Roles of sugar alcohols in osmotic stress adaptation. Replacement of glycerol by mannitol and sorbitol in yeast

For many organisms there is a correlation between increases of metabolites and osmotic stress tolerance, but the mechanisms that cause this protection are not clear. To understand the role of polyols, genes for bacterial mann itol-l-P dehydrogenase and apple sorbitol-6-P dehydrogenase were introduced into a Sacchromyces cerevisiae mutant deficient in glycerol synthesis. Sor bitol and mannitol provided some protection, but less than that generated b y a similar concentration of glycerol generated by glycerol-3-P dehydrogena se (GPD1). Reduced protection by polyols suggested that glycerol had specif ic functions for which mannitol and sorbitol could not substitute, and that the absolute amount of the accumulating osmoticum might not be crucial. Th e retention of glycerol and mannitol/sorbitol, respectively, was a major di fference. During salt stress, cells retained more of the six-carbon polyols than glycerol. We suggest that the loss of >98% of the glycerol synthesize d could provide a safety valve that dissipates reducing power, while a simi lar high intracellular concentration of retained polyols would be less prot ective. To understand the role of glycerol in salt tolerance, salt-tolerant suppressor mutants were isolated from the glycerol-deficient strain. One m utant, sr13, partially suppressed the salt-sensitive phenotype of the glyce rol-deficient line, probably due to a doubling of [Kf] accumulating during stress. We compare these results to the "osmotic adjustment" concept typica lly applied to accumulating metabolites in plants. The accumulation of poly ols may have dual functions: facilitating osmotic adjustment and supporting redox control.