ON THE COVER: Under the ground, the roots of higher plants have developed mutual relationships with microsymbionts. As representatives, arbuscular mycorrhiza (AM) symbiosis and root nodule (RN) symbiosis with rhizobial bacteria are well known. Two distinct symbiosis systems, however, share the common symbiosis pathway (CSP), which is responsible for the acceptance of different microsymbionts in legumes. Given this fact, RN symbiosis is thought to have evolved by recruiting the CSP for the ancient AM symbiosis system. In this issue, Banba et al. (pp. 1659–1671) show evidence for the direct involvement of rice CSP genes in AM symbiosis in rice. Furthermore, the results of cross-species complementation analysis between Lotus and rice indicate that rice CSP genes also manage to form normal infected nodules in the roots of Lotus. These results demonstrate that the core of the CSP has been well conserved during the evolution of RN symbiosis.
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