Antiquity and function of CASTOR and POLLUX, the twin ion channel-encoding genes key to the evolution of root symbioses in plants

Root symbioses with arbuscular mycorrhizal fungi and rhizobial bacteria share a common signaling pathway in legumes. Among the common symbiosis genes are CASTOR and POLLUX, the twin homologous genes in Lotus japonicus that encode putative ion channel proteins. Here, we show that the orthologs of CASTOR and POLLUX are ubiquitously present and highly conserved in both legumes and nonlegumes. Using rice (Oryza sativa) as a study system, we employ reverse genetic tools (knockout mutants and RNA interference) to demonstrate that Os-CASTOR and Os-POLLUX are indispensable for mycorrhizal symbiosis in rice. Furthermore, a cross-species complementation test indicates that Os-POLLUX can restore nodulation, but not rhizobial infection, to a Medicago truncatula dmi1 mutant.