Effects of pH and Osmotic Stress on Cellular Polyamine Contents in the Soybean Rhizobia Rhizobium fredii P220 and Bradyrhizobium japonicum A1017

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Homospermidine is a polyamine present in its highest concentrations in root nodule bacteria. By using the soybean rhizobia Rhizobium fredii P220 and Bradyrhizobium japonicum A1017, the effects of the pH and osmolarity of the medium on rhizobial growth and cellular polyamine contents were investigated. Elevation of medium pH repressed the growth of slowly growing B. japonicum A1017 and resulted in a slight increase in cellular putrescine, while homospermidine content was not significantly affected. In contrast, in fast-growing R. fredii P220, which showed good growth over a wide range of the medium pHs from 4.0 to 9.5, homospermidine content increased with the lowering of the medium pH. Under the acid-stressed conditions, cellular Mg2+ content in strain P220 also increased. Strain P220 was able to grow in NaCl concentrations up to 0.4 M, while strain A1017 did not grow in media containing 0.15 M NaCl. Glutamic acid and K+ contents of salt-tolerant P220 cells increased in response to NaCl concentrations, but homospermidine and Mg2+ contents were inversely related to the NaCl concentrations. External salinity had no effect on the contents of other polyamines in P220 cells. On the basis of osmotic strength, NaCl, KCl, sucrose, or glycerol induced similar decreases in cellular homospermidine content. These results suggested that the cellular levels of homospermidine in strain P220 may be regulated by mechanisms related to their pH and osmotic tolerance.

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