1. National Institute of Agrobiological Sciences, 2-1-2 Kannondai, Tsukuba, Ibaraki 305-8602, Japan
2. Bio-oriented Technology Research Advancement Institution (BRAIN), Tokyo Office, 3-18-19 Toranomon Minato-ku, Tokyo 105-0001, Japan
3. Department of Biotechnology, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
4. The Sainsbury Laboratory, John Innes Centre, Colney Lane, Norwich NR4 7UH, UK
5. John Innes Centre, Colney Lane, Norwich NR4 7UH, UK
6. Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0112, Japan
7. Kazusa DNA Research Institute, 2-6-7 Kazusa-kamatari, Kisarazu, Chiba 292-0818, Japan
8. Department of Biological Sciences, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
9. These authors contributed equally to this work
10. Present address: Genetics Institute, Ludwig Maximilians Universität (LMU), Maria-Ward-Str. 1a, D-80638 München, Germany

Correspondence to: Shinji Kawasaki^1,2 Correspondence and requests for materials should be addressed to S.K. (Email: kawasa@nias.affrc.go.jp).
The sequences have been deposited at the DNA Data Bank of Japan with the following accession numbers: LjT02K14a (AP006732), LjT02K14b (AP006733), LjT02K14c (AP006734) LjT45I15 (AP006736), LjT20F11 (AP006737), LjT46G19 (AP006735), LjT45B09a (AP006729), LjT45B09b (AP006730) and LjT45B09c (AP006731); genomic sequences (B-129 Gifu) of CASTOR (AB162016), POLLUX (AB162017), and mRNA sequences (B-129 Gifu) of CASTOR (AB162157) and POLLUX (AB162158).

The roots of most higher plants form arbuscular mycorrhiza, an ancient, phosphate-acquiring symbiosis with fungi, whereas only four related plant orders are able to engage in the evolutionary younger nitrogen-fixing root-nodule symbiosis with bacteria¹. Plant symbioses with bacteria and fungi require a set of common signal transduction components that redirect root cell development^{2, 3}. Here we present two highly homologous genes from Lotus japonicus, CASTOR and POLLUX, that are indispensable for microbial admission into plant cells and act upstream of intracellular calcium spiking⁴, one of the earliest plant responses to symbiotic stimulation. Surprisingly, both twin proteins are localized in the plastids of root cells, indicating a previously unrecognized role of this ancient endosymbiont in controlling intracellular symbioses that evolved more recently.

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