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 a t 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.

Biogenic Amines in Rhizobia and Legume Root Nodules

“…Fig. 3 shows the changes in Homospd, glutamate, K + and Mg 2+ levels in E. freii P220 cells after an osmotic down shock (43). Substantially opposite phenomena to those observed on osmotic up shock (or under high salinity) occur on osmotic down shock; i.e., there is an elevation of Homospd and Mg 2+ content accompanied by a rapid fall in levels of K + and glutamate.…”

“…Microbes in soils are continually exposed to osmotic shock, especially in nutrient-rich soils, because rainfall, drought or freezing brings about a rapid change in the osmolarity in their habitat. Such a sudden change in osmolarity (osmotic down shock or up shock) generally results in a metabolic change of Homospd in rhizobial cells (43). As when under the salt-stressed conditions described above, when fast-growing rhizobial cells are transferred from a low-osmotic buffer to a high-osmotic buffer causing an osmotic up shock, both K + ion and glutamate immediately increase and both Homospd and Mg 2+ decrease (43).…”

“…2) Removal of Mg 2+ from the growth medium results in a significant reduction in cellular Homospd (43). Such a close connection between Mg 2+ and Homospd seems to be a general phenomenon of rhizobia, since a positive correlation between the cellular level of Mg 2+ and that of Homospd was shown in various Rhizobium and Ensifer species grown in a low-or highMg 2+ medium (44).…”

“…Such a sudden change in osmolarity (osmotic down shock or up shock) generally results in a metabolic change of Homospd in rhizobial cells (43). As when under the salt-stressed conditions described above, when fast-growing rhizobial cells are transferred from a low-osmotic buffer to a high-osmotic buffer causing an osmotic up shock, both K + ion and glutamate immediately increase and both Homospd and Mg 2+ decrease (43). Dynamic changes in the cellular concentrations of these components following a rapid osmotic change are typical in fast-growing rhizobia.…”

“…The production of Homospd in rhizobial cells is closely linked to the cellular concentration not only of glutamate but also of the inorganic cation Mg 2+ (43). For the following reasons: 1) Treatment of rhizobial cells with the ionophore A23187, an ion carrier which shows high affinity for Mg…”