Plant roots form intracellular symbioses with fungi and bacteria resulting in arbuscular mycorrhiza and nitrogen-fixing root nodules, respectively. A novel receptor like-kinase has been discovered that is required for the transduction of both bacterial and fungal symbiotic signals. This kinase defines an ancient signalling pathway that probably evolved in the context of arbuscular mycorrhiza and has been recruited subsequently for endosymbiosis with bacteria. An ancestral symbiotic interaction of roots with intracellular bacteria might have emerged from such a recruitment, in the progenitor of the nodulating clade of plants. Analysis of symbiotic mutants of host plants and bacterial microsymbionts has revealed that present-day endosymbioses require the coordinated induction of more than one signalling pathway for development.