Plant Hormones
Essay title: Plant Hormones
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.
Hormone
Source
Action
Auxins
apical meristem (only moves down),
embryo of seed,
young leaves
Control of cell elongation
apical dominance (prevents lateral buds)
prevents abscission
continued growth of fruit
cell division in vascular and cork cambium
–formation of lateral roots from pericycle
–formation of adventitious roots from cuttings
Gibberellins
Roots and young leaves
Cell (stem) elongation (works in stems and leaves, but not roots)
breaking seed/bud dormancy
stimulating fruit set
Cytokinins
roots, embryos, fruits
actively growing
Promote cell division
–signal axillary/lateral bud growth
–prevent leaf abscission
chloroplast development
breaking dormancy in some seeds
enhance flowering
promote fruit development
Abscissic Acid
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
closes stomata
Ethylene
tissues of ripening fruit,
nodes of stems,
senescent leaves, flowers
Growth inhibitor
fruit ripening
leaf abscission
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
Stimulates differentiation of phloem and xylem
Stimulates root initiation on stem cuttings and lateral root development in tissue culture
Mediates the tropistic response of bending in response to gravity and light
The auxin supply from the apical bud suppresses growth of lateral buds
Delays leaf senescence
Can inhibit or promote (via ethylene stimulation) leaf and