BIOL 1120 REEDER
Plant Hormones
I. CHEMICAL MESSENGERS
A. Plant Hormone Characteristics
1. defined: an organic compound that acts as a highly specific chemical signal between cells
2. are effective in extremely small amounts
3. each hormone elicits different responses
4. effects of different plant hormones overlap (synergistic effect)
a. apical dominance: auxins, ethylene, cytokinins, abscisic acid
b. fruit development: auxins, gibberellins, cytokinins, ethylene, abscisic acid
c. seed germination: abscisic acid, gibberellins
5. may stimulate a certain response at one concentration and inhibit that same response at a
different concentration
6. ten plant hormones and hormone-like signaling molecules:
a. hormones: auxins, gibberellins, cytokinins, ethylene, abscisic acid
b. signaling molecules: brassinolide, salicylic acid, systemin, oligosaccharins, jasmonic
acid
7. similar in basic mechanism of action to animal hormones
a. bind to specific receptor proteins on the plasma membrane or in target cells
1) each three-dimensionally shaped receptor binds with only one kind of hormone
molecule
b. sometimes the binding triggers the production of a second messenger which is an
intracellular signaling molecule that affects the cell function
1) ions such as Ca2+ or cyclic AMP (cAMP)
2) the second messenger’s increase may bind to proteins and activate or inactivate
certain enzymes
3) activation or inactivation may lead to altered membrane permeability and/or altered
gene expression (transcription and/or translation)
II. THE MAJOR PLANT HORMONES
A. Auxins
1. Produced in the shoot apical meristem, young leaves, and seeds
a. naturally occurring form is called indoleacetic acid
2. Principal actions in stem elongation, apical dominance, root initiation, and fruit development
3. Plays a major role in apical dominance by the suppression of lateral bud growth as it diffuses
downward from the terminal bud’s apical meristem.
a. This is dependent on how active the growing tissue is as an auxin excess is growth
inhibiting and will starve the laterals of essential nutrients.
b. Auxin influences cells around the lateral buds to produce the hormone ethylene which
inhibits the lateral bud growth; abscisic acid accumulates during aging to suppress buds,
also.
c. The hormone, cytokinin, may also play a part in apical dominance through its mitosis-
promoting action whereas dormant laterals may contain insufficient amounts;
antagonistic to auxin: promotes growth of lateral buds while auxin inhibits.
d. When the terminal bud is removed, the lateral buds grow resulting in a bushy plant; the
number of flowers on an individual plant may be increased also in this way.
4. Stem elongation role:
a. Low concentrations of auxin promote cell wall plasticity; therefore it must be broken
down rapidly to prevent its accumulation
bio1120_plant_hormones Page 1 of 4 4/23/09
