Growth and Nodulation Responses of Rhizobium meliloti to Water Stress Induced by Permeating and Nonpermeating Solutes

Abstract

Isolates of Rhizobium meliloti , representing antigenically distinct indigenous serogroups 31 and 17, were grown in yeast extract-mannitol broth (YEM) containing NaCl or polyethylene glycol (PEG) to provide external water potentials ranging from −0.15 to −1.5 MPa. Several differences were found between representatives of the two groups in their abilities to adapt to water stress induced by the nonpermeating solute PEG. At potentials below −0.5 MPa, strain 31 had a lower specific growth rate than strain 17 and an irregular cell morphology. In contrast, neither growth nor cell morphology of either strain was affected significantly over the same range of water potentials created by a permeating solute, NaCl. Despite the superior growth of strain 17 at the low water potentials imposed by PEG, upshock of water-stressed cells (−1.0 MPa; PEG) into normal YEM (−0.15 MPa) resulted in a faster recovery of growth by strain 31 than by strain 17. Different responses of the two strains to a water potential increase were also revealed in nodulation studies. Strain 31 required significantly fewer days to nodulate alfalfa than strain 17 did when the strains were transferred from YEM with PEG at −1.0 MPa onto the roots of alfalfa seedlings in plant growth medium (−0.1 MPa). The addition of supplemental calcium (0.1 mM) to growth medium with PEG (−1.0 MPa) reduced the differences between strains in their responses to water stress. The severe growth restriction and morphological abnormalities shown by strain 31 were corrected, and the prolonged recovery time shown by water-stressed cells (−1.0 MPa; PEG) of strain 17 upon transfer to normal YEM was shortened. The latter strain also nodulated earlier and more rapidly after growth in PEG medium at −1.0 MPa in the presence of supplemental calcium ions. These results indicate that the efficacy of osmoregulation can vary among strains of the same species and that the mechanism of osmoregulation may differ depending on the nature of the water stress.

Isolation and characterization of osmotolerant bacteria from Thar Desert of Western Rajasthan (India)

“…Soil microbial communities are among the most complex, diverse, and important assemblages in the biosphere (Zhou et al, 2003) and soil moisture is a key factor for microbial activity, its biomass and diversity in soils especially in desert (Kieft, 2002), and soil moisture stress leads to severe impact on vital metabolic processes and growth. To mitigate such stresses in desert, the bacteria must maintain osmotic equilibrium across the membrane by forming desiccationresistant spores or cysts (Gould & Measures, 1977) or tolerating low water potential as vegetative cells (Busse & Bottomley, 1989).…”

Factors affecting the survival ofBradyrhizobiumapplied in liquid cultures to soya bean [Glycine max(L.) Merr.] seeds

“…Alteration of membrane composition (Boumahdi et al 2001) and membrane permeability (Bushby and Marshall 1977b) have been suggested as potential mechanisms. Effects of water potential on growth rate and cell morphology have been documented, as well (Salema et al 1982b;Busse and Bottomly 1989;Mary et al 1994). It has been suggested that increased production of extracellular polysaccharide (EPS) would increase desiccation tolerance (Deaker et al 2004), but there are mixed results on this issue (Bushby and Marshall 1977a;Cliquet and Catroux 1994).…”

Phenotypic and symbiotic characterization of rhizobia isolated from Medicago ciliaris L. growing in Zerizer from Algeria

“…Indeed, the imposition of any stress to rhizobia results in adaptive responses, which lead to changes in the regular metabolic processes that are then reflected in protein profiles. The tolerant rhizobia to osmotic stress accumulate the osmolytes, and changes their morphology and dehydration of cells (Buss and Bottomley, 1989). There was a good correlation between the tolerances to both stressors; strains that were halotolerant were in general also tolerant to PEG, suggesting that common osmoadaptation mechanisms were operating.…”