figure 39.0 a grass seedling growing toward a candle’s light chapter 39: plant hormones
TRANSCRIPT
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Figure 39.0 A grass seedling growing toward a candle’s light
Chapter 39:Plant Hormones
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Review for test
Test tomorrow all multiple choice
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HormonesChemical signalsProduced in one part of plant and effect
other partsTrigger responses in target cells and
tissuesSmall amounts produce substantial
changes
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General plant hormones
AuxinsGibberellinsCytokininsAbscisic acidEthylene
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Table 39.1 An Overview of Plant Hormones
Study This!!
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Hormonal Action
Hormone (red) binds to receptor (receptor) which activate a 2nd messenger (blue). This triggers 1.Proton pumps 2.Production of golgi
vesicles 3.Transcription and
translation
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H+
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Figure 39.7 Cell elongation in response to auxin: the acid growth hypothesis
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Acid Growth Hypothesis
1. Auxin, produced by apical mersitems in buds, trigger H+ pumps
2. H+ pumps move ions into cell wall matrix3. Acidity in wall increases and breaks bonds
in the cellulose fibers4. Cell wall weakens and water enters5. Cell elongates
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Auxin & Auxin & CytokininsCytokinins
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Auxin: Control of Apical Dominance
Effect on Stems:Auxin produced by terminal bud inhibits
growth of axillary buds. Plant grows tallerCytokinins produced by roots stimulate
axillary bud growthLower buds grow faster than those close to
terminalIf terminal bud is cut, (i.e. auxin is removed)
and axillary buds grow and plant gets fuller
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Auxin: Control of Apical Dominance
Effect on Roots:Auxin stimulates root branchingHigh concentrations of auxin INHIBIT root
growthCytokinins inhibit root branching
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Apical Dominance
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Function of cytokinins
Promotes cell division.Morphogenesis.Lateral bud development.Delay of senescence.Made in root, travel in xylem
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GibberellinGibberellin
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Discovered in association with Discovered in association with Foolish disease of rice Foolish disease of rice ((Gibberella fujikuroi)Gibberella fujikuroi)
infecteduninfected
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Effects of Gibberellins
Make in roots and young leavesGeneral cell elongation in leaves and
stems (not roots).Found in embryo of seeds and leads to
the breaking of dormancy.Promotion of flowering.Commercially used for spraying on
Thompson’s seedless grapes
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Figure 39.11 The effect of gibberellin treatment on seedless grapes
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Abscisic acidAbscisic acid
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Functions of abscisic acid
General growth inhibitor.Prevents seeds from germinating (if
ABA is removed from seeds, germinations begins!)
Causes closing of stomata“Winterization”effect on plantsProduced in response to stress.
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EthyleneEthylene
H H \ / C = C / \ H H
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Functions of ethylene
GaseousCauses fruit to ripening.Senescence and abscission.Interference with auxin transport.Initiation of stem elongation and bud
development.
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Tropism
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TropismsPhototropism:
shoots or stems bends towards light enhancing photosynthesis
Gravitropism: Plants uses statoliths to tell which way is
“up” or “down” Roots display positive gravitropism (grows
down towards the earth) and the shoot displays negative gravitropism (grows up out of the earth)
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Growth movementGrowth movement
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Phototropism
Light straight above plants
Auxin amounts equal on both sides
Cell elongation equal on both sides
Stems grow straight up!
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Phototropism
Light at an angleAuxin accumulates
(moves) to the shaded side
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PhototropismHigh auxin
concentration promotes cell growth on shaded side
Stem grows towards light source
Positive phototropism
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GravitropismShoot:
Grows away from pull of gravity
Negative gravitropism
Root: Grows toward pull of
gravity Positive gravitropism
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If plant falls over…
In roots: Statoliths (starch
grains) settle to the bottom of the cells
Triggers the movement of Ca2+
Causes lateral movement of auxin
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Roots and stems respond…
…the effects of auxin on roots and shoots are opposing!!!! IN ROOTS:
1. Low auxin, cells elongate 2. High auxin, inhibitory
response, no elongation Roots curve down IN SHOOTS 3. Low auxin inhibits
elongation 4. High auxin, stimulates cell
elongation Shoots curve up
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