The rust effector PstCFEM2 manipulates TaHA2 and TaCIPK9–mediated apoplastic acidification to promote wheat susceptibility

Apoplastic acidification represents a pivotal mechanism in the co-evolutionary dynamics between plants and pathogens. However, the mechanisms underlying this process remain largely uncharacterized. In this study, we unveil a mechanism by which the stripe rust fungal (Puccinia striiformis f. sp. tritici; Pst) effector manipulates plasma membrane (PM) H+-ATPases to promote apoplastic acidification and attenuate host immune responses. We identified a wheat (Triticum aestivum) PM H+-ATPase (TaHA2) as a key regulator of apoplastic pH and defense responses to Pst infection. The overexpression of TaHA2 exacerbated apoplastic acidification and Pst susceptibility, whereas the CRISPR-Cas9-mediated inactivation of TaHA2 in wheat conferred broad-spectrum resistance against multiple rust pathogens without compromising agronomic traits. Mechanistically, we found that the wheat calcineurin B-like interacting protein kinase 9 (TaCIPK9) phosphorylates TaHA2 at Ser-933, triggering intramolecular interactions between its C-terminal autoinhibitory domain and the central loop, thereby suppressing TaHA2 activity. Conversely, the CFEM (common in fungal extracellular membrane)-containing Pst effector PstCFEM2 competitively binds to the C-terminus of TaHA2, disrupting TaCIPK9-mediated phosphorylation and relieving autoinhibition. This effector-driven activation of TaHA2 amplifies apoplastic acidification and stomatal opening, ultimately dampening plant immunity. Our findings reveal a mechanism by which pathogens promote infection by subverting host pH regulation and provide a theoretical framework for engineering disease resistance through the manipulation of susceptibility genes.