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Plant Hormones
What kinds of things do plants
have to respond to?
 Light
- phototropism
 Touch - thigmotropism
 Gravity – gravitropism
 Turgor movements
 Biological clock - circadian rhythms
– When to open and close stomata
 Control
of flowering - photoperiodism
Figure 39.2 Review of a general model for signal-transduction pathways
Figure 39.0 A grass seedling growing toward a candle’s light
Figure 39.1 Light-induced greening of dark-sprouted potatoes: a dark-grown potato
(left), after a week's exposure to natural sunlight (right)
The Search for the Plant Hormone


Darwin and son his discovered that grass seedlings would
not bend towards the light if the tip was removed and
covered with an opaque cap.
Boysen and Jensen demonstrated that the signal was a
mobile substance.
– They placed a gelatin block between the tip and the rest of
the plant and demonstrated that the signal diffused through
the gelatin.

Went(1926) extracted the chemical messenger and
impregnated agar blocks.
– The agar blocks were places on various parts of the plant
and the plant bent in the opposite direction from which the
agar block was placed.
– Showed that cells opposite of the elongated causing the
stem to bend.
– The substance was called auxin and produced growth on the
opposite side in which it was concentrated.
Figure 39.4 Early experiments of phototropism
Figure 39.5 The Went experiments
Table 39.1 An Overview of Plant Hormones
Plant Hormones Help Coordinate Growth,
Development and Response to Stimuli

Five classes of Hormones:
–
–
–
–
–
Auxins
Cytokinins
Gibberillins
Abscisic acid
Ethylene
Hormones are effective in very small
concentrations and are the signal is
amplified
 Can affect gene expression and activity
of enzymes.

Auxins
Natural form is called indoleacetic
Acid(IAA)
 Found mostly in the apical meristem.
 Auxin only travels in one direction.
 Acid Growth Hypothesis:

– Stimulates proton pumps in the region of
growth.
– Acidic pH breaks down cell walls
– Turgor pressure causes cell to elongate
– Function:
 Stimulates
cell division
 Differentiation of secondary xylem growth
 In developing seeds promotes fruit growth.
Figure 39.6 Polar auxin transport: a chemiosmotic model (Layer 1)
Figure 39.6 Polar auxin transport: a chemiosmotic model (Layer 3)
Figure 39.6 Polar auxin transport: a chemiosmotic model (Layer 2)
Cytokinins
 Together
with auxin orchestrate root
and shoot growth.
 Stimulate cell division and
differentiation.
 Relative amounts of cytokinin and
auxin will determine what will occur.
– In equal amounts no differentiation
occurs.
– More cytokinin than auxin will result in
root buds.
– More auxin than cytokinin will result in
shoot buds.
Gibberellins
 Causes
“bolting” rapid growth of a
flower stalk.
 Involved in breaking dormancy of
apical buds in the spring.
 Stimulate growth in both leaves and
stems.
 In some plants both auxin and
gibberellins are contribute to fruit
set.
Bolting “Foolish Seed”
Abscisic Acid
Produced in the terminal bud slows
growth and inhibits cell division.
Primordial leaves develop into scales
and protect the apical bud through
the winter.
Keeps seeds dormant.
Can help plants cope with harsh
conditions by closing their stomata.
Ethylene
A
gas that promotes fruit ripening.
 Contributes to aging or
“senescence” of parts of the plant.
– Promotes degradation of cell walls and
decreases chlorophyll content that is
associated with fruit ripening.
 Involved
in leaf abscision.
Tropisms Orient Plants Toward or Away
From Stimuli

Phototropism
– Cells on the darker sides of the stem elongate
faster in response to auxin moving down from
the shoot.
– Photoreceptor is believed to be a blue light
receptor.

Gravitropism
– Roots curve downwards.
– Two theories
 Staholith
settling
 Protoplast signaling

Thigmotrpism
– Climbing plants respond to touch and grow
tendrils to grasp onto surfaces.
– Stunting height growth in windy
environments

Rapid Leaf Movements
– Rapid loss of turgor pressure in response to
touch.
 Specialized
motor organs called pulvini in the
joints of leaves.
 Lose potassium when stimulated causing water
loss.
– Signals are transmitted either through chemical or
electrical impulses called action potentials

Sleep Movements
– Daily raising or lowering of leaves

Circadian Rhythms
– Internal 24 hour cycle in which most
organism keep track of the time of day.

Photoperiodism
– Physiological response to day length.
– Control of flowering
 Short
day plants - flower during periods of short
day length.
 Long day plants - flower during periods of long day
length.
 Day neutral plants – unaffected by day length.
– Researchers have found that night length
affects plant flowering.
– Leaves detect day length