A legume cellulase required for rhizobial infection and colonization in root nodule symbiosis

In root nodule symbiosis, the accommodation of rhizobia in legumes necessitates extensive plant cell wall remodeling to build infection threads (ITs) for rhizobia travelling into nodules, and to subsequently release rhizobia from ITs to form nitrogen fixing symbiosomes. The molecular and cellular mechanisms underlying these processes are obscure. Here we report that Medicago truncatula Glycoside Hydrolase 9C2 (GH9C2) is required for both rhizobial infection and nodule colonization. The gh9c2-1 mutant exhibits incompetent nodules with disorganized ITs and defective rhizobial release, likely due to cellulose accumulation. GH9C2 localizes to IT wall and rhizobial release sites, and cellulase activity is indispensable for GH9C2 function. CBM49 domain of GH9C2 is required for rhizobial infection but not for rhizobial release. Furthermore, GH9C1and NPL act synergistically with GH9C2 in rhizobial infection. Our finding reveals transient IT-derived structures, the rhizobial release foci and uncovers a mechanism mediated by host cellulases for the symbiotic colonization by rhizobia.