Genetic analysis of calcium spiking responses in nodulation mutants of Medicago truncatula

The symbiotic interaction between Medicago truncatula and Sinorhizobium mel iloti results in the formation of nitrogen-fixing nodules on the roots of t he 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 c ause a range of early responses involved in the host developmental program. Early events in the signal transduction of NFs are not well defined. We ha ve previously reported that Medicago sativa root hairs exposed to NF displa y sharp oscillations of cytoplasmic calcium ion concentration (calcium spik ing). To assess the possible role of calcium spiking in the nodulation resp onse, we analyzed M. truncatula mutants in five complementation groups. Eac h of the plant mutants is completely Nod(-) and is blocked at early stages of the symbiosis, We defined two genes, DMI1 and DMI2, required in common f or early steps of infection and nodulation and for calcium spiking. Another mutant, altered in the DMI3 gene, has a similar mutant phenotype to dmi1 a nd dmi2 mutants but displays normal calcium spiking. The calcium behavior t hus implies that the DMI3 gene 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 NSP and Ha ge nes, which show root hair branching in response to NF, are normal for calci um spiking. This system provides an opportunity to use genetics to study li gand-stimulated calcium spiking as a signal transduction event.