How to catch the N - An inter-species exchange with the right chemistry.

While classical breeding traits have focussed on above-ground tissues, it is becoming clear that underground aspects of plant life are a hidden treasure of tools applicable for resilient crop production. Plants of the legume family develop specialized organs, called nodules, which serve as hosts for Rhizobium bacteroids. A highly specialized symbiotic relationship exists deep inside the nodules. In exchange for carbohydrates, host-specific rhizobia bacteroids can assimilate nitrogen from the air and fix it into a form that can be used by plants in a process known as biological nitrogen fixation. While we understand certain aspects of how this inter-species relationship is established, the exact biochemistry of this exchange remains dogmatic. In their recent work, Christen and colleagues (Flores-Tinoco et al, 2020) challenge the current model of nitrogen exchange and argue that that an expanded model is needed to fit experimental findings related to nitrogen fixation. The authors perform an elegant set of experiments and highlight that rather than a single-way flow of nitrogen, the N-fixing process is in fact an elaborate metabolic exchange between the nodule-dwelling bacteroids and the host plant. Importantly, this work provides an updated theoretical framework with the "catchy" name CATCH-N which delivers up to 25% higher yields of nitrogen than classical models and is suitable for rational bioengineering and optimization of nitrogen fixation in microorganisms.