Modern agriculture relies heavily on pesticides and fertilizers, negatively impacting the sustainability of global food production. While plant growth promoting bacteria provide a more sustainable alternative, natural isolates used to enrich foreign soils often yield inconsistent results. Microbial engineering can help improve the sustainability of agriculture by providing greater control of desired microbial functions. However, successful deployment of engineered microbes requires greater understanding of the robustness of engineered features in conditions that seek to mimic the rhizosphere. Here, we use surfactin biosynthesis by engineered Bacillus subtilis as a model system for interrogating robustness. Surfactin is a circular lipopeptide with many benefits to crops such as improving colonization of beneficial microbes and stimulating plant immunity. We observed that our strain can produce surfactin in both rich media and in conditions simulating plant growth environments. As we advanced towards conditions that resemble some elements of the rhizosphere, such as static incubation in plant hydroponic media rather than shaking incubation, we noticed stark differences in assimilation of inorganic nitrogen sources, which was superior under static conditions. In collaboration with the Bais Lab at the University of Delaware, we also examined how wild-type and engineered strains colonize the roots of tomato plants. We report on how exogenous L-glutamate and surfactin can improve colonization. Herein, our research aims to leverage engineering cells towards more sustainable agriculture by engineering surfactin overproduction and understanding nitrogen assimilation in more environmentally relevant conditions.
