Coordination of complex functions of Sinorhizobium meliloti throughout the establishment of symbiosis with Medicago sativa

Pathogenic and symbiotic associations with multicellular organisms provide bacteria with numerous benefits such as carbon sources and protection from harsh environmental conditions. However, the successful establishment of these relationships requires numerous complex bacterial functions as well as their precise regulation. In the case of the nitrogen-fixing symbiosis between the soil bacterium Sinorhizobium meliloti and the host plant Medicago sativa (alfalfa), motility, exopolysaccharide production, and root nodule formation are among the multitude of bacterial abilities requiring careful coordination. The process begins with the bacterial induction of root-nodule formation on the host plant. Efficient invasion of these structures requires the production of symbiotically critical exopolysaccharides, such as the low-molecular-weight (LMW) fraction of exopolysaccharide II (EPS II), and the repression of motility. Once internalized, S. meliloti differentiates into a distinct morphological form known as a bacteroid, where it fixes free nitrogen gas for the benefit of the plant. In this study, we elucidated the central role of the MucR transcriptional regulator and the ExpR/Sin quorum-sensing system in synchronizing this array of functions necessary for symbiosis. We began with an analysis of the mechanism by which quorum sensing results in the production of LMW EPS II. We demonstrated that the ExpR/Sin quorum-sensing system derepresses overall EPS II production as well as the LMW fraction from the MucR transcriptional regulator. Due to the complex nature by which the ExpR/Sin system specifically abolishes only the effects of MucR on EPS II synthesis rather than simply repress expression of this protein at the transcriptional level, other possible beneficial functions of MucR were investigated. Examination of MucR uncovered roles in activating nodule formation on host plant roots as well as preventing premature expression of genes required only during symbiosis. By precisely modulating the regulatory effects of MucR, the ExpR/Sin quorum-sensing system induces the production of LMW EPS II and invasion of the host without interfering with root nodule development and repression of bacteroid genes prior to host-plant invasion. Therefore, the MucR transcriptional regulator and the ExpR/Sin quorum-sensing system are critical in coordinating a variety of bacterial functions required throughout the process of establishing symbiosis with M. sativa.