Avraham, T., Plant Science Laboratory, Migal-Galilee Technology Center, PO Box 831, Kiryat Shmona 11016, Israel
Badani, H., Department of Agronomy and Natural Resources Department, Agricultural Research Organization, Volcani Center, Bet Dagan 50250, Israel
Galili, S., Department of Agronomy and Natural Resources Department, Agricultural Research Organization, Volcani Center, Bet Dagan 50250, Israel
Amir, R., Tel-Hai Academic College, Upper Galilee 10120, Israel

With the aim of increasing the methionine level in alfalfa (Medicago sativa L.) and thus improving its nutritional quality, we produced transgenic alfalfa plants that expressed the Arabidopsis cystathionine γ-synthase (AtCGS), the enzyme that controls the synthesis of the first intermediate metabolite in the methionine pathway. The AtCGS cDNA was driven by the Arabidopsis rubisco small subunit promoter to obtain expression in leaves. Thirty transgenic plants were examined for the transgene protein expression, and four lines with a high expression level were selected for further work. In these lines, the contents of methionine, S-methylmethionine (SMM), and methionine incorporated into the water-soluble protein fraction increased up to 32-fold, 19-fold, and 2.2-fold, respectively, compared with that in wild-type plants. Notably, in these four transgenic lines, the levels of free cysteine (the sulphur donor for methionine synthesis), glutathione (the cysteine storage and transport form), and protein-bound cysteine increased up to 2.6-fold, 5.5-fold, and 2.3-fold, respectively, relative to that in wild-type plants. As the transgenic alfalfa plants over-expressing AtCGS had significantly higher levels of both soluble and protein-bound methionine and cysteine, they may represent a model and target system for improving the nutritional quality of forage crops. © 2004 Blackwell Publishing Ltd.