Study on the pathway of glycinebetaine biosynthesis in a monocotyledonous grass, Aneurolepidium chinense, grown in semiarid area

Abstract

In some higher plants, glycinebetaine is known to be synthesized and accumulated in cells in response to salt stress or drought stress. Glycinebetaine is a compatible solute and acts as an excellent osmoprotectant. High levels of glycinebetaine are present in leaves of diverse families of dicotyledons and of some monocotyledons. The pathway of glycinebetaine biosynthesis in higher plants is as follows: Choline ? Betaine aldehyde ? Glycinebetaine. The first step is catalyzed by choline monooxygenase (CMO), being recently purified and characterized. The final step is catalyzed by betaine aldehyde dehydrogenase (BADH), which has been well characterized. Existence of BADH isozymes was suggested in spinach, mangrove, sorghum and barleys, and we reported that all those monocotyledonous plants have BADHs with a C-terminal tripeptide SKL that is known to be a signal for targeting preproteins to microbodies. We cloned and characterized a new type of sheep grass (Aneurolepidium chinense) BADH gene more homologous with dicotyledonous BADHs. Differential expression pattern was observed between the two types of sheep grass BADH genes under various stresses. Although the SKL type of BADH gene was inducible in sheep grass leaves, the new type of BADH gene was expressed constitutively under both heat and salt stresses. We also examine the localization of the new type of BADH protein. Furthermore we examine, whether or not sheep grass has chloroplastic CMO to protect photosynthetic machineries under heat or salt stresses.