Parastagonospora nodorum is a necrotrophic fungal pathogen causing the wheat disease septoria nodorum blotch which is causal to significant economic losses globally. As a necrotroph, P. nodorum secretes necrotrophic effectors that induce cell death responses in wheat cultivars harbouring dominant susceptibility genes. The Tox3 effector from P. nodorum elicits necrotic responses in wheat cultivars dominant for the Snn3 gene. Independent studies have shown that Tox3 also interacts with several

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... bers of wheat pathogenesis-related protein 1 family (TaPR1), although the role of the interaction in disease remains unclear. Therefore, I sought to dissect the interaction of Tox3 and TaPR1 with a focus on understanding the role of these proteins and their interaction in disease. Firstly, I utilized site-directed mutagenesis to identify a Tox3 variant, Tox3P173S, that was unable to interact with TaPR1 but could induce a necrotic response in Snn3 cultivars, suggesting the interaction is not required for necrosis. Additionally, wheat leaves pre-infiltrated with TaPR1 proteins prior to P. nodorum inoculation showed strong disease reduction. Subsequent gene expression profiling of known defence genes indicated TaPR1 induced a host defence response mediated by the conserved C-terminal peptide CAPE1. The priming activity of TaPR1 was compromised by the Tox3-TaPR1 interaction but unaffected by the non-interacting Tox3P173S variant. Furthermore, the incubation of Tox3 and TaPR1-1 in wheat apoplastic washing fluid (TaAWF) abolished the TaPR1 cleavage but the non-interacting Tox3P173S didn't affect the cleavage activity, suggesting Tox3 inhibits PR1-mediated immunity by preventing the cleavage of CAPE1 from PR1. Collectively, these data demonstrate that the Tox3 effector independently functions to induce necrosis and suppress host defence. Understanding that PR1 functions depends on the release of CAPE1, I sought to uncover the mechanisms of PR1 cleavage and the protease involved. A protease inhibition assay revealed that a serine [...]