Epidermal and cortical roles of NFP and DMI3 in coordinating early steps of nodulation in Medicago truncatula

Legumes have evolved the capacity to form a root nodule symbiosis with soil bacteria called rhizobia. The establishment of this symbiosis involves specific developmental events occurring both in the root epidermis (notably bacterial entry) and at a distance in the underlying root cortical cells (notably cell divisions leading to nodule organogenesis). The processes of bacterial entry and nodule organogenesis are tightly linked and both depend on rhizobial production of lipo-chitooligosaccharide molecules called Nod factors. However, how these events are coordinated remains poorly understood. Here, we have addressed the roles of two key symbiotic genes of Medicago truncatula, the lysin motif (LysM) domain-receptor like kinase gene NFP and the calcium-and calmodulin-dependent protein kinase gene DMI3, in the control of both nodule organogenesis and bacterial entry. By complementing mutant plants with corresponding genes expressed either in the epidermis or in the cortex, we have shown that epidermal DMI3, but not NFP, is sufficient for infection thread formation in root hairs. Epidermal NFP is sufficient to induce cortical cell divisions leading to nodule primordia formation, whereas DMI3 is required in both cell layers for these processes. Our results therefore suggest that a signal, produced in the epidermis under the control of NFP and DMI3, is responsible for activating DMI3 in the cortex to trigger nodule organogenesis. We integrate these data to propose a new model for epidermal/cortical crosstalk during early steps of nodulation.