Osmoregulation in Rhizobium meliloti: Mechanism and control by other environmental signals

Abstract

Bacteria commonly adapt to increases in the osmolarity of the environment by the intracellular accumulation of small organic solutes (osmolytes) that function by restoring turgor. We have investigated the mechanism of osmotic regulation in Rhizobium meliloti, the root nodule symbiont of alfalfa. As a soil microbe, this organism is subjected to variable osmolarity and a number of other environmental conditions to which it must respond appropriately. Natural abundance I3C nuclear magnetic resonance spectroscopy was used to identify the osmolytes accumulated by osmotically stressed R. meliloti. The complement of osmolytes and their concentrations were found to depend on the level of osmotic stress, growth phase of the culture, carbon source, and the presence of osmolytes in the growth medium. The osmolytes accumulated by 22 different bacterial strains are presented and compared to those of R . meliloti 102F34. o

A prominent role for glucosylglycerol in the adaptation of Pseudomonas mendocina SKB70 to osmotic stress

“…However, NAGGN is not synthesized by the relatively salt-sensitive strain P. stutzeri JM300 (this study) or by the extremely salt-tolerant species P. halosaccharolytica (which tolerates 3.3 M NaCl) (17) and P. halophila, which rely on ectoines as their major endogenous osmolytes (38). NAGGN was also detected as a major osmolyte in all strains of R. meliloti screened thus far but was not accumulated in other rhizobia (40). Minor amounts of this dipeptide were also accumulated in slightly to moderately halophilic phototrophic bacteria such as Rhodopseudomonas marina, Chromatium purpuratum, Chromatium salexigens, and Thiocapsa halophila, which were isolated from marine salterns (6,38).…”

“…Furthermore, glycine betaine suppressed the accumulation of all three endogenous osmolytes ( Table 2). This is a common phenomenon that has been observed in several other bacterial species on which betaine also confers enhanced osmotic tolerance (8,9,40,41,42). This ability to modulate the accumulation of endogenous osmolytes indicates a clear bacterial preference for glycine betaine which is highly compatible with metabolic functions at high cytoplasmic concentrations (8,11,35).…”

“…Moreover, the biosynthesis of galactosylglycerols (floridoside and isofloridoside) in eukaryotic algae involves multiple phosphorylation and dephosphorylation reactions which are tightly regulated and controlled by a variety of environmental factors (19,22). Considering these data and the probable overlap in the biosynthetic pathways for galactosylglycerols and glucosylglycerol, we speculate that the inability of P. mendocina SKB70 to accumulate glucosylglycerol at 0.5 M K2HPO4 (Table 1) (12); (ii) desiccation, heat shock, and carbon starvation in yeasts (44,46); and (iii) high osmolarity and transition to stationary phase in Escherichia coli and R. meliloti (16,40). The absence of trehalose in P. mendocina SKB70 grown to late stationary phase in 0.8 M NaCl and the lack of correlation between physiological age and trehalose levels in cultures grown in 0.7 M K2S04, 0.7 M Na2SO4, or 0.5 M K2HPO4 suggest that the accumulation of trehalose is not developmentally regulated but, rather, may be triggered by high concentrations of sulfate and phosphate ions in the medium.…”

“…Trehalose was also observed in cultures stressed with 0.7 M Na2SO4 and 0.7 M K2SO4; however, the glucosylglycerol content remained high in cultures stressed with Na2SO4 and was reduced by only 30% in cultures stressed with K2SO4. Since it has been shown that trehalose accumulation is osmoregulated and/or is growth phase dependent in other bacteria and yeasts (16,40,44) Role of glycine betaine and proline in osmoregulation of P. mendocina SKB70. In a recent review, Csonka (7) reports that both proline and glycine betaine alleviate growth inhibition of an unspecified strain of P. mendocina at high osmolarity.…”

Biogenic Amines in Rhizobia and Legume Root Nodules

“…Generally, the growth of B. japonicum is severely inhibited in media containing 100 to 200 mM NaCl, whereas Rhizobium species are often capable of growing in a 400-mM NaCl solution and sometimes grow in concentrations approaching that of seawater (68). Salt tolerance in microorganisms is closely related to the intra-cellular accumulation of organic solutes called osmolytes such as glutamate (6,42,141,175), trehalose (23,141), betaines (5), and dipeptide (140), although it also relates to extra-cellular polysaccharide production in certain rhizobia (98). The accumulation of osmolytes is thought to counteract the dehydrating effect of low water activity (11).…”