Plant hormones

1. Plant hormones • Naturally-produced nonnutrient chemical compounds involved in growth/development • Active in relatively low concentrations • Are often transported from one part of the plant to another

2. Major classes of plant hormones and their areas of involvement in plant growth and development • Auxins - cell division and rooting • Cytokinins - cell division and shoot formation • Ethylene - the ripening hormone • Abscisic acid - growth inhibition and dormancy induction • Gibberellins - germination and control of dormancy

3. Plant growth regulators (PGRs) are synthetic chemicals that often mimic plant hormonesMost PGRs used commercially for promoting rooting are auxin-type compounds (and often referred to as “rooting hormones”)

4. How natural auxins promote rooting of stem cuttings • Natural auxins are produced in shoot tips and young leaves • Auxin transport is polar (downward from the shoot tip) • Auxin will accumulate near the cut stem

5. How synthetic auxins (“rooting hormones” or PGRs) promote rooting of stem cuttings • Are applied to the cut stem surface • Move upward (into the cut stem) a short distance • Influence tissues into becoming competent or help to determine the development of root-forming tissue

6. Synthetic auxins are used primarily to: • make a stronger, quicker root system on easily rooted species • enhance rooting of hard-to-root plants

7. Some features of synthetic auxins • Indolebutyric acid (IBA) and naphthaleneacetic acid (NAA) have proven most effective • IBA and/or NAA are more stable than indoleacetic acid (IAA) • K-salt formulations can be dissolved in water, while acid formulations must be dissolved in an organic solvent (e.g., 50% ethanol)

8. Methods of application of “rooting hormones” • Talc or powder dip(0.1 to 0.8% IBA or NAA) • Dilute solution soaking method (20 to 200 ppm overnight) • Quick-dip solution (200 to 10,000 ppm for 3-5 sec)

9. Features of talc preps • Cutting is dipped (cut-end), excess is tapped off, the cutting stuck in moistened medium • Convenient for field situations (no evaporation of solution) • Variable amounts of chemical remain on each cutting • Chemical must go back into solution before it can be taken up

10. Features of liquid preps • Dilute soaking method uses less chemical but is too slow for most applications • Quick-dips provide uniform chemical uptake, more consistent rooting

11. Preparation of talc and liquid preps • Commercial preps list active ingredient (usu. an auxin) in parts per million (ppm), others in percent (%) • “Rooting hormones” are often sold as a concentrate (e.g., Dip N Grow is listed as 1% IBA, 0.5% NAA) that requires dilution

12. Things to remember when converting ppm to % and % to ppm • Grams talc are roughly equivalent to milliliters (mls) talc • % = grams/100 ml • ppm = mg/liter • Standard metric equivalents (e.g., 1 g = 1000 mg, 1 liter = 1000 ml, etc.)

13. Things to remember when diluting a “rooting-hormone” concentrate • You know the initial concentration (Ci), the desired final concentration (Cf) and the volume of the solution (Vf) you wish to make • Solve for the unknown (the initial volume or Vi) using the formula: CiVi = CfVf

14. Sample problems • A dilute-soak solution contains 200 ppm of IBA. What’s the concentration of IBA as a percent? • You want to make 1000 ml of a quick-dip solution using Dip N Grow concentrate so that the final concentration of IBA is 1000 ppm. What volume of Dip N Grow concentrate will you use to make the quick-dip solution?

15. The interaction of auxin with other plant growth regulators (PGRs) Cytokinins Adventitious shoot buds are favored when the conc. is high, auxin low; may promote rooting at low conc. Promotes stem elongation; inhibits rooting Gibberellins Abscisic acid May promote rooting; antagonistic to GA Effect on rooting varies; can stimulate rooting at later stages of initiation Ethylene

16. Auxin control of rooting - past to present: ideas about difficult-to-root plants • Rooting morphogens (e.g., orthohydroxy phenol) - some have been found, but not in all plants • Endogenous rooting inhibitors - have been shown to exist, but have not been identified • Rooting co-factors (auxin synergists) - several have been discovered (but not chemically identified) and no firm cause-and-effect relationship has been established

17. Auxin control of rooting - current molecular model (from Fig. 9-22) Auxin Auxin transport and binding Auxin-binding proteins Secondary messenger? Signal transduction Early auxin genes? Transcriptional regulators Cell cycle genes? Cell division Cell-wall proteins? Meristem organization

18. In the absence of a clear understanding of the physiology of rooting, researchers are working to make “hard-to-root” plants into “easy-to-root” plants

19. Transforming a hard-to-root plant • Infect stem cuttings with Agrobacterium rhizogenes • The bacteria inserts a piece of plasmid DNA (T-DNA) into a stem cell • The T-DNA (carrying auxin synthesis genes) is incorporated into one of the plant cell’s chromosomes • The transformed cell is stimulated to divide and form root-competent tissue

20. Percentages of microcuttings of almond (Prunus dulcis) with developed roots after infection with Agrobacterium rhizogenes and not infected* *Damiano et al (1995) Acta Hort 392:161-169