Soybean GTPase RAC1 interacts with the rhizobial effector NopC to promote root nodulation and improve yield

Rhizobial type Ⅲ effectors (T3Es) contribute to the establishment of symbiotic interactions with legume host plants in conjunction with Nod factors. However, the functions of most rhizobial T3Es, as well as the regulatory and molecular mechanisms underlying their symbiotic effects-particularly in soybean-remain poorly documented. Here, we characterize the function of the T3E nodulation outer protein C (NopC) from the broad-host-range rhizobium Sinorhizobium fredii HH103 in promoting symbiosis in soybean. The NopC genotype influences root nodulation across diverse host germplasms; this effect is further modulated by GmRAC1, which encodes a ROP/RAC family GTPase in soybean. GmRAC1 physically interacts with NopC and subsequently induces expression of the essential symbiotic genes GmNIN2a/2b and GmENOD40. Knockdown of GmNIN2a/2b results in failure of NopC to promote symbiosis, and Gmrac1 mutants develop fewer nodules than the wild type. NopC facilitates multiple stages of infection, whereas the requirement for GmRAC1 is pronounced during infection-thread progression and nodule primordium initiation. Natural variation in the GmRAC1 promoter largely determines the symbiotic contribution of NopC during symbiosis establishment. Elite GmRAC1 haplotypes associated with strong expression were artificially selected during soybean breeding. Transgenic overexpression and elite GmRAC1 haplotypes increase plant height, 100-seed weight, and overall yield. GmRAC1 functions as a key regulator of NopC-mediated symbiosis promotion and offers translational potential for enhancing symbiotic nitrogen fixation in soybean molecular breeding.