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

Abstract

The fungus Parastagonospora nodorum uses proteinaceous necrotrophic effectors (NEs) to induce tissue necrosis on wheat leaves during infection, leading to the symptoms of septoria nodorum blotch (SNB). The NEs Tox1 and Tox3 induce necrosis on wheat possessing the dominant susceptibility genes Snn1 and Snn3B1/Snn3D1, respectively. We previously observed that Tox1 is epistatic to the expression of Tox3 and a quantitative trait locus (QTL) on chromosome 2A that contributes to SNB resistance/susceptibility. The expression of Tox1 is significantly higher in the Australian strain SN15 compared to the American strain SN4. Inspection of the Tox1 promoter region revealed a 401 bp promoter genetic element in SN4 positioned 267 bp upstream of the start codon that is absent in SN15, called PE401. Analysis of the world-wide P. nodorum population revealed that a high proportion of Northern Hemisphere isolates possess PE401 whereas the opposite was observed in representative P. nodorum isolates from Australia and South Africa. The presence of PE401 removed the epistatic effect of Tox1 on the contribution of the SNB 2A QTL but not Tox3. PE401 was introduced into the Tox1 promoter regulatory region in SN15 to test for direct regulatory roles. Tox1 expression was markedly reduced in the presence of PE401. This suggests a repressor molecule(s) binds PE401 and inhibits Tox1 transcription. Infection assays also demonstrated that P. nodorum which lacks PE401 is more pathogenic on Snn1 wheat varieties than P. nodorum carrying PE401. An infection competition assay between P. nodorum isogenic strains with and without PE401 indicated that the higher Tox1-expressing strain rescued the reduced virulence of the lower Tox1-expressing strain on Snn1 wheat. Our study demonstrated that Tox1 exhibits both ‘selfish’ and ‘altruistic’ characteristics. This offers an insight into a complex NE-NE interaction that is occurring within the P. nodorum population. The importance of PE401 in breeding for SNB resistance in wheat is discussed.

The Velvet transcription factor PnVeA regulates necrotrophic effectors and secondary metabolism in the wheat pathogenParastagonospora nodorum

“…The primers used in this study are listed in Table S2 . The gene deletion construct of PnVeA including a phleomycin selectable marker was assembled using Golden Gate cloning system (Engler et al 2008) using a modified pUC19 (New England Biolabs, Ipswich, Massachusetts, USA) as the backbone vector (John et al 2022). The PnVeA complementation construct that included a 1,021 bp promoter region, the full intact gene and 290 bp of the terminator sequence was fused to a hygromycin selectable marker via fusion PCR (Hilgarth and Lanigan 2020).…”

Novel Sources of Resistance to Stagonospora nodorum and Role of Effector-Susceptibility Gene Interactions in Wheat of Russian Breeding

“…This may be due to various regulatory mechanisms that are activated by either the pathogen or the host. On the part of the pathogen, a diversity of manifestations of one interaction can be caused with epistatic or additive effects between different SnTox-Snn interactions, which are partly associated with different levels of NE gene expression during plant infection or partly with the influence of host susceptibility genes [23,35]. On the part of the host plant, mutations contained in the Snn susceptibility gene, which lead to plant insensitivity to NE, can contribute to the diversity of responses, and, as recent data have shown, the regulation of transcription of the Snn susceptibility gene can provide resistance to the cultivar [17].…”

“…Resistance to SNB is inherited quantitatively, and it is complex. The complexity of the situation lies in the fact that multiple NEs expressed by S. nodorum isolates provide functional redundancy of the pathogen on wheat cultivars that have multiple dominant susceptibility genes [23]. There are two ways that breeders and geneticists use to discover susceptibility genes.…”

Surface frustration re-patterning underlies the structural landscape and evolvability of fungal orphan candidate effectors

“…Another layer of complexity emerges from interference between the activity of several effectors, either through epistatic interactions 24 or by cross-suppression of immune recognition 25 , 26 . Finally, effectors may promote virulence not by directly targeting host processes but rather by altering the host microbiota 27 , 28 .…”