Unraveling the rhizobial infection thread

Most legumes can form an endosymbiotic association with soil bacteria called rhizobia, which colonize specialized root structures called nodules where they fix nitrogen. To colonize nodule cells, rhizobia must first traverse the epidermis and outer cortical cell layers of the root. In most legumes, this involves formation of the infection thread, an intracellular structure that becomes colonized by rhizobia, guiding their passage through the outer cell layers of the root and into the newly formed nodule cells. In this brief review, we recount the early research milestones relating to the rhizobial infection thread and highlight two relatively recent advances in the symbiotic infection mechanism, the eukaryotically conserved 'MYB-AUR1-MAP' mitotic module, which links cytokinesis mechanisms to intracellular infection, and the discovery of the 'infectosome' complex, which guides infection thread growth. We also discuss the potential intertwining of the two modules and the hypothesis that cytokinesis served as a foundation for intracellular infection of symbiotic microbes. This review provides a brief history of rhizobial infection, and highlights two recent advances in the infection mechanism regarding the involvement of cell cycle components and the exocyst 'infectosome' complex.