Collaborative Research: Chemotactic signaling in Sinorhizobium meliloti symbiotic plant host interaction

Chemotaxis enables motile bacteria to move away from harmful and towards beneficial chemicals. One unique group of motile soil bacteria are rhizobia, which can 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 allows rhizobia to recognize and move towards its host, which improves the symbiotic relationship, and consequently, enhances plant growth. Multiple chemoreceptor proteins are being used by rhizobia to sense a wide spectrum of chemical compounds secreted by the host plants. However, little is known about the specific processes involved in the regulation of sensitivity and adaptation, which are essential features for an effective chemotactic response.

The overarching goal of this project is to decode the pathway that controls stimuli adaptation. Outcomes will transform our current concepts of the biology of bacterial chemotaxis and their interaction with their eukaryotic hosts. The results of this research can directly benefit future agricultural and environmental issues by potentially reducing the use of artificial fertilizers, resulting in less expensive and less polluting agriculture.

Broader Impacts activities include the intrinsic merit of the research as nitrogen fixation is one of the most important processes in the biosphere. In addition, the project will involve the interdisciplinary training of graduate students. The team is committed to mentoring graduate and undergraduate students, especially underrepresented populations and women.

Public outreach activities include hands-on demonstrations for elementary and high school students, both on- and off-campus, and involvement of undergraduate and high school students in research.

The chemotaxis system of the model plant symbiont Sinorhizobium meliloti evolved a greater complexity than that of enteric bacteria. Although several unique components and features controlling chemotaxis have been recently uncovered, regulation of pathway sensitivity and stimuli adaptation are unknown. The aim of this research project is to elucidate the receptor modification system, which plays a pivotal role in this process.

Specifically, the function of a novel chemotaxis protein, CheT, which unprecedentedly interacts with a conserved element of the adaptation pathway, will be deciphered. First, the investigators will characterize the chemotactic pathway controlling adaptation. Two chemoreceptors with a central role in host-plant sensing, namely amino acid sensor McpU and betaine sensors McpX, will serve as models because they are expected to exhibit different modes of interaction with the adaptation pathway.

Behavioral, microscopic, and mutational analyses, as well as mass spectrometry will be used to uncover the molecular basis for stimuli adaptation. Second, the role of CheT in chemotactic signaling will be elucidated through genetic, behavioral, phosphorylation, and structural analyses. The proposed research will advance knowledge of complex sensory systems.

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.