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Control
Systems
in Plants
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Plant Hormones
• What is a Plant Hormone ?
• Compound produced by one
part of an organism that is
translocated to other parts
where it triggers a response in
target cells and tissues.
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Functions of Plant Hormones
• Control plant growth and
development by affecting division,
elongation, and cell differentiation
• Effect depends on size of action,
stage of plant growth and hormone
concentration
• Hormonal signal is amplified by
gene expression, enzyme activity,
or membrane properties
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Table 39.1 An Overview of Plant Hormones
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Auxins
(IAA) indoleacetic acid:
natural auxin in plants
• Promotes elongation & secondary growth
• Apical meristem is the major site of auxin production
• Inhibits lateral growth
• Induces female floral parts & fruit
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Figure 39.8 Apical dominance: with apical bud (left), apical bud removed (right)
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Figure 39.7 Cell elongation in response to auxin: the acid growth hypothesis
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Cytokinins• Move from the roots to tissues
by moving up xylem
• Stimulates protein synthesis
• Made in roots
• Functions:
–1. Cell division and differentiation
–2. Apical dominance
–3. Anti-aging hormones
• slow protein deterioration
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Gibberellins
• Stimulate elongation of cells
• Inhibits root growth
• Stimulate flower part
development- bolting
• Signals seeds to break
dormancy and germinate
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Figure 39.9 “Foolish seedling disease” in rice
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Figure 39.11 The effect of gibberellin treatment on seedless grapes
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Abscisic Acid (ABA)
• Growth inhibitor
• returns seeds to dormancy
• inhibits cell division in vascular
cambium
• causes rapid closing of stoma
during dry periods
• promotes positive geotropism
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Figure 39.12 Precocious germination of mutant maize seeds
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Ethylene
• Gaseous hormone
• High [auxin] induces release of
ethylene
• Causes senescence (aging)
• Fruit ripening – one bad apple
does spoil the whole bunch
• Abscission – loss of leaves on
deciduous trees
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Figure 39.16 Abscission of a maple leaf
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Which hormones cause the following….
• Apical dominance from apical bud
• Abscission
• Stimulates growth of axillary buds
• Root growth
• Stimulates closing of stomata
• Causes fruit ripening
• Stimulates seeds to break dormancy and germinate
• Growth inhibitor
• Cell division and differentiation
• Cell elongation
• Seedless fruit
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Answers to Previous Slide:
•Auxin
•Ethylene
•Cytokinins
•Cytokinins
•Abscisic acid
•Ethylene
•Gibberellins
•Abscisic acid
•Auxins and cytokinins
•Auxins
•Auxins
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Which hormone is made at each
location?
• Made in roots and transported
upwards
• Found in meristems of apical
buds and seed embryos
• Found in tissues of ripening fruit
• Leaves stems, roots and green
fruit
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Answers to Previous Slide:
•Cytokinins
•Auxins
•Ethylene
•Abscisic Acid
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Plant Movement• A. Tropisms:
– growth response toward or away from stimuli
• 1. Phototropism– cells on darker side of
shoot elongate faster than cells on bright side due to auxin distribution
– auxin move laterally across the tip from the bright to dark side by an unknown mechanism.
– Cells on the dark side grow
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• 2. Gravitropism (geotropism)– gravity
– roots --> positive geotropism
– stems---> negative geotropism
• Statoliths– starch grains in root
cap cells, they trigger calcium redistribution which results in auxin movement in root
– auxin inhibits cell elongation
– upperside of root elongates faster than bottom
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3. Thigmotropism
• growth in response to touch
• tendril contacts solid and coils
• increased production of ethylene
4. Hydrotropism
• growth toward water
• willow tree