OSMOADAPTATION BY RHIZOSPHERE BACTERIA

The osmolality of rhizosphere soil water is expected to be elevated in relation to bulk-soil water osmolality as a result of the exclusion o f solutes by plant roots during water uptake, the release of plant roo t exudates, and the production of exopolymers by plant roots and rhizo bacteria. In contrast, the osmolality of water within highly hydrated bulk soil is low (less than 50 Osm/kg); thus the ability to adapt to e levated osmolality is likely to be important for successful rhizospher e colonization by rhizobacteria. The present review focuses on the osm oadaptive responses of three gram-negative rhizobacterial genera: Rhiz obium, Azospirillum, and Pseudomonas. Specifically, we examine the com patible solutes and osmoprotectants utilized by various species within these genera. The adaptation of rhizobacteria to hypoosmotic environm ents is also examined in the present review. In particular, we focus o n the biosynthesis and accumulation of periplasmic glucans by rhizobac teria. Finally, the relationship between rhizobacterial osmoadaptation and selected plant-microbe interactions is considered.