RESUMO
The soybean root system is complex. In addition to being composed of various cell types, the soybean root system includes the primary root, the lateral roots, and the nodule, an organ in which mutualistic symbiosis with the N-fixing rhizobia occurs. A mature soybean root nodule is characterized by a central infection zone where the atmospheric nitrogen is fixed and assimilated by the symbiont, resulting from the close cooperation between the plant cell and the bacteria. To date, the transcriptome of individual cells isolated from developing soybean nodules has been established, but the transcriptomic signatures of the cells of the mature soybean nodule have not yet been characterized. Applying single nucleus RNA-seq and Molecular CartographyTM technologies, we precisely characterized the transcriptomic signature of the soybean root and mature nodule cell types and revealed the co-existence of different sub-populations of B. diazoefficiens-infected cells in the mature soybean nodule including those actively involved in nitrogen fixation, and those engaged in senescence. The mining of the single cell-resolution nodule transcriptome atlas and associated gene co-expression network confirmed the role of known nodulation-related genes and identified new genes controlling the nodulation process. For instance, we functionally characterized the role of GmFWL3, a plasma membrane microdomain-associated protein controlling rhizobia infection. Our study reveals the unique cellular complexity of the mature soybean nodule and helps redefine the concept of cell types when considering the infection zone of the soybean nodule.