A triple threat: the Parastagonospora nodorum SnTox267 effector exploits three distinct host genetic factors to cause disease in wheat

Abstract

Parastagonospora nodorum is a fungal pathogen of wheat. As a necrotrophic specialist, it deploys effector proteins that target dominant host susceptibility genes to elicit programmed cell death (PCD). Here we identify and functionally validate the effector targeting the host susceptibility genes Snn2, Snn6 and Snn7.We utilized whole-genome sequencing, association mapping, gene-disrupted mutants, gain-of-function transformants, virulence assays, bioinformatics and quantitative PCR to characterize these interactions.A single proteinaceous effector, SnTox267, targeted Snn2, Snn6 and Snn7 to trigger PCD. Snn2 and Snn6 functioned cooperatively to trigger PCD in a light-dependent pathway, whereas Snn7-mediated PCD functioned in a light-independent pathway. Isolates harboring 20 SnTox267 protein isoforms quantitatively varied in virulence. The diversity and distribution of isoforms varied between populations, indicating adaptation to local selection pressures. SnTox267 deletion resulted in the upregulation of effector genes SnToxA, SnTox1 and SnTox3.We validated a novel effector operating in an inverse-gene-for-gene manner to target three genetically distinct host susceptibility genes and elicit PCD. The discovery of the complementary gene action of Snn2 and Snn6 indicates their potential function in a guard or decoy model. Additionally, differences in light dependency in the elicited pathways and upregulation of unlinked effectors sheds new light onto a complex fungal necrotroph-host interaction.

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

“…New Phytologist K2PP effector interaction for further study of this class of effectors. Outram et al (2020) presented the crystal structure of SnTox3, which was used by the protein structuring server PHYRE2 to model the 3D structure of both SnTox5 and SnTox267 (Richards et al, 2021). PHYRE2 was able to model 98% of the structure of the mature SnTox5 with 100% confidence, whereas it was able to model only 35% of the mature SnTox267 with 92.6% confidence, which suggests structural homology between SnTox3 and SnTox5 and, to a lesser extent, SnTox3 and SnTox267.…”

“…To date, nine such interactions have been identified, including SnToxA-Tsn1 (Friesen et al, 2006;Faris et al, 2010), SnTox1-Snn1 (Liu et al, 2004;Shi et al, 2016), SnTox2-Snn2 (Friesen et al, 2007), SnTox3-Snn3-B1 (Friesen et al, 2008), SnTox3-Snn3-D1 (Zhang et al, 2011;Zhang et al, 2021), SnTox4-Snn4 (Abeysekara et al, 2009), SnTox5-Snn5 (Friesen et al, 2012), SnTox6-Snn6 (Gao et al, 2015) and SnTox7-Snn7 (Shi et al, 2015). Even though SnTox2, SnTox6 and SnTox7 were initially hypothesized to be three different effectors that targeted three separate sensitivity genes, Richards et al (2021) recently showed that these three effectors were in fact the same protein, which was designated as SnTox267. Owing to the number of characterized interactions, the wheat-P. nodorum system is recognized as a model for the study of necrotrophic specialist pathogens (Oliver et al, 2012;Faris & Friesen, 2020).…”

“…Liu et al (2016) showed that in addition to targeting Snn1, SnTox1 had the ability to bind chitin and protect the fungal cell wall from wheat chitinases. The most recent necrotrophic effector gene that was cloned in this system was SnTox267, which encoded for a 27.4 kDa mature protein that targeted three sensitivity genes in two different pathways to induce necrosis (Richards et al, 2021).…”

Reactive Oxygen Species in Host Plant Are Required for an Early Defense Response against Attack of Stagonospora nodorum Berk. Necrotrophic Effectors SnTox

“…To date, eight Snn-SnTox interactions are known [17]. In addition to the three main interactions Snn1-SnTox1, Snn3-SnTox3, and Tsn1-SnToxA, several other interactions have been identified, such as Snn2-SnTox2, Snn4-SnTox4, Snn5-SnTox5, Snn6-SnTox6 and Snn7-SnTox7 [18][19][20][21][22][23]. The effect of each Snn-SnTox interaction is incomplete and is complemented by other interactions.…”

Variability in an effector gene promoter of a necrotrophic fungal pathogen dictates epistasis and effector-triggered susceptibility in wheat

“…nodorum- wheat pathosystem. These are ToxA- Tsn1 [ 13 , 14 ], Tox1- Snn1 [ 15 , 16 ], Tox267- Snn2 / Snn6 / Snn7 [ 17 ], Tox3- Snn3B1 / Snn3D1 [ 18 , 19 ], Tox4- Snn4 [ 20 ], Tox5- Snn5 [ 21 ] and Tox2A- Qsnb . cur–2AS1 [ 22 ].…”