Transcript PLANT HORMONES - University of Missouri

PLANT HORMONES
“Plant hormones are a group of naturally occurring, organic
substances which influence physiological processes at low
concentrations”
Peter J Davies, ed
PLANT HORMONES (2004)
CLASSES OF PLANT HORMONES
THE FIVE “CLASSICAL” HORMONES
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AUXIN
CYTOKININ
GIBBERELLIN
ETHYLENE
ABSCISIC ACID
OTHER PLANT HORMONES
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BRASSINOSTEROIDS
JASMONATE
SALICYLIC ACID
POLYAMINES
SIGNAL PEPTIDES
A BRIEF AUXIN HISTORY
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Charles Darwin 1880s-observed plant shoots bending towards the
light; hypothesized a “substance” from the tip responsible; response
called phototropism
Fritz Went 1920s-plant shoot decapitation studies to demonstrate the
existence of a growth promoting substance-the term auxin (“to grow”)
first used
1930s-chemical structures of substances found in human urine that
promote plant growth defined (called auxins)
The predominant auxin in plants identified as indole-3-acetic acid
NATURALLY OCCURRING AUXIN
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CH2
COOH
N
H
INDOLE-3-ACETIC ACID
A SYNTHETIC AUXIN
O--CH2--COOH
Cl
Cl
2,4 DICHLOROPHENOXYACETIC ACID (2,4-D)
ROLE OF AUXIN IN PLANT GROWTH AND
DEVELOPMENT
• CELL ELONGATION -cell enlargement and stem growth
• CELL DIVISION -in the cambium; tissue culture
• DIFFERENTIATION -vascular system (xylem and phloem)
• ROOT INITIATION
• TROPISMS-light and gravity
AUXIN SYNTHESIS AND TRANSPORT
• Pathway precursors-INDOLE AND TRYPTOPHAN
• Synthesized primarily in leaf primordia and young leavesapical parts of the plant; also, in some seeds and in roots
• Transported cell-to-cell, mainly from the apex to the base
(root) of the plant
BIOASSAYS FOR AUXIN
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Bending of dark grown seedling in response to applied auxin
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Rapid elongation of stem sections
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Effects on callus: auxins promote rooting of callus cells; auxin
plus cytokinin induce shoot formation
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Inhibition of root elongation; promotion of lateral root formation
AGRONOMIC USES OF AUXIN
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Production of seedless fruit in some species(tomato, grapes);
called parthenocarpy (fruit without fertilization)
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Rooting on stem cuttings (mainly ornamentals)-used to maintain
genetic purity
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Herbicides- at high concentrations, auxins such as 2,4-D will kill
broadleaf dicots (“weeds” in cereal grain fields and lawns)
TAKE HOME MESSAGES
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Auxins are absolutely crucial for many aspects of growth and
development
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Auxins are effective at very low concentrations
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Auxins (applied exogenously) have many agronomic uses
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Auxins don’t work in a “vacuum”-there are interactions and
“crosstalk” with other plant hormones. The well-documented
example is the balance of auxin and cytokinins to control the
production of roots and shoots from callus
REFERENCES
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Plant Hormones (Davies PJ, ed) 2004 Kluwer Academic Publishers
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Google search (images) auxin, IAA, 2,4-D, etc
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[email protected]