Transcript Auxins

Plant Hormones
What kinds of things do
plants have to respond to?
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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
Auxins
Plant Hormones Help Coordinate
Growth, Development and to Stimuli
 Five classes of Hormones:
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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
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Natural form is called indoleacetic Acid(IAA)
Found mostly in the apical meristem.
Auxin only travels in one direction.
Acid Growth Hypothesis:
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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 cintribute 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 l;ength affects
plant flowering.
 Leaves detect day length