Abstract: Mitogen-activated protein kinase (MAPK) cascades play a fundamental role in plant immunity by transducing external signals inside plant cells. Here, we defined a wheat MAPK cascade, composed of the mitogen-activated protein kinase kinase (MAPKK) TaMKK2 and its downstream MAPK TaMAPK6, which phosphorylates the core immune regulator TaSGT1 (suppressor of G2 allele of Skp1), resulting in enhanced nuclear entry of TaSGT1, thereby conferring resistance against the devastating wheat pathogen Puccinia striiformis f. sp. tritici (Pst). Hence, we identified a TaMKK2-TaMAPK6-TaSGT1 signaling cascade that contributes to wheat stripe rust resistance. During infection, Pst secrets a haustorium-associated secreted protein 215 (HASP215), that targets TaMKK2 and interferes with the interaction of TaMKK2 with TaMAPK6 to suppress TaMAPK6 phosphorylation and activation, thereby leading to reduced capacity of TaMAPK6 to phosphorylate TaSGT1. Consequently, inhibition of TaMAPK6-mediated TaSGT1 phosphorylation resulted in decreased nuclear translocation of TaSGT1 and suppressed plant immunity. Our work elucidates the positive function of TaMKK2-TaMAPK6 cascade in wheat immunity by regulating the immune component TaSGT1, and its regulation by the rust effector HASP215, providing new insights into the MAPK cascade on crop immunity and the pathogenicity mechanism of obligate biotrophic fungus.

Key words: effector, MAPK cascades, Puccinia striiformis f. sp. Tritici, SGT1, wheat