Galactinol synthase (GS): increased enzyme activity and levels of mRNA due to cold and desiccation

Abstract

Galactinol synthase (GS) catalyzes the first committed step in the biosynthesis of the raffinose family oligosaccharides (RFO) and could play a key regulatory role in the carbon partitioning between sucrose and RFO in the developing seed. RFO have been proposed to be osmoprotectants when plants are exposed to environmental water deficit stresses, such as cold and desiccation, and during the developmentally induced desiccation in the late maturation stage of seed development. We present the following evidences obtained from three plant species in support of these proposed roles for GS and RFO: (a) GS activity increased in kidney bean seeds upon exposure of plants to cold; (b) GS mRNA levels increased significantly upon cold exposure in the vegetative tissues of Arabidopsis thaliana, a chill resistant species, with the transcripts diminishing upon re-exposure to room temperature. A smaller increase due to cold was also observed in the siliques with the transcripts likewise disappearing upon re-exposure of the plants to room temperature; (c) we established by protein sequence comparison that a previously unidentified gene belonging to a group of ABA-independent, desiccation stress inducible genes isolated from rice with acquired chill resistance, encodes the rice homologue of the GS gene.