Specificity of chemotaxis-driven motility in Sinorhizobium meliloti host interaction

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Chemotaxis enables motile bacteria to move away from harmful and towards beneficial chemicals. One unique group of motile soil bacteria are rhizobia, which can live freely in the soil, as well as engage in specific symbiotic relationships with leguminous plants such as peas, soy beans, and alfalfa. This symbiosis supplies the host plant with nitrogen, which is the most limiting nutrient for plant growth. The process of chemotaxis in the model rhizobium Sinorhizobium meliloti allows the organism to recognize its host, which improves the symbiotic relationship, and consequently, enhances plant growth. The PI discovered multiple and diverse chemoreceptors in S. meliloti, which are used to sense chemical compounds. However, little is known about the attractant spectrum or the specificity of rhizobial chemoreceptors. The overarching goal of this project is to characterize the specific adaptations of S. meliloti chemoreception to the range of host-derived attractants. Results from this research will transform our current concepts of bacterial behavior in soil through discovery of emergent properties of legume-rhizobia interactions. Chemotaxis of the soil bacterium Sinorhizobium meliloti is a critical prerequisite for the establishment of its symbiotic relationship with alfalfa. The discovery of a new type of bacterial chemotaxis, namely attraction to plant-borne betaines, and the identification of the rhizobial chemoreceptor for these compounds expands our understanding of plant-bacteria communication. The results of this research will open avenues for developing novel and innovative strategies to enhance chemoreception in the symbiotic bacterium S. meliloti, which will increase symbiotic efficiency, result in higher level of nitrogen fixation, and may ultimately increase crop yields. This project can directly benefit future agricultural and environmental issues by reducing the use of chemical fertilizers. Broader Impacts activities will involve the interdisciplinary training of graduate students. The team is committed to mentoring graduate and undergraduate students, especially underrepresented minorities and women. Public outreach activities include hands-on demonstrations for elementary and high school students, both on- and off-campus, and involvement of high school students in research.Using the Alfalfa-S. meliloti interaction as a model system, the investigators aim to elucidate the molecular mechanisms that govern legume-rhizobia communications. Understanding these mechanisms could open important new avenues for addressing daunting agricultural and environmental issues. This research will characterize S. meliloti chemoreceptors for which functional understanding is lacking. In an orthogonal approach, the investigators will define the nature of plant-derived compounds responsible for the recruitment of S. meliloti to the host rhizosphere. The composition of compounds exuded by plants into the rhizosphere has specifically evolved to attract symbiotic microbes to the roots of their corresponding hosts. First, the investigators propose to characterize the diverse phytochemicals secreted by the S. meliloti host alfalfa using mass-spectrometric metabolome analysis and quantitative chemotaxis assays. This study will uncover the S. meliloti attractome and establish the contributions of individual compounds to chemotaxis. Second, they propose to identify receptor ligands for individual chemoreceptors and determine receptor-ligand interaction through behavioral, genetic, biochemical, and structural analyses. The already highly advanced biochemical and structural characterization of McpX serves as blueprint for these analyses. The proposed research will advance our knowledge of the role of exuded root metabolites in the dialogue between plants and microbes.The Project is jointly funded by Molecular and Cellular Biosciences (CDF Cluster) and Integrative Organismal Systems (PBI Cluster), with additional support provided by the Directorate's Rule of Life Venture Fund.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.