Response of Rhizobium fredii P220 to osmotic shock: Interrelationships between K+, Mg2+, glutamate and homospermidine

Abstract

The response of Rhizobium fredii P220, a salt-tolerant strain of soybean rhizobia, to osmotic shock was investigated by using non-growing washed cells. Rapid changes in K+, Mg2+, glutamate and homospermidine were observed in strain P220 cells subjected to sudden changes in the osmolarity of incubation buffer. Osmotic upshock resulted in elevation of cellular K+ and glutamate, and reduction in cellular homospermidine and Mg2+. When the cells were transferred to upshock buffer lacking K+, the reduction in Mg2+ was totally blocked, but the elevation of glutamate and the reduction in homospermidine were only partially repressed. Osmotic downshock resulted in the opposite phenomenon: There was an elevation of homospermidine and Mg2+, and a rapid fall of K+ and glutamate. When the cells were transferred to downshock buffer lacking Mg2+, the elevation of homospermidine was partially repressed, but the decrease in K+ and glutamate was not repressed at all. Lowering of the cellular K+ by treatment with ionophores nigericin and monensin resulted in a slight decrease in glutamate and a slight increase in homospermidine and Mg2+, possibly due to a pH effect caused by the K+-H+ exchange. Raising the cellular Mg2+ content by treatment with ionophore A23187 brought about an increase in homospermidine. The homospermidine content of Mg2+-deficient cells grown with low-Mg2+ medium reduced to 35% of those grown with the basal medium. These results indicate that in R. fredii, K+ strictly controls Mg2+ flux during osmotic shock whereas the reverse is not true, and that glutamate and homospermidine essentially escape direct control by K+. We also suggest that Mg2+, which has no effect on the pool size of glutamate, is one of the factors which regulate homospermidine content in rhizobial cells.