Osmoadaptation by Rhizosphere Bacteria

Abstract

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.

Effect of exo-polysaccharides producing bacterial inoculation on growth of roots of wheat (Triticum aestivum L.) plants grown in a salt-affected soil

“…A higher growth of roots than shoot of the inoculated plants and a highly significant positive correlation of all the root growth parameters (except number of seminal roots) with dry mass and water insoluble soil saccharides content of the RS (Table 4) indicated that influence of the RS factors on growth of roots was more prominent and stronger than a decrease in Na + uptake by inoculated wheat plants. Since the EPS-producing bacteria were isolated from the saltaffected soils and were adapted to the salts stress (Miller, et al, 1996), their inherent salt tolerant capability helped the plants to withstand initial effect of osmotic stress (Munns, 2002). Moreover, a higher EPS-producing bacterial population harboured initially on roots of the inoculated than un-inoculated plants was enable to stimulate and enhance the extent of the root exudates (Tisdall, 1994;Miller, et al, 1996;Wittenmayer and Merbach, 2005).…”

“…Since the EPS-producing bacteria were isolated from the saltaffected soils and were adapted to the salts stress (Miller, et al, 1996), their inherent salt tolerant capability helped the plants to withstand initial effect of osmotic stress (Munns, 2002). Moreover, a higher EPS-producing bacterial population harboured initially on roots of the inoculated than un-inoculated plants was enable to stimulate and enhance the extent of the root exudates (Tisdall, 1994;Miller, et al, 1996;Wittenmayer and Merbach, 2005). Thus in consequence to intensive release of the plant root exudates in the rhizosphere, the EPS-producing bacterial population was increased further and the extent of EPS synthesis was fostered.…”

“…inoculation, an improved biomass yield of shoot portion of the inoculated wheat plants resulted in a higher uptake of water from the soil which decreased the negative effects of the salts stress on growth of the plants. No doubt an improved RS permeability as a result of a higher soil aggregation around roots (Alami, et al 2000), a decreased soluble salts content of the rhizosphere soil resulted from binding of the excessive soluble cations into the rhizosheath-root complex through EPS-cation-soil bridging (Marshall, 1975;Morel, et al, 1991) and a positive water potential of the bacterial EPS (Miller, et al, 1996) content of the RS could all support to the plants to draw a greater quantity of water from the rhizosphere soil. However, as the increase in growth and biomass yield of the plants grown in the stressed environments follow the decrease in stress on growth (Munns, 2002), a higher soil water uptake by a well grown shoot portion of the plants could not be solely responsible for improved growth of the wheat plants inoculated with the EPS-producing bacterial isolates.…”

Stenotrophomonas rhizophila sp. nov., a novel plant-associated bacterium with antifungal properties.

“…S. maltophilia isolates could grow well at 5 % NaCl, whereas for members of the B1 group, 4n5 % was the highest salinity allowing growth. Tolerance to high and changing salinities is important for adaptation in both microenvironments (the rhizosphere and the human body) (Miller & Wood, 1996). All strains of Stenotrophomonas synthesized trehalose, but only strains of the B1 group accumulated glucosylglycerol as an additional substance.…”

The two-component system PrlS/PrlR of Brucella melitensis is required for persistence in mice and appears to respond to ionic strength

“…Osmoadaptation also induces remodelling of the cell envelope and cell surface structures i.e. membrane or periplasmic protein composition, lipid composition, periplasmic glucan level and EPS synthesis (Miller & Wood, 1996). Ultimately, some of these modifications could lead to biofilm (or aggregate) formation (Karatan & Watnick, 2009).…”