A functionally conserved Zn₂Cys₆ binuclear cluster transcription factor class regulates necrotrophic effector gene expression and host‐specific virulence of two major Pleosporales fungal pathogens of wheat

Abstract

SUMMARYThe fungus Parastagonospora nodorum is the causal agent of Septoria nodorum blotch of wheat (Triticum aestivum). The interaction is mediated by multiple fungal necrotrophic effector-dominant host sensitivity gene interactions. The three bestcharacterized effector-sensitivity gene systems are SnToxATsn1, SnTox1-Snn1 and SnTox3-Snn3. These effector genes are highly expressed during early infection, but expression decreases as the infection progresses to tissue necrosis and sporulation. However, the mechanism of regulation is unknown. We have identified and functionally characterized a gene, referred to as PnPf2, which encodes a putative zinc finger transcription factor. PnPf2 deletion resulted in the down-regulation of SnToxA and SnTox3 expression. Virulence on Tsn1 and Snn3 wheat cultivars was strongly reduced. The SnTox1-Snn1 interaction remained unaffected. Furthermore, we have also identified and deleted an orthologous PtrPf2 from the tan spot fungus Pyrenophora triticirepentis which possesses a near-identical ToxA that was acquired from P. nodorum via horizontal gene transfer. PtrPf2 deletion also resulted in the down-regulation of PtrToxA expression and a near-complete loss of virulence on Tsn1 wheat. We have demonstrated, for the first time, evidence for a functionally conserved signalling component that plays a role in the regulation of a common/horizontally transferred effector found in two major fungal pathogens of wheat.

Evaluating the importance of the tan spot ToxA–Tsn1 interaction in Australian wheat varieties

“…Another more intriguing possibility is whether ToxA recognition by Tsn1 leads to the suppression of other host effector sensitivity genes required for additional symptoms. It is certainly evident that the ToxA– Tsn1 interaction alone is not a prerequisite for pathogenicity of race 1 Ptr isolates, and pathologists have started to recognize that race 1 Ptr isolates harbour additional uncharacterized effectors in addition to ToxA and ToxC (Moffat et al ., ; Manning & Ciuffetti, ; Rybak et al ., ).…”

“…For example, atypical disease phenotypes following infection of differential wheat lines have been noted for some Ptr isolates and postulated to be caused by novel effectors, including an American tan spot isolate that produced typical race 1 necrotic symptoms even though the isolate does not possess the ToxA gene (Ali et al ., ). In previous work, it was demonstrated that a race 1 isolate incapacitated in ToxA production, either by targeted gene deletion (Moffat et al ., ) or by disruption to the ToxA regulatory pathway via the deletion of a Zn2ys6 binuclear cluster transcription factor (Rybak et al ., ), was still able to induce chlorosis, suggesting the presence of ToxC or other chlorosis‐inducing effectors. Additionally, infection of the ToxA‐sensitive wheat line TAM 105 with a ToxA knockout mutant resulted in necrotic lesions comparable to wildtype Ptr (Manning & Ciuffetti, ).…”

The global regulator of pathogenesis PnCon7 positively regulates Tox3 effector gene expression through direct interaction in the wheat pathogen Parastagonospora nodorum

“…One possibility is that PnCon7 may act upstream of ToxA and Tox1 and influence other regulatory proteins. Indeed, previous studies have identified multiple transcription factors have a role regulating ToxA and Tox3 expression (IpCho et al ., ; Rybak et al ., ). Moreover, there is no evidence to suggest a transcription factor regulates Tox1 expression.…”

“…More recently, Rybak et al . () showed that the GAL4‐like PnPf2 transcription factor was required for both ToxA and Tox3 expression. Unlike the SnStuA mutants, the PnPf2 mutants in P. nodorum appeared phenotypically comparable to the wild‐type strain implying that the regulatory role of the transcription factor is more specific.…”

“…Several reports have now provided evidence that the expression of ToxA and Tox3 is regulated, either directly or indirectly, by transcription factors. A Zn2Cys6 zinc finger transcription factor (PnPf2) has been found to positively regulate the expression of ToxA and Tox3 and isolates of P. nodorum lacking PnPf2 results in loss of pathogenicity on the ToxA and Tox3 sensitive wheat lines (Rybak et al ., ). PnPf2 is an ortholog of the Alternaria brassicicola AbPf2 which is also a Gal4‐like Zn2Cys6 zinc finger transcription factor; similarly, deletion of this gene causes loss of pathogenicity (Cho et al ., ).…”