A Lipopolysaccharide Lipid A Acyltransferase Gene msbB Is Involved in Soybean Rhizobial Intracellular Colonization...

Three major components of lipopolysaccharide in rhizobia, namely core polysaccharide, o-antigen, and lipid A, often act as microbe-associated molecular patterns to participate in the symbiosis between rhizobia and legumes. 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), 27-hydroxyoctacosanoic acid. VLCFAs are usually transferred to lipid A by an acyltransferase, MsbB. In this research, we constructed an msbB deletion mutant and 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. Promoter-GUS assays revealed that msbB was mainly expressed at the early stage of nodulation. Quantitative analysis of early infection events revealed that the mutation of msbB significantly reduced root hair curling, infection threads, and nodule primordia, suggesting impairment of the symbiotic infection process. Nodulation assay and transmission electron microscopy analysis of the nodule ultrastructure further showed that msbB deletion led to the formation of small and ineffective root nodules without colonization of rhizobia, thereby causing a loss of nitrogen fixation capacity. RNA-seq analysis indicated that HH103ΩmsbB inoculation may impair early symbiotic signaling and 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. [Formula: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.