Regulation of the immunity-related VIK–APK–EDS1 pathway in <i>Medicago</i> for resistance to <i>Phytophthora</i>

Root rot, induced by Phytophthora medicaginis, causes devastating damage to perennial alfalfa (Medicago sativa). However, the mechanism by which P. medicaginis infects Medicago remains elusive. Here, we identified the VASCULAR HIGHWAY 1-INTERACTING KINASE (VIK)-ANKYRIN PROTEIN KINASE (APK)-ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1) pathway during P. medicaginis infection in Medicago truncatula. MtAPK is an autoimmune gene, and Mtapk-mediated autoimmunity depends on MtEDS1. P. medicaginis infection triggers MtVIK to phosphorylate Ser20 of MtAPK, enhancing the interaction between MtAPK and MtEDS1 in the cytoplasm and constraining the nuclear resistance of MtEDS1. Disease resistance could be enhanced not only by knocking out MtVIK but also by the Ser20Ala site mutation of MtAPK. Interestingly, we found that alfalfa germplasms with lower MsVIK expression after inoculation with P. medicaginis exhibited greater disease resistance. Furthermore, CRISPR/Cas9 editing of MsVIK mutants in alfalfa resulted in stronger disease resistance without growth or yield penalties. Taken together, VIK is a negative regulator of Medicago immunity and has significant potential for cultivating durable resistance in crops through genetic modification.