Yael Hacham, Jong Yoon, Ifat Matityahu, Alex Kaplan, Lilach Kachan, David Oliver and Rachel Amir
Chairperson and Research Scientist, Laboratory of Plant Science, Migal Galilee Technology Center, P.O. Box 831, Kiryat Shmona 11016, Israel
Cysteine (Cys), the first organic sulfur-containing metabolite, serves as a precursor for the synthesis of glutathione and methionine (Met), two metabolites that are central to plant growth and survival. Glutathione plays a crucial role in the defence against a wide variety of environmental stresses, while Met is a protein constituent, and through its first metabolite, S-adenosylMet (SAM), regulates essential processes required for plant growth. To reveal the relations between glutathione and Met, we used tobacco plants overexpressing the regulatory enzyme of Met biosynthesis pathway, cystathionine γ-snthase (CGS), and those overexpressing the yeast gene encoding a feedback-insensitive O-acetylserine (thiol)lyase (OASTL) in the plastids and in the cytosol that regulate the levels of Cys and glutathione. We crossed between the two transgenic lines to determine that the level of Met can significantly increase in plants overexpressing the plastidic OASTL with AtCGS, accompanied by a reduction in glutathione. The results strongly suggest that the flux towards Met is relatively high, and thus Met can be considered as an intermediate metabolite in the pathways leading to its various associated metabolites. In addition, flux and metabolic profiling analyses indicated the existence of metabolic competition between the biosynthesis pathways of Met and glutathione on their common precursor, Cys, and that this competition is more crucial under oxidative conditions when more Cys is required for the synthesis of glutathione. Plants overexpressing AtCGS with or without the yeast enzyme were significantly more sensitive to oxidative stress, indicating the reason why the levels of Met remained low during the evolution.