Soil acidity constraints grain legume production in tropical soils, both limiting Rhizobium survival and reducing nodulation. Strains of rhizobia with greater tolerance to hydrogen-ion concentration have been identified, but the basis for strain differences in pH tolerance has yet to be determined. In this study, strains of Rhizobium leguminosarum by phaseoli which differed in their tolerance to acidity were exposed to acid pH, then cell levels of potassium and calcium determined, and specific 'acid-shock' proteins identified. Lowering the external pH to 4.6-4.7 resulted in an immediate efflux of calcium from the cell of both acid tolerant and sensitive bean strains. Change in cell potassium levels on exposure to acidity varied with the strain. Strain UMR1899 and an acid-sensitive mutant derived from it maintained high cytoplasmic potassium at acid pH, whereas an acid-sensitive strain UMR1632 underwent a marked decline in cell potassium at pH 4.6. Exposure of these strains to pH 4.5 in the presence of [S-35]-labeled methionine enhanced production of a number of proteins, while synthesis of other proteins at this pH was significantly reduced. Differences in banding pattern were also evident between UMR1899 and the Tn5-induced pH-sensitive mutant UMR5005 derived from it, and between cells grown in the presence and absence of calcium and phosphorus.
