Mechanisms to account for maintenance of the soluble methionine pool in transgenic Arabidopsis plants expressing antisense cystathionine gamma-synthase cDNA

To investigate the role of cystathionine gamma -synthase (CGS) in the regul ation of methionine synthesis Arabidopsis plants were transformed with a fu ll-length antisense CGS cDNA and transformants analysed. Plants that were h eterozygous for the transgene showed a 20-fold reduction of CCS activity th at was accompanied by severe growth retardation and morphological abnormali ties, from germination to flowering. Application of exogenous methionine to the transgenic lines restored normal growth. Surprisingly, transformed Ara bidopsis plants exhibited a modest decrease in methionine content (35% redu ction of the wild-type level) but a seven-fold decrease in the soluble pool of S-methylmethionine (SMM), a compound that plays a major role in storage and transport of reduced sulphur and labile methyl moieties. Several mecha nisms can account for the maintenance of the soluble pool of methionine. Fi rst, the observed 20-fold increase in O-phosphohomoserine, a substrate of C GS, could compensate for the depressed level of CGS polypeptide by increasi ng the net rate of catalysis supported by the remaining enzyme. Second, the transgenic plants exhibited a two-fold increased level of cystathionine be ta -lyase, the second enzyme in the methionine biosynthetic pathway. This i ndicates that enzymes other than CGS are subjected to a regulatory control by methionine or one of its metabolites. In addition to these mechanisms af fecting de novo methionine synthesis, the recruitment of SMM to produce met hionine may account for the small change of methionine levels in transgenic lines. (C) 2000 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.