Metabolic Analysis of Nitrogen Fixation in the Soybean Micro-Symbiont Sinorhizobium Fredii Using Constraint-Based Modeling

Rhizobia are soil bacteria known for being able to establish symbiosis with diverse host plants. In this context, Sinorhizobium fredii is a soil bacterium able to form nitrogen-fixing nodules with diverse legumes such as soybean. In particular, S. fredii strain CCBAU45436 represents one of the dominant sublineage of S. fredii that nodulates soybeans in alkaline-saline soils in the Huang-Huai-Hai Plain region in China. Here, we present a genome-scale metabolic model for the soybean micro-symbiont Sinorhizobium fredii strain CCBAU45436. The model was manually curated based on physiological, biochemical information and integration of high-throughput data. A symbiosis reaction was defined to represent the specific exchange of nutrients between the bacteroid and soybean. The symbiosis reaction together with the nitrogen fixation reaction defines the objective function to describe the stage of nitrogen fixation. The model developed can be used to systematically analyze symbiotic factors and predict essential genes for nitrogen fixation. In addition, the model can be expanded to represent different life stages of the bacteria to try to understand niche adaptation. No reconstructions are currently available for S. fredii strains despite of their potential benefits to sustainable agriculture and ecosystem.

This work is supported by the Hong Kong Research Grants Council GRF (14108014) and Area of Excellence Scheme (AoE/M-403/16) awarded to H.-M.L.