A LuxR/LuxI-Type Quorum-Sensing System in a Plant Bacterium, Mesorhizobium tianshanense , Controls Symbiotic Nodulation

Abstract

The ability of rhizobia to symbiotically fix nitrogen from the atmosphere when forming nodules on their plant hosts requires various signal transduction pathways. LuxR-LuxI-type quorum-sensing systems have been shown to be one of the players in a number of rhizobium species. In this study, we found that Mesorhizobium tianshanense , a moderate-growth Rhizobium that forms nodules on a number of licorice plants, produces multiple N -acyl homoserine lactone (AHL)-like molecules. A simple screen for AHL synthase genes using an M. tianshanense genomic expression library in Escherichia coli , coupled with a sensitive AHL detector, uncovered a LuxI-type synthase, MrtI, and a LuxR-type regulator, MrtR, in M. tianshanense . Deletions of the mrtI or mrtR locus completely abolished AHL production in M. tianshanense . Using lacZ transcriptional fusions, we found that expression of the quorum-sensing regulators is autoinduced, as mrtI gene expression requires MrtR and cognate AHLs and mrtR expression is dependent on AHLs. Compared with the wild-type strains, quorum-sensing-deficient mutants showed a marked reduction in the efficiency of root hair adherence and, more importantly, were defective in nodule formation on their host plant, Glycyrrhiza uralensis . These data provide strong evidence that quorum sensing plays a critical role in the M. tianshanense symbiotic process.