Homoserine kinase and threonine synthase in methionine-overproducing soybean tissue cultures

To gain understanding of the regulation of methionine level in plants, we assayed homoserine kinase and threonine synthase in extracts of wild type and several methionine-overproducing soybean [Glycine max (L.) Merr.] callus lines. The specific activity of homoserine kinase was depressed by 45-73%, and that of threonine synthase by 26-43% in the high methionine lines. Cysteine inhibited threonine synthase in wild type and variant lines. Threonine synthase in two variant lines showed significantly less inhibition by cysteine and in one line was inhibited by threonine. Depressed threonine synthase activity may increase the availability of homoserine phosphate to the competing methionine biosynthetic pathway.