An <i>Azorhizobium caulinodans</i> ORS571 mutant with deletion of a gene encoding a TIGR02302 family protein overproduces exopolysaccharides and is defective in infection into plant host cells
<p><i>Azorhizobium caulinodans</i> is a microsymbiont of <i>Sesbania rostrata</i> Bremek. & Oberm., and is able to fix nitrogen in both the free-living and symbiotic states. In this study, we focused on the <i>ggm</i> gene (locus tag, AZC_4606) that encodes a putative membrane protein belonging to the TIGR02302 family. Although the genes encoding TIGR02302 family protein are distributed in a wide range of alpha-proteobacteria including rhizobia, the functions of this protein are still unknown. To investigate the functions of this protein in <i>A. caulinodans</i>, we made a <i>ggm</i> mutant, and analyzed its phenotypes. The <i>ggm</i> mutant produced more bubbles than the wild-type strain in L3 + N medium liquid cultures, and formed mucoid colonies on L3 + N medium agar plates, suggesting that the <i>ggm</i> mutant overproduced exopolysaccharides (EPSs). The amounts of EPSs produced by the <i>ggm</i> mutant on L3 + N plates were about 1.3-fold higher than those by the wild-type strain, and expression levels of EPS production-related genes in the <i>ggm</i> mutant grown in L3 + N liquid medium were about 2- to 4-fold higher than those of the wild-type strain. In addition, the stem nodules formed by the <i>ggm</i> mutant on the stems of <i>S. rostrata</i> showed little or no nitrogen-fixing activity. By microscopic analyses, large infection pockets and a few infected cells were observed in the stem nodules formed by <i>ggm</i> mutant, suggesting that the <i>ggm</i> mutant is defective in invasion into plant cells. Taken together, our results suggest that Ggm is involved in EPS production and that adequate levels of EPS production are required for <i>A. caulinodans</i> to invade into host cells.</p
