Mutation in the threonine synthase gene results in an over-accumulation ofsoluble methionine in Arabidopsis

In higher plants, O-phosphohomoserine (OPH) represents a branch point betwe en the methionine (Met) and threonine (Thr) biosynthetic pathways. It is be lieved that the enzymes Thr synthase (TS) and cystathionine gamma-synthase (CGS) actively compete for the OPH substrate for Thr and Met biosynthesis, respectively. We have isolated a mutant of Arabidopsis, designated mto2-1, that over-accumulates soluble Met 22-fold and contains markedly reduced lev els of soluble Thr in young rosettes. The mto2-1 mutant carries a single ba se pair mutation within the gene encoding TS, resulting in a leucine-204 to arginine change. Accumulation of TS mRNA and protein was normal in young r osettes of mto2-1, whereas functional complementation analysis of an Escher ichia coli thrC mutation suggested that the ability of mto2-1 TS to synthes ize Thr is impaired. We concluded that the mutation within the TS gene is r esponsible for the mto2-1 phenotype, resulting in decreased Thr biosynthesi s and a channeling of OPH to Met biosynthesis in young rosettes. Analysis o f the mto2-1 mutant suggested that, in vivo, the feedback regulation of CGS is not sufficient alone for the control of Met biosynthesis in young roset tes and is dependent on TS activity. In addition, developmental analysis of soluble Met and Thr concentrations indicated that the accumulation of thes e amino acids is regulated in a temporal and spatial manner.