Correlation between NaCl Sensitivity of Rhizobium Bacteria and Ineffective Nodulation of Leguminous Plants

A sodium chloride (NaCl)-sensitive mutant of Rhizobium fredii USDA191, which contained a single copy of Tn5-Mob transposed into chromosomal DNA, was obtained by Tn5-Mob random insertion. The growth rate of this mutant was lower than that of the wild type in the presence of 0.2 M NaCl and it seemed to lack the inductive ATP production in response to the addition of NaCl. This mutant induced the formation of small and whitish nodules on lateral roots of soybeans, which were negative for acetylene reduction activity, indicating that the nodules were ineffective for nitrogen fixation. The mutant also reduced the weight of above-ground portions and roots to 64 and 55%, respectively, compared with the weight of the plants inoculated with the wild-type cells. These results suggest that NaCl sensitivity of Rhizobium bacteria is one of the important factors for nodule formation and nitrogen fixation.