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. Orthologs of CASTOR and POLLUX are ubiquitously present in both legumes and non-legumes, but their function in non-legumes remains to be elucidated. Here, we use reverse genetic approaches to demonstrate that the rice (Oryza sativa) ortholog of POLLUX, namely Os-POLLUX, is indispensible for mycorrhizal symbiosis in rice. Furthermore, we show that Os-POLLUX can restore nodulation, but not rhizobial infection, to a M. truncatula dmi1 mutant.