Co-expression of ApGSMT and ApDMT promotes biosynthesis of glycine betaine in rice (Oryza sativa L.) and enhances salt and cold tolerance

Glycine betaine (GB) is an osmoprotectant that protects plants from abiotic stress. GB is biosynthesized by choline dehydrogenation/oxidation and glycine methylation. The latter pathway involves glycine as a substrate and produces GB via successive methylation of glycine by two different N-methyltransferase enzymes; glycine sarcosine methyltransferase (GSMT), and sarcosine dimethylglycine methyltransferase (SDMT). In this study, Agrobacterium-mediated gene transformation was used to produce transgenic rice (Oryza sativa L., cv. Nipponbare) plants containing ApGSMT and ApDMT genes isolated from Aphanothece halophytica. The co-expression of both ApGSMT and ApDMT transgenes resulted in a significant increase of GB biosynthesis and enhanced tolerance to salt and cold stresses in the transgenic rice plants. These results demonstrate the potential of bioengineering for glycine N-methyltransferase genes in crop plants tolerance to abiotic stress.