Various plant-microbe and microbe-microbe interactions take place in the rhizosphere, the outcomes of which influence plant health and productivity. The molecular communication between bacteria and host plants that allow the establishment of specific symbiotic or pathogenic plant-bacteria interactions are known in detail. However, commensal plant-microbe associations that dominate in the rhizosphere have not been studied to the same extent and relatively little is known regarding the strategies used by commensal soil bacteria to initiate a loose association with plant root surfaces or the spatiotemporal dynamics of these associations. This research aims to characterize key bacterial determinants implicated in the establishment and maintenance of commensal bacteria-plant associations and the spatiotemporal dynamics of these associations, in real-time. Experiments will characterize the role of sensing and chemotaxis in the association of Azospirillum brasilense with wheat roots, which is an excellent model to quantitatively analyze beneficial commensal plant-microbe associations at high temporal and spatial resolution scales. Using iterations between experiments and mathematical modeling, the research will characterize the selective advantage that multiple chemotaxis pathways provide to the ability of bacteria to sense and respond to gradients relevant to their lifestyle in the soil and in the rhizosphere. Next, the role of receptors capable of integrating metabolic status with sensing in mediating rhizosphere colonization will be determined. The project will also use a novel expression system to track how changes in the intracellular concentration of a key metabolite (c-di-GMP) affect locomotor behaviors in real time. Last, a quantitative approach to monitor, in real-time, the role of sensing and chemotaxis in the spatiotemporal dynamics of commensal plant-root colonization will be implemented. Most soil bacteria form commensal associations with the roots of plants and the sequenced genomes of these bacteria encode at least two Che pathways. Results obtained will be directly transposable to other motile soil bacteria and will provide much needed quantitative insights into the molecular mechanisms involved in the establishment of commensal plant-microbe associations. This knowledge is a prerequisite to a systems-level understanding of the diverse plant-microbe associations that exist in the rhizosphere. The approach to real-time and quantitative monitoring of commensal plant-microbe associations will document the spatiotemporal dynamics of these associations for the first time and ultimately inform future effective strategies to manipulate the rhizosphere to improve plant health and productivity.
Broader Impacts: This project will provide new tools and methods to quantitatively analyze the spatiotemporal dynamics of plant-microbe associations, including approaches to track real-time root-microbe associations. The mathematical model derived from this research will be useful in future systems-level simulations of plant-microbe associations for rhizosphere manipulation and promotion of plant health. The project will also characterize sensory modules that will enhance the synthetic biology toolkit. In addition, cross-disciplinary interactions in biology and mathematics for graduate students and undergraduate students, including members of underrepresented groups in the sciences, will be established.
