SAM serves as a precursor of the plant hormone ethylene, implicated in the control of numerous developmental processes (Lieberman, 1979; Yang and Hoffman, 1984; Yang, 1989; Kende, 1993), and methionine adenosyltransferase (S-adenosyl-L-methionine synthase; SAM synthetase) [EC 2.5.1.6] expression is up-regulated under conditions inducing ethylene biosynthesis (see. e.g. Gomez-Gomez and Carrasco, 1998). SAM is converted to the amino acid 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase, which is then oxidized to ethylene by ACC oxidase (Kende, 1993):

The HCN generated from the catalytic action of ACC oxidase is detoxified by rapid metabolism to the amino acid B-cyanoalanine which can then be hydrated to asparagine or conjugated to gamma-glutamyl-B-cyanoalanine (Yang, 1989). 5'-methylthioadenosine is metabolized back to methionine by the methionine salvage pathway (Miyazaki and Yang, 1987).

ACC synthase may be regulated by its substrate SAM via SAM-induced inactivation. This mechanism may be responsible for the short half-life of this enzyme in vivo (Yang, 1989). ACC oxidase requires Fe²⁺ for catalytic activity, and in some systems is activated by CO₂ (Kende, 1993). The plant species Potamogeton pectinatus is unusual in that it lacks ACC oxidase and ethylene biosynthesis; this species is consequently rich in ACC (Summers et al, 1996). ACC synthase is encoded by a divergent multigene family, the members of which are differentially regulated in a tissue-specific manner during plant development (Bui and O'Neill, 1998).

ACC synthase is induced by a number of environmental stresses, including mechanical stress (wounding or mechanical impedance), drought, chilling, anoxia and hypoxia (Yang, 1989; Kende, 1993; He et al, 1996a). ACC oxidase is also induced by hypoxia and by mechanical impedance in maize roots (He et al, 1996a); however, with the combination of hypoxia plus mechanical impedence, ethylene formation and ACC synthase induction exhibit a greater synergism than ACC oxidase induction, cellulase induction and aerenchyma formation (He et al, 1996a). Aerenchyma formation accelerates the transfer of O₂ from the aerial tissues to the O₂-deficient tissues of the stem base and root of flooded plants (He et al, 1996b). Ethylene production in response to hypoxia is implicated in the signal transduction pathway leading to cell death and lysis in the root cortex resulting in aerenchyma formation in maize roots (He et al, 1996b; Drew, 1997; Pennell and Lamb, 1997). However, in Arabidopsis, developmental and molecular responses to mechanical stimulation do not require the ethylene response pathways controlled by ETR1 and EIN2 (Johnson et al, 1998).

Because ACC oxidase is oxygen-dependent, ACC can accumulate markedly in roots exposed to anoxia. In flooded tomato plants, the accumulated ACC is transported from the root to the shoot (via the xylem) where it is then rapidly oxidized to ethylene, inducing epinasty (Bradford and Yang, 1980; English et al, 1995) and adventitious root formation (Visser et al, 1996). The latter may in part involve an ethylene-mediated increase in the sensitivity of root-forming tissue to endogenous indole acetic acid (Visser et al, 1996).

Young et al (1997) conclude that ethylene is involved in triggering programmed cell death in developing maize endosperm and is responsible for the aberrant phenotype of sh2 kernels.

Trebitsh et al (1997) have identified an ACC synthase gene that is linked to the female (F) locus that enhances female sex expression in cucumber.

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He C-J, Morgan PW, Drew MC 1996b Transduction of an ethylene signal is required for cell death and lysis in the root cortex of maize during aerenchyma formation induced by hypoxia. Plant Physiol. 112: 463-472.

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Trebitsh T, Staub JE, O'Neill SD 1997 Identification of a 1-aminocyclopropane-1-carboxylic acid synthase gene linked to the female (F) locus that enhances female sex expression in cucumber. Plant Physiol. 113: 987-995.

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Young TE, Gallie DR, DeMason DA 1997 Ethylene-mediated programmed cell death during maize endosperm development of wild-type and shrunken2 genotypes. Plant Physiol. 115: 737-751.