Esther Chen

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.

Selected publications

Chen, E. J., Fisher, R.F., Perovich, V.M., Sabio, E.A., and Long, S.R. Identification of direct transcriptional target genes of ExoS/ChvI two-component signaling in Sinorhizobium meliloti. J Bacteriol 2009 191 6833-6842

Chen, E.J., Sabio, E.A., and Long, S.R. The periplasmic regulator ExoR inhibits ExoS/ChvI two-component signalling in Sinorhizobium meliloti. Mol Microbiol 2008 69 1290-1303

Wells, D.H., Chen, E.J., Fisher, R.F., and Long, S.R. ExoR is genetically coupled to the ExoS-ChvI two-component system and located in the periplasm of Sinorhizobium meliloti. Mol Microbiol 2007 64 647-664