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

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

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

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

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

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

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)

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)

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

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

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

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.)

Things to remember when diluting a “rooting- hormone” concentrate You know the initial concentration (C i ), the desired final concentration (C f ) and the volume of the solution (V f ) you wish to make Solve for the unknown (the initial volume or V i ) using the formula: C i V i = C f V f

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?

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

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

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

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

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

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: