- Virginia Tech
Chemotaxis enables motile bacteria to detect and respond to chemicals in their surroundings. Avoiding harmful compounds and seeking beneficial substances not only aids survival in a rapidly changing environment, but also facilitates optimal interaction with eukaryotic host organisms. In terrestrial ecosystems, the nutrient-rich rhizosphere is colonized by a specific microbial community, which is shaped through chemical communication between plants and microbes. Rhizobia, a unique group among these rhizosphere-colonizing bacteria, can live freely in the soil or engage in specific symbiotic relationships with legumes, such as soybeans, peas, and alfalfa. This symbiosis leads to the conversion of atmospheric nitrogen to biologically available forms of nitrogen, which can be utilized by the host legume and subsequent crops thus reducing the need for not only expensive, but also environmentally deleterious, synthetic fertilizer. The establishment and maintenance of a successful legume-rhizobia symbiosis requires a sequence of highly regulated and coordinated events between the organisms. The secretion of a wide spectrum of small biomolecules by plant roots initiates and modulates host-microbe dialogues, attracts bacteria to the rhizosphere, and ensures specificity of symbiotic interaction. The symbiosis between alfalfa and Sinorhizobium meliloti constitutes an important example of such a relationship and an ideal model for studying host-microbe interaction.
In S. meliloti, a varying soil milieu, metabolic diversity, and specific adaptations to host signals have led to the evolution of a more complex chemosensory system compared to that of enteric bacteria. Several unique components controlling S. meliloti chemotaxis have only been recently uncovered. S. meliloti’s eight chemoreceptors play a pivotal role in directing the bacterium toward nutrients sources and its plant host. Yet, remarkably little is known about the nature of their ligands, receptor specificity, ligand-receptor interactions, and chemosensory adaptation.
Using the alfalfa-S. meliloti interaction as a model system, we 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. Using a diverse set of genetic and molecular tools to study chemoreception, signal transduction, and motility, we have made significant progress toward our long range goals. We uncovered the central role of the chemoreceptor McpU in host-plant sensing. McpU is a direct sensor of most amino acids, all of which are exuded in millimolar concentrations by germinating alfalfa seeds. In our latest studies, we discovered that S. meliloti McpX constitutes the first known bacterial betaine chemoreceptor. Betaines protect plants against environmental stress, but their role in communication with bacteria is a new discovery. Building on these advances, we propose to examine the molecular basis for the novel McpX-betaine interaction and to characterize S. meliloti chemoreceptors for which we lack a functional understanding. In an orthogonal approach, we will define the nature of plant-derived compounds responsible for the recruitment of S. meliloti to the host rhizosphere.