Control
Systems
in Plants
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.
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
Table 39.1 An Overview of Plant Hormones
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
Figure 39.8 Apical dominance: with apical bud (left), apical bud removed (right)
Figure 39.7 Cell elongation in response to auxin: the acid growth hypothesis
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
Gibberellins
• Stimulate elongation of cells
• Inhibits root growth
• Stimulate flower part
development- bolting
• Signals seeds to break
dormancy and germinate
Figure 39.9 “Foolish seedling disease” in rice
Figure 39.11 The effect of gibberellin treatment on seedless grapes
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
Figure 39.12 Precocious germination of mutant maize seeds
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
Figure 39.16 Abscission of a maple leaf
Which hormones cause the following….
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•
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•
•
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•
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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
Answers to Previous Slide:
•Auxin
•Ethylene
•Cytokinins
•Cytokinins
•Abscisic acid
•Ethylene
•Gibberellins
•Abscisic acid
•Auxins and cytokinins
•Auxins
•Auxins
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
Answers to Previous Slide:
•Cytokinins
•Auxins
•Ethylene
•Abscisic Acid
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
• 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
Thigmotropism
• growth in response to touch
• tendrils contacts solid and coils
• increased production of ethylene
3.
• `
Hydrotropism
• growth toward water
• willow tree
4.