An HMA-like integrated domain in the wheat tandem kinase WTK4 recognises an RNase-like pathogen effector

Proteins with a tandem kinase structure have recently emerged as players in race-specific resistance in cereal crops. However, the molecular understanding of these immune receptors' resistance mechanisms is limited by the lack of knowledge about the pathogen effectors that they recognise. In this work, we identify AvrWTK4, the wheat powdery mildew RNase-like effector recognised by the wheat tandem kinase protein WTK4, through a combination of bi-parental genetic mapping and mutagenesis. We demonstrate that mutations in the AvrWTK4 gene or a reduction of its expression lead to virulence on WTK4. Transfection of AvrWTK4 specifically induced cell death in WTK4-expressing Aegilops tauschii protoplasts. The avirulent AvrWTK4 variant interacts more strongly than the virulent variant with the N-terminal heavy metal-associated (HMA)-like domain of WTK4. We found that the integrated HMA-like domain possibly serves as a direct effector-binding decoy for WTK4, providing a mechanistic paradigm for effector recognition by tandem kinase proteins.