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PLANT HORMONES

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 AUXIN CYTOKININ GIBBERELLIN ETHYLENE

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PLANT HORMONES

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  1. 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)

  2. CLASSES OF PLANT HORMONES THE FIVE “CLASSICAL” HORMONES • AUXIN • CYTOKININ • GIBBERELLIN • ETHYLENE • ABSCISIC ACID

  3. OTHER PLANT HORMONES • BRASSINOSTEROIDS • JASMONATE • SALICYLIC ACID • POLYAMINES • SIGNAL PEPTIDES

  4. 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

  5. NATURALLY OCCURRING AUXIN

  6. A SYNTHETIC AUXIN

  7. 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

  8. 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

  9. 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

  10. 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)

  11. 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 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

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

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