I. Terminology and classification
A. Plant growth regulator - broad category of organic
substances that, in minute amounts, promote, inhibit, or otherwise modify
physiological processes.
B. Other terms - plant hormones, phytohormones, plant growth
substances, and plant growth regulators
1. PGRs - usually a broad term
encompassing plant hormones
C. Classes of PGRs (more specifically, plant hormones)
1. Auxins (IAA, NAA) - cell
enlargement (with cell-wall modification)
2. Gibberellins (GA) - cell
enlargement (less cell-wall modification)
3. Cytokinins (Kinetin, BA) - cell
division
4. Growth inhibitors (ABA) - leaf
abscision & fruit drop
5. Ethylene (ethylene) - senescence
& ripening
II. Actions of the five plant hormones
A. Auxins (found in meristematic tissue)
1. Mechanism: IAA - cell-wall softening - extension -
membrane RNA synthesis - cell-wall enzymes - new wall material - elongation
2. Auxin moves to the cells on the
lower side of a horizontal organ, stimulating cell elongation and curvature
3. Actions of auxin
a) Apical dominance mediated by auxin
b) Auxin inhibits axillary buds
c) Auxin stimulates adventitious roots
d) Induction of seedless fruits by
auxin (parthenocarpy)
e) High concentrations can lead to
distortion - retardation
B. Gibberellins (found in fruit seeds, buds, young leaves,
root tips)
1. Mechanism can be synergistic -
works with auxin in cell elongation
2. Stimulates internode elongation
3. Can assist germination, sprouting,
and flowering
4. Can lead to seedless fruits - male
sterility
C. Cytokinins (found in developing fruits and seeds)
1. Mechanism is generally cell
division ("cytokinesis")
2. With auxin, cytokinin aids growth
AND differentiation of tissue culture
a) Parenchyma cells are generally used
for tissue culture
3. Responses to cytokinin include
shoot initiation, leaf enlargement, lateral bud growth, adventitious root
formation, delay of senescence, and stimulation of germination
D. Growth inhibitors - abscisic acid - (located near
abscision layers)
1. Mechanism: increased RNA - enzymes - pectin hydrolysis -
cellulase - organ "drop"
2. Also a stress hormone - ABA triggers
stomatal closure
3. Abscisic acid stimulates senescence
(aging) and dehiscence (splitting)
4. Can be used to shorten or retard
plant growth
E. Ethylene (present throughout the plant and especially in
stressed tissue and in ripe fruits)
1. Ethylene is a gas which promotes
senescence
2. Ethylene can be closely associated
with auxin (With high 2,4-D, high ethylene....death)
3. Associated with rapid senescence in
diseased and injured tissues
4. Can be used to increase girth in
trees
5. Increases fruit ripening
III. Important point: the growth regulators exist and work together
to regulate growth & development
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