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 “rootinghormone” 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
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
Adventitious shoot buds are favored
when the conc. is high, auxin low; may
promote rooting at low conc.
Gibberellins
Promotes stem elongation; inhibits
rooting
Abscisic acid
May promote rooting; antagonistic to GA
Ethylene
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
Auxin-binding proteins
Secondary messenger?
Early auxin genes?
Auxin transport and
binding
Signal transduction
Transcriptional
regulators
Cell cycle genes?
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*
Days
Infected
Not infected
IAA-treated
10
45
0
0
20
87
0
3
30
97
0
3
*Damiano et al (1995) Acta Hort 392:161-169