Physiological Roles of Betaines in Sinorhizobium meliloti and other Members of the Rhizobiaceae Subjected to Salt Stress

Sinorhizobium meliloti is able to survive osmotically stressful environments by the intracellular accumulation of compatible solutes, which restore the osmotic balance between the cell and the environment. Among these solutes, glycine betaine (the N-trimethyl derivative of glycine) has been shown to be a very efficient osmoprotective compound which strongly stimulates the growth rate of this bacterium in high-salt medium (Le Rudulier, Bernard, 1986). Unlike enteric bacteria, S. meliloti also catabolizes glycine betaine and uses it both as a carbon and a nitrogen source for growth. Glycine betaine either can be taken up directly from the environment by specific transport systems or synthesized from choline-O-sulfate or choline (Osteras et al., 1998; Smith et al., 1988). The osmoadaptative responses of other rhizobial species have not been thoroughly studied in comparison to those of S. meliloti. However, we have recently analyzed choline and glycine betaine uptake capacities and investigated the occurrence of glycine betaine biosynthesis pathway in various Rhizobiaceae strains which display a large variation in salt tolerance (Boncompagni et al., 1999).