Nucleotide Sequence of a cDNA Clone Encoding 1-Aminocyclopropane-1-Carboxylate Synthase in Mustard (Brassica juncea [L.] Czern & Coss)

Ethylene, a gaseous plant hormone, is involved in regulation of various physiological responses during plant growth and development. These include seed germination, abscission, fruit ripening, and plant senescence (Yang and Hoffman, 1984). Plants also produce high levels of ethylene when they are under environmental stresses or pathogen attacks. In ethylene biosynthesis, the precursor Met is converted to SAM via the catalysis of the enzyme SAM synthetase. SAM is further converted to ACC by ACC synthase (EC 4.4.1.14), which is the rate-limiting step of ethylene production. This reaction can be inhibited by exogenous application of aminoethoxyvinylglycine or aminooxyacetic acid, both of which are inhibitors of pyridoxal phosphate, which is required in ACC synthase activity. The last step of ethylene biosynthesis, ACC-ethylene, is catalyzed by ACC oxidase (ethyleneforming enzyme) (Yang and Hoffman, 1984). ACC synthase has been purified to homogeneity from tomato fruits, and the corresponding genes have been isolated (Van Der Straeten et al., 1990), although the enzyme usually is present at low concentrations. To date, genes encoding ACC synthase have been cloned from severa1 plant species including monocots and dicots (Theologis, 1992). In tomato, at least six different genes for ACC synthase have been identified, indicating that the enzyme is encoded by a multigene family (Rottmann et al., 1991). Two of the genes have been shown to be differentially expressed in ripening fruits after wounding. In transgenic experiments, tomato plants expressing antisense ACC synthase RNA displayed a great reduction of ethylene production, concomitant with a delay of the fruit-ripening process (Oeller et al., 1991). We have previously shown the regulatory role of ethylene in shoot morphogenesis of recalcitrant Brassica species in vitro (Chi et al., 1991; Pua, 1993; Pua and Chi, 1993) and have also cloned a cDNA encoding ACC oxidase from mustard (Pua et al., 1992). We report in this study a cDNA clone encoding ACC synthase, designated pMACC, isolated from mustard (Brassica juncea). The cDNA contained one open reading frame encoding a