Ethylene in Plant Growth

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Ethylene inhibits cell division, DNA synthesis, and growth in the meristems of roots, shoots, and axillary buds, without influencing RNA synthesis. Apical dominance often is broken when ethylene is removed, apparently because the gas inhibits polar auxin transport irreversibly, thereby reducing the shoot's auxin content just as if the apex had been removed. A similar mechanism may underly ethylene-induced release from dormancy of buds, tubers, root initials, and seeds. Often ethylene inhibits cell expansion within 15 min, but delays differentiation so that previously expanding cells eventually grow to enormous size. These cells grow isodiametrically rather than longitudinally because their newly deposited cellulose microfibrils are laid down longitudinally rather than radially. Tropistic responses are inhibited when ethylene reversibly and rapidly prevents lateral auxin transport. In most of these cases, as well as certain other instances, ethylene action is mimicked by application of an auxin, since auxins induce ethylene formation. Regulation by ethylene extends to abscission, to flower formation and fading, and to fruit growth and ripening. Production of ethylene is controlled by auxin and by red light, auxin acting to induce a labile enzyme needed for ethylene synthesis and red light to repress ethylene production. Numerous cases in which a response to red light requires an intervening step dependent upon inhibition of ethylene production have been identified. Ethylene action requires noncovalent binding of the gas to a metal-containing receptor having limited access, and produces no lasting product. The action is competitively inhibited by CO2, and requires O2. Ethylene is biosynthesized from carbons 3 and 4 of methionine, apparently by a copper-containing enzyme in a reaction dependent upon an oxygen-requiring step with a Km = 0.2% O2. The oxidative step appears to be preceded by an energy-requiring step subsequent to methionine formation.

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