Plant hormones are specialized chemical substances produced by plants. They are the main internal factors controlling growth and development. Hormones are produced in one part of a plant and transported to others, where they are effective in very small amounts. Depending on the target tissue, a given hormone may have different effects. Plant hormones play an integral role in controlling the growth and development of plants. A plant hormone is generally described as an organic compound synthesized in one part of the plant and translocated to another part, where in low concentrations elicits a physiological response. There are five generally recognized classes of plant hormones; some of the classes are represented by only one compound, others by several different compounds. They are all organic compounds, they may resemble molecules which turn up elsewhere in plant structure or function, but they are not directly involved as nutrients or metabolites.
apical meristem (only moves down),
embryo of seed,
Control of cell elongation
apical dominance (prevents lateral buds)
continued growth of fruit
cell division in vascular and cork cambium
--formation of lateral roots from pericycle
--formation of adventitious roots from cuttings
Roots and young leaves
Cell (stem) elongation (works in stems and leaves, but not roots)
breaking seed/bud dormancy
stimulating fruit set
roots, embryos, fruits
Promote cell division
--signal axillary/lateral bud growth
--prevent leaf abscission
breaking dormancy in some seeds
promote fruit development
leaves, stems, green fruit
Reduces cell division (helps maintain dormancy of seeds and buds)
prepare plants for winter
decreasing cell division
developing protective scales
deposition of waterproofing substances
tissues of ripening fruit,
nodes of stems,
senescent leaves, flowers
initiation of flowering
apical hook of some dicots
AUXINS: Nature of Auxins
Compounds are generally considered auxins if they can be characterized by their ability to induce cell elongation in stems and otherwise resemble indoleacetic acid (the first auxin isolated) in physiological activity. Auxins usually affect other processes in addition to cell elongation of stem cells but this characteristic is considered critical of all auxins and therefore defines the hormone. Functions:
Stimulates cell elongation
Stimulates cell division in the cambium and, in combination with cytokinins in tissue culture 3.
Stimulates differentiation of phloem and xylem
Stimulates root initiation on stem cuttings and lateral root development in tissue culture 5.
Mediates the tropistic response of bending in response to gravity and light 6.
The auxin supply from the apical bud suppresses growth of lateral buds 7.
Delays leaf senescence
Can inhibit or promote (via ethylene stimulation) leaf and fruit abscission 9.
Can induce fruit setting and growth in some plants
Involved in assimilate movement toward auxin possibly by an effect on phloem transport 11.
Delays fruit ripening
Promotes flowering in Bromeliads
Stimulates growth of flower parts
Promotes (via ethylene production) femaleness in dioecious flowers 15.
Stimulates the production of ethylene at high concentrations Gibberellins: The Nature of Gibberellins
Unlike the classification of auxins which are classified on the basis of function, gibberellins are classified on the basis of structure as well as function. All gibberellins are derived from the ent-gibberellane skeleton. All gibberellins are acidic compounds and are therefore also called gibberellic acids (GA) with a different subscript to distinguish between...
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