A Lipopolysaccharide Lipid A Acyltransferase Gene msbB Is Involved in Soybean Rhizobial Intracellular Colonization and Symbiotic Nitrogen Fixation

Three major components of lipopolysaccharide (LPS) in rhizobia, namely core polysaccharide, o-antigen, and lipid A, act as microbe-associated molecular patterns (MAMPs) to participate in the symbiosis between rhizobia and legume. Rhizobia have a different lipid A structure from other Gram-negative bacteria. The 3-hydroxy group on the 2' or 3' myristate acyl chain of its lipid A is substituted by a unique very long chain fatty acid (VLCFA). VLCFAs are transferred to lipid A by an acyltransferase MsbB. In this research, we constructed the msbB deletion mutant, complementary, and overexpression strains of Sinorhizobium fredii HH103, and investigated their free-living and symbiotic phenotypes. The findings revealed that deletion of msbB had no impact on the autonomous growth of HH103, yet significantly reduced the resistance of rhizobia to abiotic stresses. The promoter-GUS assays revealed that msbB was mainly expressed at the early stage of nodulation. Quantitative analysis of early infection revealed that the mutation of msbB significantly reduced root hair curling, infection threads, and nodule primordia, suggesting impairment of the symbiotic infection process. The nodulation assay and transmission electron microscopy analysis of nodule ultrastructure showed that msbB deletion led to the formation of ineffective root nodules without colonization of rhizobia, thereby causing a loss of nitrogen fixation capacity. RNA-seq analysis indicated that HH103ΩmsbB inoculation trigger a localized defense response in the soybean root to result in symbiotic deficiencies. Taken together, these results reveal the important role of VLCFAs in soybean rhizobia in the establishment of effective symbiosis and nodule nitrogen fixation.