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

10. 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

11. 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)

12. 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

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