Raffinose and stachyose accumulation, galactinol synthase expression, and winter injury of contrasting alfalfa germplasms

Large differences in winter hardiness exist among alfalfa (Medicago sativa L.) cultivars, but the physiological and molecular bases for these differences are not understood. Our objective was to determine how raffinose family oligosaccharide (RFO) accumulation and steady state mRNA levels for galactinol synthase (GaS) in roots relate to genetic variation in alfalfa winter survival. A GaS cDNA was isolated that possesses over 70% identity with GaS clones from other plant species. Induction of GaS transcripts in crowns of winter hardy alfalfa cultivars occurred within 8 h of exposure to 2degreesC, and was intensified by exposing plants to -2degreesC for 2 wk. Galactinol synthase transcripts increased in November in crown and root tissues of winter hardy alfalfa plants. This increase was accompanied by large increases in root RFO concentrations between October and December. A close positive association between RFO accumulation in roots in December and genetic differences in winter survival was observed in these alfalfa populations. Although roots and crowns of nondormant alfalfa cultivars accumulated both Gas transcripts and RFO, accumulation was delayed until December and these cultivars did not survive winter. Understanding the mechanisms regulating GaS gene expression and subsequent RFO accumulation in roots and crowns provides opportunity to genetically improve alfalfa winter hardiness.