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 of solutes by plant roots during water uptake, the release of plant root exudates, and the production of exopolymers by plant roots and rhizobacteria. In contrast, the osmolality of water within highly hydrated bulk soil is low (less than 50 Osm/kg); thus the ability to adapt to elevated osmolality is likely to be important for successful rhizosphere colonization by rhizobacteria. The present review focuses on the osmoadaptive responses of three gram-negative rhizobacterial genera: Rhizobium, Azospirillum, and Pseudomonas. Specifically, we examine the compatible solutes and osmoprotectants utilized by various species within these genera. The adaptation of rhizobacteria to hypoosmotic environments is also examined in the present review. In particular, we focus on the biosynthesis and accumulation of periplasmic glucans by rhizobacteria. Finally, the relationship between rhizobacterial osmoadaptation and selected plant-microbe interactions is considered.