The Parastagonospora nodorum necrotrophic effector SnTox5 targets the wheat gene Snn5 and facilitates entry into the leaf mesophyll

Authors: KARIYAWASAM, GAYAN - North Dakota State University; RICHARDS, JONATHAN - Louisiana State University Agcenter; WYATT, NATHAN - Orise Fellow; RUNNING, KATHERINE - North Dakota State University; Xu, Steven; LIU, ZHAOHUI - North Dakota State University; BOROWICZ, PAWEL - North Dakota State University; Faris, Justin; Friesen, Timothy

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/27/2021
Publication Date: 7/6/2021
Citation: Kariyawasam, G.K., Richards, J.K., Wyatt, N.A., Running, K., Xu, S.S., Liu, Z., Borowicz, P., Faris, J.D., Friesen, T.L. 2021. The Parastagonospora nodorum necrotrophic effector SnTox5 targets the wheat gene Snn5 and facilitates entry into the leaf mesophyll. New Phytologist. https://doi.org/10.1111/nph.17602.
DOI: https://doi.org/10.1111/nph.17602

Interpretive Summary: Parastagonospora nodorum is a destructive pathogen causing septoria nodorum blotch (SNB) disease of wheat. This pathogen causes disease by inducing necrotic spots on the leaves that reduce the photosynthetic area, resulting in yield and quality losses to growers. Understanding how this pathogen induces disease to complete its pathogenic life cycle to cause epidemics is critical to successful resistance breeding and the control of the SNB disease problem. In the current work, we identified and characterized the protein SnTox5 to show that this protein targets the wheat gene Snn5 to induce leaf necrosis. The SnTox5 protein was important to the movement of the pathogen past the epidermal layer of the cell into the mesophyll (middle) layer as well as being critical in promoting colonization of this layer to obtain nutrients. The characterization of the SnTox5-Snn5 interaction is important for other scientists working on this and other necrotrophic pathosystems as well as for breeders trying to breed for durable resistance to septoria nodorum blotch of wheat.

Technical Abstract: Parastagonospora nodorum, causal agent of septoria nodorum blotch, is a destructive necrotrophic fungal pathogen of wheat. P. nodorum is known to secrete several necrotrophic effectors that target wheat susceptibility genes that trigger classical biotrophic resistance responses but resulting in susceptibility rather than resistance. SnTox5 targets the wheat susceptibility gene Snn5 to induce necrosis. In this study, we used full genome sequences of 197 P. nodorum isolates collected from the US and their disease phenotyping on the Snn5 differential line LP29, to perform genome wide association study analysis to localize the SnTox5 gene to chromosome 8 of P. nodorum. SnTox5 was validated using gene transformation and CRISPR-Cas9 based gene disruption. SnTox5 encoded for a small secreted protein with a 22 and 45 amino acid secretion signal and a pro sequence, respectively. The SnTox5 gene is under purifying selection in the Upper Midwest but under strong diversifying selection in the South/East regions of the US. Comparison of wild type and SnTox5-disrupted strains on wheat lines with and without the susceptibility target Snn5 showed that SnTox5 has two functions, 1) facilitating colonization of the mesophyll layer and 2) targeting Snn5 to induce PCD to provide cellular nutrient to complete its necrotrophic life cycle.