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Auxins
The plant hormone stimulates cell elongation. It stimulates the Wall Loosening Factors, for example, elastins, to loosen the cell walls. If gibberellins are also present, the effect is stronger. It also stimulates cell division if cytokinins are present. When auxin and cytokinin was applied to callus, rooting can be generated if the auxin concentration is higher than cytokinin concentration while xylem tissues can be generated when the auxin concentration is equal to the cytokinins.
It participates in phototropism, geotropism, hydrotropism and other developmental changes. The uneven distribution of auxin, due to environmental cues (for example, unidirectional light and gravity force), results in uneven plant tissue growth.
It also induces sugar and mineral accumulation at the site of application.
It induces formation and organization of phloem and xylem. When the plant is wounded, the auxin can induce the Cell differentiation and regeneration of the vascular tissues.
Auxin induces new root formation by breaking root apical dominance induced by cytokinins
It induces shoot apical dominance
In low concentrations, auxin can inhibit ethylene formation and transport of precursor in plants
Auxin delays fruit senescence
Auxin plays a minor role in the initiation of flowering.
Gibberellins :
* Stimulates shoot and cell elongation
* Delays senescence of leaves
* Produces seed germination
* Breaking of dormancy
* Stimulates bolting and flowering in biennials
* Regulates production of hydrolytic enzymes for digesting starches
* Inhibits root growth
* Antagonist promotes root growth and GA reverses this
* Promotes root initiation in low concentration in pea cuttings
* Inhibits CK bud growth on calluses
* Inhibits bud formation
* Inhibits leaf formation
* Used to increase fruit size, increase cluster size (in grapes), delay ripening of citrus fruits, speed up flowering of strawberries, and stimulate starch break down in barley (for beer making).
* Used at high concentration to promote growth of male flowers on female plants; allows production of female-only seeds
Cytokinins :
Cytokinins generally promote shoot development and inhibit root development, although they are necessary for cell division in both shoot and root apical meristems.
* Promotes chlorophyll production and leaf unrolling
* Promotes photosynthesis
* Stimulates cell broadening
* Promotes shoot formation
* Promotes the unloading of sugar from phloem
* Causes the outgrowth of secondary shoot buds ? breaks shoot apical dominance/lateral bud development
* Delays leaf senescence
* Stimulates cell division with auxin
* Participates in morphogenesis
* Induces creation of xylem and phloem
* Inhibits C4 Photosynthesis
* Promotes stomatal opening (theoretical)
* Directly induces GA/BA at high levels (theoretical)
* Stimulates the rate of metabolism of cells in the shoot (that are not at their peak metabolism rates) in response to an increase in the levels minerals and water (theoretical)
* Inhibits root growth (theoretical)
Abscisic acid :
# Stimulates stomatal closure, in order to reduce transpiration and prevent water loss.
# Inhibits fruit ripening
# Encourages seed dormancy by inhibiting cell growth ? inhibits seed germination
# Inhibits the uptake of Kinetin
# Activates the pathogen resistance response defense
# Induces senescence in already-damaged cells and their proximate neighbours
# Quickly puts a plant, organ, tissue or individual cell in a defensive posture (whatever this entails) in response to rapidly-developing nutrient or environmental stress that threatens their survival
# Decreases metabolism in response to a newly-developing deficiency of nutrient or adverse environmental condition, such that the condition becomes survivable at the new lower level of metabolism
# Possibly induces cell dormancy or senescence by a climactic increase or sustained level, stimulating the synthesis of GA and/or Ethylene
Ethylene :
It exists as a gas and acts at trace levels throughout the life of the plant by stimulating or regulating the ripening of fruit, the opening of flowers, and the abscission (or shedding) of leaves.
Other plant responses:
* Stimulates leaf and flower senescence
* Induces leaf abscission mainly in older leaves.
* Induces seed germination
* Induces root hair growth ? this increases the efficiency of water and mineral absorption
* Stimulates epinasty ? leaf petiole grows out, leaf hangs down and curls into itself
* Stimulates fruit ripening
* Induces the growth of adventitious roots during flooding
* Affects neighboring individuals
* Disease/wounding resistance
* Triple response when applied to seedlings ? root ? and shoot growth inhibition and pronounced hypocotyl hook bending
* Inhibits stem swelling or Stimulates cell broadening and lateral root growth (some sources are in disagreement)
* Interference with auxin transport (with high auxin concentrations)
* Induces flowering in pineapples
Florigen :
Florigen or Flowering hormone are the terms used for the hypothesized hormone-like molecules that control and/or trigger flowering in plants. Its precise identity and mechanism are not known; only its function.
The flowering hormone has eluded scientists for over sixty years, as the flowering response has been found to be increasingly complex.A possible hypothesis is that florigen does not exist; rather, a particular ratio of other hormones must be achieved for the plant to flower.
However, recent experiments suggest that florigen does exist. Its existence is substantiated by experiments that indicate that the substance that triggers blooming is produced or activated in the leaves of the plant, and must be given time to pass out of the leaves before the plant can flower.
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