Occurrence of choline and glycine betaine uptake and metabolism in the family Rhizobiaceae and their roles in osmoprotection

The role of glycine betaine and choline in osmoprotection of various Rhizob ium, Sinorhizobium, Mesorhizobium, Agrobacterium, and Bradyrhizobium refere nce strains which display a large variation in salt tolerance was investiga ted. When externally provided, both compounds enhanced the growth of Rhizob ium tropici, Sinorhizobium meliloti, Sinorhizobium fredii, Rhizobium galega e, Agrobacterium tumefaciens, Mesorhizobium loti, and Mesorhizobium huakuii , demonstrating their utilization as osmoprotectants. However, both compoun ds were inefficient for the most salt-sensitive strains, such as Rhizobium leguminosarum (all biovars), Agrobacterium rhizogenes, Rhizobium etli, and Bradyrhizobium japonicum. Except for B. japonicum, all strains exhibit tran sport activity for glycine betaine and choline, When the medium osmolarity was raised, choline uptake activity was inhibited, whereas glycine betaine uptake was either increased in R. leguminosarum and S, meliloti or, more su rprisingly, reduced in R. tropici, S. fredii, and M. loti. The transport of glycine betaine was increased by growing the cells in the presence of the substrate. With the exception of B. japonicum, all strains were able to use glycine betaine and choline as sole carbon and nitrogen sources. This cata bolic function, reported for only a few soil bacteria, could increase compe titiveness of rhizobial species in the rhizosphere. Choline dehydrogenase a nd betaine-aldehyde dehydrogenase activities were present in the cells of a ll strains with the exception of M. huakuii and B,japonicum. The main physi ological role of glycine betaine in the family Rhizobiaceae seems to be as an energy source, while its contribution to osmoprotection is restricted to certain strains.