P-440 - Integrative microscopy of the soybean symbiosome - Deciphering molecular interactions at the symbiosome interface of the soybean-Bradyrhizobium symbiosis.

Biological nitrogen fixation is crucial to the cultivation of leguminous crops, where bacteria capture atmospheric nitrogen and deliver nitrogen compounds to the plant. Soybean forms a symbiosis with Bradyrhizobium species, where bacteria enter specific root organs (“nodules") and transform into nitrogen fixing organelles, the “symbiosomes”. Whilst the initial steps of symbiosis are increasingly understood, the molecular landscape of proteins at play at the symbiosome interface remains poorly characterised. Here, I combine plant cell biology, electron microscopy and biophysics to structurally characterise this plant-microbe interface, focussing on protein complexes involved in nutrient exchange and communication. I set up a pipeline for explorative characterisation of nodule development, using cell sorting and cryo-FIB/SEM to visualise the nanometre-scale structures at play at the symbiosome interface. In addition, specific protein complexes active at the symbiosome interface are identified from a single nuclei transcriptomic atlas and structurally characterised using confocal microscopy, biophysics and single particle cryo-EM. Together, these approaches yield unprecedented detail of molecular processes and protein composition at the symbiosome membranes offering the foundation to guide future efforts in optimising biological nitrogen fixation in soybean.