RoL: Regulation of cell envelope homeostasis in the alpha-proteobacterium Sinorhizobium meliloti

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Organisms adapt, survive and even thrive. Some function more successfully by interacting with another completely different kind of organism, where the two do something together that neither can do on its own. This may benefit both organisms, making it a mutualistic symbiosis. How do organisms manage to cooperate instead of one causing disease in the other? Our project addresses such questions in the case of a plant and its bacterial symbiont. With the bacteria providing usable nitrogen, this partnership is fundamental to environmental and agricultural sustainability. Our work will employ several approaches to discover how the plant host accommodates and communicates with its bacterial partner. Beyond this particular symbiosis, our work pushes back the frontier of understanding the general rules for microbe-host interactions. How prevalent are symbioses in the living world? How do communities of microbes interact with hosts, and what are the costs and benefits? The experimental approaches we develop in this project, and the discoveries we make in our research, will advance the general field of symbiosis and will also contribute to microbiome and disease research. The project supports the STEM community and will increase its diversity by incorporating new high school science curricula on nitrogen-fixing symbiosis. These classroom and laboratory modules will tie together plant science with ecology and nutrition and provide historical and cross-cultural context on the use of legumes in sustainable agriculture. The new curricula will be of particular interest to students of Latinx and native American ancestry. Rhizobia-legume symbioses proceed through a series of stages with ongoing recognition and communication between the plant host and bacterial partner. While early stages of symbiosis have been well studied, and signal molecules and molecular responses have been analyzed, less is known about the stages of late infection and early differentiation. Previous work from several research groups including ours have identified Medicago truncatula dnf mutants (defective in nitrogen fixation) that are defective in Sinorhizobium meliloti bacteroid maturation. One plant mutant, dnf1, is required for processing of nodule cysteine-rich (NCR) peptides, and other dnf mutants encode NCR peptides themselves. By transcription and mutant analyses, we identified a set of bacterial genes, the "L2 cluster", which appear to be downregulated just prior to bacteroid differentiation. The L2-cluster gene set is enriched for genes predicted to function in cell envelope homeostasis. We concurrently discovered that cell envelope stress responses are tied to regulation of extracellular polysaccharides, which are required for host plant infection. With a combination of genetic, molecular, and cell biology approaches, we will dissect the mechanism for cell envelope stress response and define its relationship to symbiosis. We will further explore how the plant host controls the bacterial L2 cluster by exploiting plant mutation analysis together with in vivo cell and molecular reporter analysis of bacterial infection and differentiation. Co-funding Programs: Rules of Life; Plant Biotic Interactions; Cellular Dynamics and Function.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.