The Cysteine Rich Necrotrophic Effector SnTox1 Produced by <em>Stagonospora nodorum</em> Triggers Susceptibility of Wheat Lines Harboring <em>Snn1</em>

<div><p>The wheat pathogen <em>Stagonospora nodorum</em> produces multiple necrotrophic effectors (also called host-selective toxins) that promote disease by interacting with corresponding host sensitivity gene products. SnTox1 was the first necrotrophic effector identified in <em>S. nodorum</em>, and was shown to induce necrosis on wheat lines carrying <em>Snn1</em>. Here, we report the molecular cloning and validation of <em>SnTox1</em> as well as the preliminary characterization of the mechanism underlying the SnTox1-<em>Snn1</em> interaction which leads to susceptibility. <em>SnTox1</em> was identified using bioinformatics tools and verified by heterologous expression in <em>Pichia pastoris</em>. <em>SnTox1</em> encodes a 117 amino acid protein with the first 17 amino acids predicted as a signal peptide, and strikingly, the mature protein contains 16 cysteine residues, a common feature for some avirulence effectors. The transformation of <em>SnTox1</em> into an avirulent <em>S. nodorum</em> isolate was sufficient to make the strain pathogenic. Additionally, the deletion of <em>SnTox1</em> in virulent isolates rendered the <em>SnTox1</em> mutated strains avirulent on the <em>Snn1</em> differential wheat line. <em>SnTox1</em> was present in 85% of a global collection of <em>S</em>. <em>nodorum</em> isolates. We identified a total of 11 protein isoforms and found evidence for strong diversifying selection operating on <em>SnTox1</em>. The SnTox1-<em>Snn1</em> interaction results in an oxidative burst, DNA laddering, and pathogenesis related (PR) gene expression, all hallmarks of a defense response. In the absence of light, the development of SnTox1-induced necrosis and disease symptoms were completely blocked. By comparing the infection processes of a GFP-tagged avirulent isolate and the same isolate transformed with <em>SnTox1</em>, we conclude that SnTox1 may play a critical role during fungal penetration. This research further demonstrates that necrotrophic fungal pathogens utilize small effector proteins to exploit plant resistance pathways for their colonization, which provides important insights into the molecular basis of the wheat-<em>S. nodorum</em> interaction, an emerging model for necrotrophic pathosystems.</p>