What We Study
Legume-rhizobia symbioses are keystone examples of inter-species cooperation that can greatly enhance the quality of soils and crops. Establishing these symbioses requires complex developmental processes for which many of the underlying molecular mechanisms are unknown.
Schematic of symbiosis development in the Aeschynomene (joint- and deer- vetch) genus of legumes.
During the early stages of the symbiosis, legume roots release chemical compounds that attract rhizobia living in the soil. These compounds simulate rhizobia to travel towards legume roots and to secrete additional signals that help them colonize and invade the root surface. When the legume root cells detect these signals, symbiosis-specific developmental pathways are triggered that lead to the formation of a new plant organ, known as a root nodule.
Root nodule biogenesis requires the coordination of plant and bacterial development. Legume root cells must differentiate into new nodule-specific cell types and undergo coordinated cell proliferation to populate the new organ. In turn, the rhizobia, must navigate the root tissue to reach the new nodule cells, where the rhizobia are engulfed by the legume nodule cells as intracellular (endo-) symbionts. Both partners then shift their metabolic output for mutual benefit: rhizobial endosymbionts shut down most basal metabolic activities to become powerhouses for nitrogen fixation (the conversion of nitrogen gas into ammonia), while legumes allocate sugars derived from photosynthesis to support the endosymbionts.
How do lipids regulate rhizobial cell organization and differentiation?
Phylogenetic tree of hopanoid-producing organisms (left) and super-resolution imaging of Bradyrhizobium membranes
Lipid membranes are defining features of cells. They are dynamic structures and undergo rapid remodeling in different environments, including during the transition from free-living to endosymbiotic lifestyles. Common lipids in nitrogen-fixing plant symbionts include the hopanoids, which are the cholesterol analogs of the bacterial domain. Hopanoid lipids are required for efficient Bradyrhizobium-legume symbiosis, and they appear to facilitate bacterial survival of root nodule-related stresses. They are also important regulators of the biophysical properties of the bacterial membrane. We are using a combination of biochemical, computational, and microscopy-based approaches to understand how hopanoids and other rhizobial lipids affect the organization and function of the endosymbiotic membrane.
How do rhizobia sense and response to plant signals in the rhizosphere?
Interkingdom signaling in the Bradyrhizobium-soybean symbiosis (left) and plate motility of various Bradyrhizobium strain (right)
The rhizosphere contains a complex community of bacterial species. To identify compatible symbionts in this microbial milieu, legumes and rhizobia express specialized receptors that bind signals from their preferred partners. This chemical dialogue is the basis of symbiotic specificity, and while many essential signals have been identified, these are a few of the thousands of molecules present in the rhizosphere. Where and when specificity-related signals are produced, and how they are delivered to a compatible partner, is not clear. We use bacterial genetics and a variety of -omics approaches to understand rhizobial signal secretion and response in the competitive legume rhizosphere.