Heterologous Expression of Rhizobial CelC2 Cellulase Impairs Symbiotic Signaling and Nodulation in Medicago truncatula

The infection of legume plants by rhizobia is tightly regulated to ensure accurate bacterial penetration, infection, and development of functionally efficient nitrogen-fixing root nodules. Rhizobial Nod factors (NF) have key roles in the elicitation of nodulation signaling. Infection of white clover roots also involves the tightly regulated specific breakdown of the noncrystalline apex of cell walls in growing root hairs, which is mediated by Rhizobium leguminosarum bv. trifolii cellulase CelC2. Here, we have analyzed the impact of this endoglucanase on symbiotic signaling in the model legume Medicago truncatula. Ensifer meliloti constitutively expressing celC gene exhibited delayed nodulation and elicited aberrant ineffective nodules, hampering plant growth in the absence of nitrogen. Cotreatment of roots with NF and CelC2 altered Ca2+ spiking in root hairs and induction of the early nodulin gene ENOD11. Our data suggest that CelC2 alters early signaling between partners in the rhizobia-legume interaction.