Sinorhizobium meliloti nifA gene exerts a pleiotropic effect on nodulation through the enhanced plant defense response

Sinorhizobium meliloti nifA gene is required for the expression of a bunch of nif and fix genes. Here, we report its pleiotropic effects on the nodule formation. Compared with wild type strain, nifA mutant sig- nificantly reduced nodule suppression rate in split-root system. The plants inoculated with mutant strain produced lower amount of daidzein and less necrotic cells on their roots. In addition, the defense genes failed to be evoked by nifA mutant at the early nodulation stage. These findings indicated that host defense response was one of the mechanisms mediated by nifA gene to regulate nodule formation during symbiosis. Even though nifA mutant could increase the number of nodules in host plant, it synthesized lower Nod factors than wild type. This suggested that nifA gene mediated multiple and diverse instances in nodulation formation.     

The enoyl-ACP reductase gene, <Emphasis Type="Italic">fabI1</Emphasis>, of <Emphasis Type="Italic">Sinorhizobium meliloti</Emphasis> is involved in salt tolerance,swarming mobility and nodulation efficiency

Our previous work showed that an enoyl-ACP reductase gene fabI1 of Sinorhizobium meliloti was down-regulated in the nifA mutant nodule bacteria. To gain a better understanding of fabI1 gene, a single site insertion mutant was constructed in this study. The fabI1 mutant was retarded in cell growth, and its ability to grow on media with high concentration of NaCl was reduced. In addition, the mutant was completely defective in swarming phenotype. During symbiosis, the fabI1 mutant had delayed nodule formation...