Activation of calcium- and calmodulin-dependent protein kinase (CCaMK), the central regulator of plant root endosymbiosis

► AM fungi produce lipo-chitooligosaccharides structurally closely related to rhizobial Nod factors that induce lateral root formation in plants. It is thus conceivable that production of LCOs by rhizobia was a key step during the evolution of the root nodule symbiosis. ► RNAi of an NFR5-like LysM receptor like kinase (RLK) from the non-legume Parasponia led to reduced AM formation, pinpointing LysM RLKs as candidates for fungal LCO receptors. ► High frequency calcium-spiking is observed during AM fungal and rhizobial plant cell infection, thus making specific spiking signatures less likely to be responsible for differential gene induction in both symbioses. ► CCaMK emerges as the central regulator upstream of nodule organogenesis and infection. ► A direct uptake system for bacteria by plant root cortex cells was discovered that is activated by deregulated CCaMK and in this situation does not require Nod factor.