
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Specialized plastids containing dense starch grains (statoliths) aggregate in the
low points of root cap cells.
Aggregating statoliths trigger calcium redistribution, which results in lateral
transport of auxin in the root.
Calcium and auxin accumulate on the lower side of the elongation zone.
Roots curve down, because at high concentrations, auxin inhibits root cell
elongation, so cells on the upper side elongate faster than those on the lower side.
Thigmotropism
 Growth response to touch
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Tendrils (garden peas and beans) wind around on object they are using for support
Thigmomorphogenesis: Morphological response to chronic mechanical
stimulation
Plants that are exposed to wind grow shorter and thicker stems. This helps prevent
them from being blown down in the wind.
V. Turgor movements
Turgor movements: Reversible movements caused by changes in turgor pressure of
specialized cells
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Sleep movements
 Sleep movements: Lowering of leaves to a vertical position in evening and raising
of leaves to a horizontal position in morning
 Occurs in many legumes.
 Due to daily changes in turgor pressure of motor cells of pulvini (special motor
organs located in leaf joints).
 Cells on one side of the pulvinus are turgid while those on the other side are
flaccid.
 Migration of potassium ions from one side of the pulvinus to the other is the
osmotic agent leading to reversible uptake and loss of water by motor cells.
VI. Control of Daily and Seasonal Responses
1. Biological clocks and circadian rhythms in plants
 Biological clocks: internal oscillators that keep accurate time to control many
rhythmic phenomena.
 Plants display sleep movements and a rhythmic pattern of opening and closing
stomata.
 Circadian rhythm: A physiological cycle with a frequency of about 24 hours
 Most biological clocks are cued to the light-dark cycle resulting from the Earth’s
rotation.
2. Photoperiodism
 Photoperiodism: A physiological response to day length such as seed germination,
flowering
Photoperiodism
 Flowering in many plants is induced by length of day and night
 Shortday: Generally flower in late summer, fall and winter such as poinsettias
and potatoes.
 Long-day plants: Generally flower in late spring and summer such as spinach,
radish, clover and corn.
 Day-neutral plants are unaffected by photoperiod such as tomatoes and rice,
Flowering hormone?
 Most plant physiologists believe an unidentified hormone (florigen or flowering
hormone) is produced in the leaves and moves to the buds
 If all leaves are removed, no photoperiod detection occurs.
Phytochromes
 Phytochrome = A protein containing a chromophore (light-absorbing component)
that helps plants to measure the length of darkness in a photoperiod.
 Phytochrome exists in two interconvertible forms, Pr (inactive form) and Pfr
(active form)
 If the phytochrome is illuminated (sunlight), some Pr is converted to Pfr.
 Pfr triggers many plant responses to light (e.g., seed germination and flowering).
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