Effects of Pathogen Load and Host Genetic Background on Tandem Kinase-Mediated Resistance in Wheat

Wheat is a critical global food source, yet its production is threatened by stripe rust caused by Puccinia striiformis f. sp. tritici (Pst), which reduces yields by ~5 Mt annually. The resistance gene Yr15, derived from wild emmer wheat, encodes the tandem kinase protein WTK1 and confers wide-spectrum resistance to more than 2,000 Pst isolates. Here, we examined how WTK1-mediated resistance is shaped by pathogen load, isolate identity, and host genetic background, with emphasis on the histopathological dynamics of infection. Susceptible genotypes Kronos S (tetraploid) and Avocet S (hexaploid) showed stable levels of susceptibility across inoculum gradients, whereas their near-isogenic lines carrying WTK1 (Kronos R and Avocet R) exhibited dose-dependent hypersensitive responses, with Avocet R showing slightly stronger reactions. An inoculum of 10 mg/ml consistently distinguished susceptible from resistant responses, providing a reliable threshold for phenotyping. Importantly, WTK1-carrying lines resisted all tested isolates, including the highly virulent Pst#5006. A 336-hour post-inoculation (hpi) time-course revealed that fungal growth diverged between resistant and susceptible plants beginning at 144 hpi. In Kronos R, fungal colonies were detectable up to 96 hpi but were subsequently curtailed by localized programmed cell death. Biomass quantification confirmed no significant increase in fungal load in WTK1 lines from 12-336 hpi. Microscopic analysis further showed that defense activation occurred after haustorium formation, indicating a post-haustorial mechanism of resistance. Together, these findings provide the first detailed temporal map of tandem kinase protein mediated defense in wheat and underscore the robustness of WTK1 across pathogen pressures.