Inside a pea or bean plant, a microscopic tug-of-war is underway: nitrogen-fixing bacteria must be pulled into root cells to do their work, but the molecular signals that trigger this entry have remained unknown for decades. This matters because modern agriculture depends on synthetic nitrogen fertilisers, which are energy-intensive to produce and pollute waterways. Legumes like peas, beans, and clover already host rhizobia bacteria inside specialised root nodules, where the bacteria convert atmospheric nitrogen into a form the plant can use. Understanding exactly how the plant signals these bacteria to enter its cells could allow scientists to transfer this ability to crops like wheat or rice. If this research succeeds, it could reduce agriculture’s reliance on synthetic fertilisers, cutting both production costs and environmental damage. The project is fundamental science—it seeks to uncover a core biological mechanism rather than deliver an immediate product. But past discoveries in plant-microbe signalling have already transformed farming practices, and a complete molecular map of rhizobia internalisation could eventually enable engineered crops that feed themselves.
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Nitrogen is one of the most limiting nutrients for plants. Despite its abundance in the atmosphere, plants cannot directly utilize atmospheric dinitrogen (N2). Their growth relies entirely on fixed nitrogen obtained either from soil nitrogen-fixing bacteria, a process called biological nitrogen fixation, or from the intensive application of nitrate and ammonium fertilizers. The latter has emerged as the main method to boost crop productivity since the mid-1900s. However, this practice has inflicted considerable damage on our environment. As a result, a significant challenge is the reduction of inorganic fertilizer usage within agricultural practice. Root legume endosymbiosis is one of the most efficient biological nitrogen fixation processes in which nitrogen fixing bacteria, so called rhizobia, are directly delivered within the cells of a new plant organ, the root nodule. The root nodule provides cellular microaerobic conditions for bacterial nitrogenase activity and atmospheric dinitrogen fixation. Although root legume endosymbiosis has been studied for decades, the signalling mechanism controlling the internalization of rhizobia within the root nodule cells has remained a central enigma and represents a cross-kingdom interaction that defies conventional boundaries in biological relationship. This biological process draws parallels to the endosymbiotic theory, which explains the evolution of eukaryotic cells by incorporation of free-living procaryotic cells. However, in root endosymbiosis, the internalization is not an historical relic but an ongoing, dynamic process with ecological and agricultural implications. Here, we aim to uncover and characterize the signalling pathway and mechanism allowing internalization of nitrogen-fixing rhizobia into root nodule cells. By combining state-of-the-art cell biology, molecular genetics, proteomics and single cell RNA sequencing technologies, we will identify the key factors and the minimal cellular program that enable rhizobia internalization into nodule cells. The significance of this research directly aligns with the BBSRC strategic priorities, encompassing "Understanding the rules of life" by delving into molecular processes governing bacteria endocytosis into host membrane-bound compartments. This research also resonates with BBSRC goals of "Bioscience for sustainable agriculture and food". Understanding and harnessing the potential of nitrogen-fixing bacteria have implications for improving crop production, promoting environmental sustainability, and advancing agricultural practices that are both productive and ecologically responsible.
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