Figure 3.

Alternative models for placement of calcium spiking with respect to NF signal transduction. (A) Data in this paper show that DMI1 and DMI2 encode genes required in common upstream of calcium spiking and of multiple nodulation behaviors. Calcium spiking may lie directly upstream of one or more of the known nodulation responses, such as transcription or morphogenesis. In this case, DMI3 may lie downstream of calcium spiking and could potentially function as a calcium-sensitive effector protein that translates calcium signals to other cellular components. (B) Calcium spiking may occur as a branch off of the pathway that ultimately leads to root hair branching, early nodulin (ENOD) gene expression, and cortical cell division (ccd). The DMI1 and DMI2 loci lie upstream of the branch point, and DMI3 must lie downstream, as should NSP. HCL might act downstream of earlier NF signal transduction events or in a separate pathway altogether. The events or components required for NF-induced root hair swelling in the dmi1, dmi2, and dmi3 mutants must lie upstream of calcium spiking. If calcium spiking is not in the main signal transduction pathway but is stimulated by NF and by early steps in signal transduction, it may nonetheless have a function related to nodulation.