Understanding the role of 27-hydroxyoctacosanoic acid, a lipid A component of rhizobial LPS in the Rhizobium-pea symbiosis
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The initial stages of the Rhizobium-legume symbiosis involve several processes of finely tuned exchange of molecular signals, and ultimately result in the formation of root nodules on the host plant, where atmospheric nitrogen is fixed. Proper establishment of the symbiotic process is dependent on the structural uniqueness of surface molecules of the bacterium such as lipopolysaccharides. The gene encoding the specialized acyl carrier protein (acpXL) responsible for transferring 27-hydroxyoctacosanoic acid in Rhizobium leguminosarum was mutated in this study. The physiological and symbiotic phenotype of this mutant strain was characterized. Although the lab-grown mutant strain lacked this residue on the lipid A region, analysis of bacteroids from nodules infected with the mutant strain revealed reappearance of the fatty acid molecule. This is indicative that an in planta condition is capable of turning on an alternate pathway which is independent of the acpXL machinery for the addition of the 27- hydroxyoctacosanoic acid molecule. The effect of the mutation was perhaps most strongly observed at the ultrastructural level, which revealed misshaped bacteroids that were defective in their ability to divide synchronously with the symbiosome membrane hence resulting in multiple bacteroids per membrane. Also, the inability of the mutant strain to grow on media with high osmolarity indicates defective/altered outer membrane properties.