Introduction

Ethylene is the plant growth regulator that controls ripening and senescence in plants. It is produced from methionine via the formation of S-adenosyl-l-methionine, which, in turn, forms the non-protein amino acid, 1-aminocyclopropane-1-carboxylic acid (ACC) (Fig. 1). When ACC was first recognized as the immediate substrate for ethylene formation, it became apparent that all kinds of plant tissues had an active and constitutive enzyme capable of catalysing the conversion of ACC to ethylene, as high rates of ethylene production were observed when plant tissues were supplied with ACC (Cameron et al., 1979). The rate of ethylene production when tissues were fed ACC was always far greater than when they were fed methionine under the same conditions (Yang & Hoffman, 1984). During climacteric fruit ripening and flower senescence an initially low ACC oxidase activity increased in line with the rise of ACC synthesis and ethylene production (Yang & Hoffman, 1984). However, despite the substantial ACC oxidase activity that was readily observed with plant tissues, in vitro activity could not be detected. When tissues were homogenized, methionine was readily converted enzymatically to ACC, but there was no ethylene formation, and ACC supplied to tissue homogenates was not converted to ethylene. These observations led to the notion that ethylene formation required membrane integrity (Yang & Hoffman, 1984). This view was supported by the sensitivity of ACC oxidase to osmotic shock, detergents and protonophores, and by the discontinuities in the Arrhenius plots for ethylene production (see Yang & Hoffman, 1984; Kende, 1993).