RESPONSE OF RHIZOBIUM-FREDII P220 TO OSMOTIC SHOCK - INTERRELATIONSHIPS BETWEEN K+, MG2+, GLUTAMATE AND HOMOSPERMIDINE

The response of Rhizobium fredii P220, a salt-tolerant strain of soybe an rhizobia, to osmotic shock was investigated by using non-growing wa shed cells. Rapid changes in K+, Mg2+, glutamate and homospermidine we re observed in strain P220 cells subjected to sudden changes in the os molarity of incubation buffer. Osmotic upshock resulted in elevation o f 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 glut amate and the reduction in homospermidine were only partially represse d. Osmotic downshock resulted in the opposite phenomenon: there was an elevation of homospermidine and Mg2+. and a rapid fall of K+ and glut amate. When the cells were transferred to downshock buffer lacking Mg2 +, the elevation of homospermidine was partially repressed, but the de crease in K+ and glutamate was not repressed at all. Lowering of the c ellular K+ by treatment with ionophores nigericin and monensin resulte d in a slight decrease in glutamate and a slight increase in homosperm idine and Mg2+, possibly due to a ph effect caused by the K+-H+ exchan ge. Raising the cellular Mg2+ content by treatment with ionophore A231 87 brought about an increase in homospermidine. The homospermidine con tent 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 essent ially escape direct control by K+. We also suggest that Mg2+, which ha s no effect on the pool size of glutamate, is one of the factors which regulate homospermidine content in rhizobial cells.