PLANT HORMONES

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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)

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CLASSES OF PLANT HORMONES

• THE FIVE CLASSICAL HORMONES
• AUXIN
• CYTOKININ
• GIBBERELLIN
• ETHYLENE
• ABSCISIC ACID

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OTHER PLANT HORMONES

• BRASSINOSTEROIDS
• JASMONATE
• SALICYLIC ACID
• POLYAMINES
• SIGNAL PEPTIDES

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A BRIEF AUXIN HISTORY

• 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

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NATURALLY OCCURRING AUXIN
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A SYNTHETIC AUXIN
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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

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AUXIN SYNTHESIS AND TRANSPORT

• Pathway precursors-INDOLE AND TRYPTOPHAN
• Synthesized primarily in leaf primordia and young
  leaves-apical 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

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BIOASSAYS FOR AUXIN

• Bending of dark grown seedling in response to
  applied auxin
• Rapid elongation of stem sections
• Effects on callus auxins promote rooting of
  callus cells auxin plus cytokinin induce shoot
  formation
• Inhibition of root elongation promotion of
  lateral root formation

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AGRONOMIC USES OF AUXIN

• Production of seedless fruit in some
  species(tomato, grapes) called parthenocarpy
  (fruit without fertilization)
• Rooting on stem cuttings (mainly
  ornamentals)-used to maintain genetic purity
• Herbicides- at high concentrations, auxins such
  as 2,4-D will kill broadleaf dicots (weeds in
  cereal grain fields and lawns)

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TAKE HOME MESSAGES

• Auxins are absolutely crucial for many aspects of
  growth and development
• Auxins are effective at very low concentrations
• Auxins (applied exogenously) have many agronomic
  uses
• Auxins dont 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

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REFERENCES

• Plant Hormones (Davies PJ, ed) 2004 Kluwer
  Academic Publishers
• Google search (images) auxin, IAA, 2,4-D, etc
• HagenG_at_missouri.edu