The symbiotic interaction between
Medicago truncatulaand Sinorhizobium melilotiresults in the formation of nitrogen-fixing nodules on the roots of the host plant. The early stages of nodule formation are induced by bacteria via lipochitooligosaccharide signals known as Nod factors (NFs). These NFs are structurally specific for bacterium–host pairs and are sufficient to cause a range of early responses involved in the host developmental program. Early events in the signal transduction of NFs are not well defined. We have previously reported that Medicago sativaroot hairs exposed to NF display sharp oscillations of cytoplasmic calcium ion concentration (calcium spiking). To assess the possible role of calcium spiking in the nodulation response, we analyzed M. truncatulamutants in five complementation groups. Each of the plant mutants is completely Nod− and is blocked at early stages of the symbiosis. We defined two genes, DMI1and DMI2, required in common for early steps of infection and nodulation and for calcium spiking. Another mutant, altered in the DMI3gene, has a similar mutant phenotype to dmi1and dmi2mutants but displays normal calcium spiking. The calcium behavior thus implies that the DMI3gene acts either downstream of calcium spiking or downstream of a common branch point for the calcium response and the later nodulation responses. Two additional mutants, altered in the NSPand HCLgenes, which show root hair branching in response to NF, are normal for calcium spiking. This system provides an opportunity to use genetics to study ligand-stimulated calcium spiking as a signal transduction event.