Arbuscular mycorrhizal (AM) fungi establish mutualistic symbiosis with a wide range of terrestrial plants, including rice. However, the mechanisms underlying the initiation of AM symbiosis have yet to be elucidated, particularly in nonleguminous plants. We previously demonstrated that OsCERK1, a lysin motif (LysM) receptor-like kinase essential for chitin-triggered immunity, also plays a key role in AM symbiosis in rice. However, the mechanisms underlying the regulation of switching between immunity and symbiosis by OsCERK1 have yet to be fully elucidated. SYMRK/DMI2 is a leucine-rich repeat receptor-like kinase associated with both root nodule symbiosis and AM symbiosis in legumes. The homologue of SYMRK in rice, OsSYMRK, has a shorter form than in legumes because OsSYMRK lacks a malectin-like domain (MLD). The MLD reportedly contributes to symbiosis in Lotus japonicus; however, the contribution of OsSYMRK to AM symbiosis in rice remains unclear. Phylogenetic analyses indicated that the MLD of SYMRK/DMI2 is widely conserved even in mosses and ferns but absent in commelinids, including rice. To understand the function of OsSYMRK, we produced an Ossymrk knockout mutant using CRISPR/Cas9 technology. AM colonization was mostly abolished in Ossymrk with a more severe phenotype than Oscerk1. Ca2+ spiking against chitin tetramer was also diminished in Ossymrk. In contrast, comparable defense responses against chitin heptamer to wild type were observed in Ossymrk. Bimolecular fluorescence complementation studies demonstrating an interaction between OsSYMRK and OsCERK1 indicate OsSYMRK may play an important role in switching from immunity to symbiosis through the interaction with OsCERK1 in rice.
Keywords: AM symbiosis; Chitin-triggered immunity; Malectin-like domain; OsCERK1; Rice (Oryza sativa); SYMRK/DMI2.
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