Defense Recognition of a Stripe Rust Fungal Effector Is Uncoupled from Disease Outcomes in Wheat

Plant resistance (R) and pathogen avirulence (Avr) gene interactions are central to pathogen recognition and disease resistance in crops. Functional characterization of recognized Avr effectors of Puccinia striiformis f. sp. tritici (Pst) lags other key fungal pathogens of wheat. Here, we used a wheat protoplast-based screen to identify Avr/R interactions via the proxy of effector-induced defense responses in a set of diverse wheat cultivars. We identified an Avr candidate, termed AvrPstB48, that triggers defense responses in 16 out of 24 cultivars tested. AvrPstB48 is hemizygous, and the Pst genome carries four divergent paralogs within a gene cluster. Analysis of these paralogs revealed partial redundancy in their ability to activate wheat defenses and enabled us to identify a single amino acid in AvrPstB48 that is necessary but not sufficient for defense activation. Notably, the activation of defense signaling by AvrPstB48 in protoplasts did not directly correlate with disease outcomes. Whole-plant infection assays revealed that some cultivars that exhibited strong defense activation in the protoplast assay are susceptible to the Pst isolate Pst104E137A, from which AvrPstB48 is derived. Comparison of the infection dynamics of two wheat cultivars that differ in their AvrPstB48 recognition capacity revealed a delay in disease progression in the recognizing cultivar Avocet S compared with the non-recognizing cultivar Morocco. Although correlative only, our observations, combined with other recent reports, support a "recognize-then-suppress" model of plant-pathogen interaction in which disease outcomes are driven not only by simple Avr/R interactions but also by pathogen effectors that suppress defense signaling downstream of effector recognition. [Formula: see text] Copyright © 2026 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.