Nitrogen is an essential nutrient for all life, required to generate the fundamental building blocks of the cell. Although nitrogen gas is abundant in the atmosphere, it is largely inert and cannot be broken down by the vast majority of organisms. Instead, most species depend on more easily metabolized, “bioavailable” nitrogen sources such as nitrates and ammonia to survive. Bioavailable nitrogen is often limiting in the environment, and many species-rich ecosystems have evolved to survive within a narrow range of nitrogen levels.

The rise of modern agriculture has disrupted the natural balance of bioavailable nitrogen through the widespread application of nitrogen-rich fertilizers. While these industrial fertilizers have yielded massive gains in agriculture, their use has also contributed to greenhouse gas emissions and the prevalence of toxic species and compounds in soils and the water supply. As the population continues to rise over the next century, demanding higher and higher levels of agricultural productivity, a key scientific challenge will be providing crops with the nitrogen they need while mitigating the release of excess nitrogen in the environment.

Overview of the symbiosis between Bradyrhizobium diazoefficiens and Aeschynomene spp. tropical legumes. In the cartoon representation, Bradyrhizobia cells (blue) colonize the surface of Aeschynomene roots (left), invade plant root cells to become intracellular symbionts, and stimulate in proliferate of roots cells into a spherical nodule (center inset). Within these root nodules, bacteria fix atmospheric nitrogen to produce ammonia that supports plant growth (right). Brightfield images of roots before and after nodule formation (far right and far left images) and fluorescence images of Bradyrhizobia (green) within host plant cells (cell walls in blue, DNA in red) (center images) are shown below. Figure adapted from Belin et al. Nat. Rev. Microbiol. 2018 with images from Belin et al. MPMI 2019 and Bonaldi et al. MPMI  2011.

Nature has already evolved a sustainable solution to this challenge: “nitrogen-fixing” bacteria, the only organisms on Earth with the metabolic capacity to directly convert atmospheric nitrogen gas into bioavailable forms. Many nitrogen-fixing bacteria form close associations with plants, including the mutually beneficial symbioses between soil bacteria known as rhizobia and legumes. During these symbioses, rhizobia invade and reside within specialized organs known as nodules on legume roots, exchanging bioavailable nitrogen for carbohydrates derived from host plant photosynthesis. Studying the genetic factors that determine the efficiency of legume-rhizobia metabolic exchanges has the potential to enhance sustainability in agriculture through development of more productive strains of rhizobia. The legume-rhizobia symbiosis is also a keystone example of inter-species cooperation, through which we can uncover fundamental properties of host-bacteria associations that may be relevant to human health.

Dr. Belin’s group investigates the legume-rhizobia symbiosis by focusing on the cell biology of the Bradyrhizobia, globally abundant soil bacteria that form nitrogen-fixing symbioses with legumes including soybeans, peanuts, acacias, and vetches. Current research questions include:

1.     What are the fundamental principles guiding the organization of the Bradyrhizobium cell membranes, and is the spatial regulation of membrane-based processes important for Bradyrhizobium-legume symbioses?

2.     What adaptations allow Bradyrhizobia successfully navigate legume root tissues prior to becoming incorporated into host cells, and how much do these adaptations vary between legume hosts?

3.     What are the chemical and/or biophysical cues involved in the control of rhizobial proliferation by legume hosts – and to what extent can symbiotic bacteria manipulate these control mechanisms?

If you are interested in joining the Belin lab, or would like to learn more about our research, please contact Dr. Belin at belin@carnegiescience.edu.