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Plant Hormones- Cytokinins, Ethylene & Gibberellins

Cytokinin

A group of plant hormones (phytohormones) that, together with other plant hormones, induces plant growth and development. Since the isolation of the first naturally occurring cytokinin, zeatin, from corn seeds in 1961, more than 25 different cytokinins have been isolated from plants.


Oversynthesis of cytokinins in plant tissue causes abnormal growth: crown gall tumor disease caused by the bacterium Agrobacterium tumefaciens is an example of excessive production of cytokinins in local tumor tissue. Tissue from crown gall tumors can grow on a simple medium lacking plant hormones because the tumor tissue overproduces both cytokinin and auxin. Roots have been shown to be the major site of cytokinin biosynthesis, but stems and leaves are also capable of synthesizing cytokinins. It is possible that all actively dividing cells are capable of cytokinin biosynthesis.


Cytokinins exhibit a wide range of physiological effects when applied externally to plant tissues, organs, and whole plants. Exogenous applications of this hormone induce cell division in tissue culture in the presence of auxin. The formation of roots or shoots depends on the relative concentrations of auxin and cytokinin added to the culture medium. High auxin and low cytokinin concentrations lead to root formation, while low auxin and high cytokinin concentrations give shoots. Tissue culture techniques have been employed by plant biotechnologists to grow genetically engineered plant cells into whole plants. Cytokinins appear to be necessary for the correlated phenomena of mitosis and nucleic acid synthesis. Cytokinins delay the aging process in detached leaves by slowing the loss of chlorophyll. Cytokinin effects also include breaking of dormancy, promotion of seed germination, stimulation and nutrient mobilization, enhanced anthocyanin and flavanoid synthesis, increased resistance to disease, and stimulation of the opening of stomata.


Ethylene

Ethylene acts physiologically as a hormone in plants. 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. Its biosynthesis starts from methionine with 1-aminocyclopropane-1-carboxylic acid (ACC) as a key intermediate.


List of Plant Responses to Ethylene

  • Seedling triple response, thickening and shortening of hypocotyl with pronounced apical hook. This thought to be a seedlings reaction to an obstacle in the soil such a stone, allowing it to push past the obstruction.
  • In pollination, when the pollen reaches the stigma, the precursor of the ethylene, ACC, is secreted to the petal, the ACC releases ethylene with ACC oxidase.
  • Stimulates leaf and flower senescence.
  • Stimulates senescence of mature xylem cells in preparation for plant use.
  • Inhibits shoot growth except in some habitually flooded plants like rice.
  • Induces leaf abscission.
  • Induces seed germination.
  • Induces root hair growth - increasing the efficiency of water and mineral absorption.
  • Induces the growth of adventitious roots during flooding.
  • Stimulates epinasty - leaf petiole grows out, leaf hangs down and curls into itself.
  • Stimulates fruit ripening.
  • Induces a climacteric rise in respiration in some fruit which causes a release of additional ethylene. This can be the one bad apple in a barrel spoiling the rest phenomenon.
  • Affects neighboring individuals.
  • Disease/wounding resistance.
  • Triple response when applied to seedlings - stem elongation slows, the stem thickens, and curvature causes the stem to start growing horizontally. This strategy is thought to allow a seedling grow around an obstacle.
  • Inhibits stem growth outside of seedling stage.
  • Stimulates stem and cell broadening and lateral branch growth also outside of seedling stage.
  • Synthesis is stimulated by auxin and maybe cytokinin as well.
  • Ethylene levels are decreased by light.
  • The flooding of roots stimulates the production of ACC which travels through the xylem to the stem and leaves where it is converted to the gas.
  • Interference with auxin transport (with high auxin concentrations).
  • Inhibits stomatal closing except in some water plants or habitually flooded ones such as some rice varieties, where the opposite occurs (conserving CO2 and O2)
  • Where ethylene induces stomatal closing, it also induces stem elongation
  • Induces flowering in pineapples.

Gibberellin


GA1

Gibberellins (GAs) are plant hormones involved in promotion of stem elongation, mobilization of food reserves in seeds and other processes. Its absence results in the dwarfism of some plant varieties. Chemically all known gibberellins are diterpenoid acids that are synthesized by the terpenoid pathway in plastids and then modified in the endoplasmic reticulum and cytosol until they reach their biologically-active form. All gibberellins are derived from the ent-gibberellane skeleton, but are synthesised via ent-kauren. The gibberellins are named GA1....GAn in order of discovery. Gibberellic acid, which was the first gibberellin to be structurally characterised, is GA3. There are currently 136 GAs identified from plants, fungi and bacteria.


Much of our knowledge of the biosynthesis and molecular mechanisms of gibberellins comes from research on their role in triggering a-amylase release by the aleurone layer in seed germination. Gibberellin was first isolated in 1935 by Japanese scientist, Teijiro Yabuta (1888-1977) from the fungus Gibberella fujikuroi.


There There are a number of applications of plant hormones in agriculture, horticulture, and biotechnology. Synthetic auxins are used as weed killers. Auxins are also used to counteract the effects of hormones that promote the dropping of fruit from trees. Gibberellins are used extensively to increase the size of seedless grapes: when applied at the appropriate time and with the proper concentration, gibberellins cause fruits to elongate so that they are less tightly packed and less susceptible to fungal infections. Gibberellins are also used by some breweries to increase the rate of malting because they enhance starch digestion. They have also been sprayed on fruits and leaves of navel orange trees to prevent several rind disorders that appear during storage. They are used commercially to increase sugarcane growth and sugar yields. Cytokinins and auxins are used in plant cell culture, particularly in cultivating genetically engineered plants. The ability of cytokinins to retard senescence also applies to certain cut flowers and fresh vegetables. Ethylene has been used widely in promoting pineapple flowering; flowering occurs more rapidly and mature fruits appear uniformly, so that a one-harvest mechanical operation is possible. Because carbon dioxide in high concentrations inhibits ethylene production, it is often used to prevent overripening of picked fruits. Ethylene is also used for accelerating fruit ripening.

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