A New ToxA Haplotype in the Wheat Fungal Pathogen Bipolaris sorokiniana

Abstract

The necrotrophic effector ToxA is a well-studied virulence factor produced by several fungal necrotrophs. Initially cloned from the wheat tan spot pathogen Pyrenophora tritici-repentis in 1996, ToxA was found almost a decade later in another fungal pathogen Parastagonospora nodorum and its sister species Parastagonospora pseudonodorum. In 2018, ToxA was detected in a third wheat fungal pathogenic species: Bipolaris sorokiniana that causes spot blotch disease. However, unlike the case with P. tritici-repentis and P. nodorum, the ToxA in B. sorokiniana has been investigated in recent years only. In this report, five Australian B. sorokiniana isolates were assessed for the presence of ToxA. Four isolates were found to contain ToxA. While one isolate harboured the previously reported ToxA haplotype sequence (ToxA19), three isolates contain a different haplotype, designated herein as ToxA25 that has a non-synonymous mutation resulting in an amino acid change of glycine to arginine at position 168. Both B. sorokiniana ToxA isoforms, when heterologously expressed in Escherichia coli, exhibited the classic ToxA-necrosis inducing activity on ToxA sensitive Tsn1 cultivars. Preliminary analysis of the B. sorokiniana isolates on Australian wheat cultivars showed that isolates with ToxA19, ToxA25 or ToxA-deficient displayed varying degrees of virulence, with the most aggressive isolates observed for those producing ToxA. Differences in spot blotch disease severity between Tsn1 and tsn1 cultivars were observed, however this was not limited to the ToxA-producing isolates. The overall results suggests that the virulence of the Australian B. sorokiniana isolates is diverse with the significance of ToxA-Tsn1 interactions depending on individual isolates.