Function of GRAS Proteins in Root Nodule Symbiosis is Retained in Homologs of a Non-Legume, Rice

Root nodule (RN) symbiosis in legumes shares genes involved in the early signaling pathway with more ancient arbuscular mycorrhiza (AM) symbiosis, which is widespread in higher plants. The non-legume homologs of such genes have been well documented to be not only essential for the AM symbiosis in non-legume mycorrhizal plants but also functional in the RN symbiosis in legume plants. In contrast, it has not been investigated in detail whether RN symbiosis-specific genes, which are not essential for AM symbiosis, are functionally conserved in non-legumes. Two GRAS-domain transcription factors, NSP1 and NSP2, have been shown to be required for RN symbiosis, but not for AM symbiosis. In this study, we demonstrated that their homologs, OsNSP1 and OsNSP2, from rice are able to fully rescue the RN symbiosis-defective phenotypes of the mutants of corresponding genes in the model legume, Lotus japonicus. Our results indicate that some of the genes essential for RN symbiosis conserve their functions in homologs from non-legumes, which do not nodulate.