|Plant Physiology (Biology 327) - Dr. Stephen G. Saupe; College of St. Benedict/ St. John's University; Biology Department; Collegeville, MN 56321; (320) 363 - 2782; (320) 363 - 3202, fax; email@example.com|
Plant Hormones - Auxin
Auxin is a general name for a group of hormones that are involved with growth responses (i.e., elongate cells, stimulate cell division in callus). Not surprisingly, the term "auxin" is derived from the Greek word "to increase or grow". This was the first group of plant hormones discovered.
A. Naturally Occurring Auxins
The most important auxin found in plants is indole-3-acetic acid (IAA). IAA is comprised of an indole ring linked to acetic acid (see overhead). Other auxins that have been isolated from plants include indole ethanol, indole acetaldehyde, indole acetonitrile, phenylacetic acid (PAA), and 4-chloro-indoleacetic acid. These are probably converted to IAA in vivo.
B. Synthetics with Auxin Activity
There are a variety of substances that are not known to occur in plants that have auxin activity. These include indolebutyric acid (IBA); naphthalene acetic acid (NAA); 2,4- dichloro-phenoxyacetic acid (2,4-D), and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). The exact mechanism of action of these compounds is not known, but they may inhibit nucleic acid synthesis.
C. Chemistry of action
Although a variety of molecular structures have auxin activity (i.e., derivatives of indole, benzene and naphthalene), these molecules seem to share a few features that appear to be required for activity. Auxin activity seems to be correlated with a flat, hydrophobic ring system that separates negatively-charged (acidic, carboxyl group) side chain and positively charged region. There is a charge separation of about 0.5 nm. Note that the indole unit isn't required for activity, but a planar ring system is.
D. Conjugated forms
Auxins, as do other hormones, occur in a free or conjugated (bound to sugars, alcohols or other molecules) form. In fact, up to 98% of the auxin may be bound. Auxins may be conjugated with inositol, coenzyme A or glucosides (sugars).
PCIB (p-chlorophenoxyisobutryic acid) is a compound that competes with auxin for binding sites. However, it doesn't cause any growth response.
Auxin is made in actively growing tissue which includes young leaves, fruits, and especially the shoot apex. Made in cytosol of cells
There are two major routes to the production of IAA.
A. Basipetal (or Polar) Transport
Auxin is transported in a basipetal (towards the base, base-seeking) direction. In other words, auxin moves from the shoot tip towards the roots and from the root tip towards the shoot.
Evidence - (1) Seedling vs. [auxin]; and (2) 14C labeled IAA applied to the top of a stem section is recovered only from the bottom of the stem section. When auxin is applied to the end of stem segments, it is only transported from the "top" of the section to the "bottom" as demonstrated in these data:
|14C-IAA in donor block (dpm)C-IAA in donor block (dpm)||10,000||10,000|
|14C-IAA in receiver block (dpm)C-IAA in receiver block (dpm)||7,500||300|
B. Mechanism of polar transport
There are four classic bioassays for auxin. These tests, which are all based on the ability of auxin to stimulate shoot growth (or inhibit root growth) are:
A. Avena coleoptile curvature
Pioneered by F. Went. The angle of curvature of a decapitated oat coleoptile is measured after placing an agar block containing auxin on one side. Then, angle of curvature vs. [IAA] is plotted.
B. Avena coleoptile elongation
The ratio of final length/original length for oat coleoptiles or pea stem sections is measured after the tissues are floated in solutions containing different concentrations of IAA. Elongation vs. [IAA] is plotted.
C. Split pea curvature test
A section of pea hypocotyl is obtained and split halfway down the middle. After incubating the sections in solutions of known IAA concentration, the angle of curvature is measured. Note that only the epidermal cells are responding to the treatment.
D. Cress root inhibition
This bioassay is based on the ability of auxin to stop root growth. A ratio of treatment/control growth is plotted vs. log [IAA].
VI. Auxin responses
IAA is involved in the following responses:
A. Cell elongation and wall
Discussed earlier in the semester. Basis of several bioassays and the discovery of auxin. NOTE: Normally exogenous application of IAA has little effect on plants.
B. Cell differentiation
Promotes differentiation of vascular tissue (i.e., xylem & phloem).
C. Ethylene production
IAA apparently stimulates the production of ethylene.
D. Inhibition of root growth
[IAA] > 10-6 M inhibit root elongation. However, very low (>10-8 M) favor root elongation.
E. Stimulate root initiation (lateral roots, adventitious roots)
Roots always form at the basal end of cutting
Although most plants dont initiate the production of flowers after auxin treatment, pineapple and its relatives (Bromeliaceae) do. Once flowers are initiated, in many species, IAA promotes the formation of female flowers, especially in cucurbits (gourd family).
G. Parthenocarpic fruit development - as per RCBr Lab
H. Apical dominance
The apical meristem (apex) controls or dominates the development of the lateral buds. (PS - a bud is an embryonic shoot with immature leaves and stem). Apical dominance also occurs in roots. It is responsible for the Xmas tree shape of many trees and prevents an individual from becoming too top-heavy.
- Function: (a) maintain upward growth; (b) maximize light absorption; (c) prevent top-heaviness; and (d) provide mechanism to replace the apical bud if it is removed by grazing or damage.
- Thimann & Skoog (1934) - first suggested a correlation between [IAA] and apical dominance. This idea was further developed by Cholodny-Went who proposed that plant tissues responded to different concentrations of IAA (see figure). For example, hi [IAA] stimulates shoot growth but inhibits bud and roots. Thus, IAA is produced at the tip of the plant and is transported downward. The high concentrations near the apex inhibit lateral buds. As the concentration decreases it frees the buds from the inhibition and they develop.
- every gardener knows that removing the apical meristem results in bushier plants. This is consistent with the Cholodny-Went hypothesis. If IAA is applied to a de-tipped plant it prevents lateral bud development
- IAA stimulates the formation of ethylene which is known to inhibit lateral bud formation.
- branching mutants of tomato don't export 14C-IAA
- [IAA] in buds after the tip is removed should decrease, and they don't
- [IAA] that act to replace the tip are much higher than levels in the intact plant
- 14C-IAA doesnt enter the lateral buds when applied at tip
- Some plants dont respond to exogenous IAA
I. Tropic responses - such as gravitropism and phototropism
Formation of the abscission layer is correlated with the [IAA] in leaf. As long as the [IAA] in the leaf is high relative to the stem, then the abscission layer doesnt form. When the [IAA] in the leaf drops, which occurs normally during the growing season, it stimulates the formation of the abscission layer forms and the leaf falls. The IAA presumably works by decreasing the sensitivity of the cells to ethylene which is the primary hormone involved in initiating leaf drop.
VII. Regulating Endogenous Levels
All of the general methods (i.e., degradation, conjugates, regulate rate of synthesis, sequestering in different regions) of regulation are probably operative. In particular, IAA is probably shuffled back and forth between the two main areas, or pools, where it is stored the cytosol and chloroplast. IAA oxidase, a type of peroxidase, converts IAA to inactive metabolites. There are several degradative pathways.
01/07/2009 � Copyright by SG