The soybean GTPase RAC1 interacts with the rhizobial effector NopC to promote root nodulation and increase yield

Rhizobial type-Ⅲ effectors (T3Es) contribute to establishing symbiotic interactions with legume host plants, alongside Nod factors. However, the functions of most rhizobial T3Es, as well as the regulatory and molecular mechanisms underlying their symbiotic effects on hosts, particularly in soybean, are poorly documented. Here, we characterize the function of the T3E Nodulation Outer Protein C (NopC) in the broad-host-range rhizobium Sinorhizobium fredii HH103 for promoting symbiosis in soybean. NopC genotype influences root nodulation across diverse host germplasm and this is further influenced by GmRAC1, encoding a ROP/RAC family GTPase in soybean. GmRAC1 physically interacts with NopC to subsequently induce the expression of the essential symbiotic genes GmNIN2a/2b and GmENOD40. Knock-down of GmNIN2a/2b results in NopC failing to promote symbiosis, and Gmrac1 mutants have fewer nodules than the wild type. NopC facilitates multiple infection stages whereas the requirement for GmRAC1 is pronounced for infection-thread progression and nodule-primordia initiation. Natural variation in the GmRAC1 promoter largely dictates the symbiotic contribution of NopC during symbiotic establishment, and elite GmRAC1 haplotypes with strong expression were artificially selected in soybean breeding. Transgenic over-expression level and elite GmRAC1 haplotypes increase plant height, 100-seed weight and soybean yield. GmRAC1 serves as a key regulator of NopC-mediated symbiosis promotion and offers translational potential for enhanced symbiotic nitrogen fixation in molecular breeding of soybean.