Research Areas

Molecular biology of microbe-host interactions; genes and signals in the symbiosis between Sinorhizobium meliloti and its host, Medicago sativa.

Bacterial genes and signals critical for a plant symbiosis. The availability of "fixed" nitrogen is often a limiting factor in agricultural productivity and food abundance. Atmospheric dinitrogen gas must be "fixed" or reduced before it can be used to sustain plant growth. Biological nitrogen fixation is performed by root-associated bacteria (rhizobia) in symbiosis with legume plants. Establishment of the symbiosis involves the development of root nodules where the bacteria live as endosymbionts and provide fixed nitrogen to the host plant. Since the rhizobia must infect and persist inside root cells for the symbiosis, this beneficial symbiosis also serves as a model for studying the infection process of related bacteria, Brucella, that cause disease in mammals.

We study the ExoS/ChvI signaling pathway in the rhizobial bacteria Sinorhizobium meliloti, a symbiont with alfalfa. Remarkably, not only are the ExoS/ChvI genes essential for the S. meliloti-plant symbiosis, but the orthologs of the ExoS/ChvI genes in Brucella are essential for causing disease in mammals. This shared requirement for the ExoS/ChvI genes for infection of different hosts underscores their importance in microbe-host interactions. In free-living S. meliloti bacteria, ExoS/ChvI signaling also controls exopolysaccharide synthesis, motility, biofilm formation, metabolism, and growth.

Our aim is to elucidate the downstream and upstream regulation of ExoS/ChvI signaling. Specifically, which genes are transcriptional targets of ExoS/ChvI signaling? Do these genes have roles in symbiosis or in the other functions mediated by ExoS/ChvI signaling? How does the novel inhibitor protein ExoR regulate ExoS/ChvI activity? Our investigation of ExoS/ChvI signaling will provide crucial insight into the molecular mechanisms underlying a symbiosis with relevance to agriculture and to disease.