Bacteria are the most abundant organisms in soil, and they can have major impacts on plant health. Pathogenic bacteria can devastate the productivity and quality of crops, whereas other bacterial species directly promote plant development. These "beneficial bacteria" can act in diverse ways, including making soil nutrients more accessible to plants, limiting the spread of pathogens, and helping plants survive extreme weather. Harnessing the functions of these beneficial bacteria has the potential to transform agriculture, and a major challenge of the next century will be to determine how they can be used to help feed the planet.
We study beneficial bacteria known as rhizobia that can promote the growth of legumes like peanuts and soybeans. These legume-rhizobia symbioses are keystone examples of inter-species cooperation, and they involve complex developmental processes. To initiate the symbiosis, legume roots release chemical compounds that attract soil rhizobia, which then colonize and invade the root surface. This triggers developmental pathways in the plants that lead to the formation of a new, symbiosis-specific plant organ known as a root nodule.
During root nodule formation, legume root cells differentiate into nodule-specific cell types that then proliferate to form the new organ. Rhizobia penetrate the root tissue until they reach these new nodule cells, where they are engulfed as intracellular (endo-) symbionts. Both partners then shift their metabolic output for mutual benefit: rhizobia shut down basal metabolic activities to become powerhouses for ammonia production, while legumes allocate sugars derived from photosynthesis to “feed” their rhizobial endosymbionts.
In the Belin lab, we try to understand the basic cell biology of rhizobia known as Bradyrhizobia, a genus of bacteria found in soils worldwide. Bradyrhizobia can form a symbiosis with many economically important legumes, including soybeans, peanuts, and acacia trees, and are the most globally dominant rhizobia, yet they are relatively poorly studied. We hope to better understand these beneficial bacteria and how they can be used to improve sustainability in agriculture.
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.
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.