Department of Life Sciences, School of Agriculture, Meiji University, 1-1-1, Higashimita, Tama-ku, Kawasaki, Kanagawa, 214-8571 Japan.
School of Biological and Environmental Sciences, Kwansei Gakuin University, Gakuen Uegahara 1, Sanda, Hyogo, 669-1330 Japan.
Plant Cell Physiol. 2023 Apr 17;64(4):378-391. doi: 10.1093/pcp/pcad006.
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 are yet to be elucidated, particularly in nonleguminous plants. We previously demonstrated that chitin elicitor receptor kinase 1 (OsCERK1), a lysin motif 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 are yet to be fully elucidated. SYMBIOSIS RECEPTOR-LIKE KINASE (SYMRK)/DOES NOT MAKE INFECTIONS 2 (DMI2) is a leucine-rich repeat receptor-like kinase associated with both root nodule symbiosis and AM symbiosis in legumes. The homolog of SYMRK in rice, OsSYMRK, has a shorter form than that 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 genome editing 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 the wild type were observed in Ossymrk. Bimolecular fluorescence complementation studies demonstrating an interaction between OsSYMRK and OsCERK1 indicate that OsSYMRK may play an important role in switching from immunity to symbiosis through the interaction with OsCERK1 in rice.
丛枝菌根（AM）真菌与包括水稻在内的广泛陆地植物建立共生关系。然而，AM 共生起始的机制仍有待阐明，特别是在非豆科植物中。我们之前的研究表明，几丁质激发受体激酶 1（OsCERK1），一种对几丁质触发免疫至关重要的溶菌酶基序受体样激酶，在水稻的 AM 共生中也发挥关键作用。然而，OsCERK1 调控免疫与共生之间转换的机制仍有待充分阐明。共生受体样激酶（SYMRK）/不产生感染 2（DMI2）是一种富含亮氨酸重复受体样激酶，与豆科植物的根瘤共生和 AM 共生都有关。水稻中 SYMRK 的同源物 OsSYMRK 的形式比豆科植物的短，因为 OsSYMRK 缺乏甘露糖结合凝集素样结构域（MLD）。据报道，MLD 有助于 Lotus japonicus 的共生；然而，OsSYMRK 对水稻 AM 共生的贡献尚不清楚。系统发育分析表明，SYMRK/DMI2 的 MLD 甚至在苔藓植物和蕨类植物中广泛保守，但在包括水稻在内的鸭跖草目中不存在。为了了解 OsSYMRK 的功能，我们使用基因组编辑技术产生了一个 Ossymrk 敲除突变体。与 Oscerk1 相比，Ossymrk 的 AM 定殖大部分被消除，表型更严重。对几丁质四聚体的 Ca2+ 爆发也在 Ossymrk 中减弱。相比之下，在 Ossymrk 中观察到与野生型相当的几丁质七聚体的防御反应。证明 OsSYMRK 与 OsCERK1 之间相互作用的双分子荧光互补研究表明，在水稻中，OsSYMRK 可能通过与 OsCERK1 的相互作用在从免疫到共生的转换中发挥重要作用。
