A new ToxA haplotype in the wheat fungal pathogen Bipolaris sorokiniana.

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.

A Race Profile of Tan Spot in Australia Reveals Race 2 Isolates Harboring ToxC1.

Tan spot disease is caused by Pyrenophora tritici-repentis (Ptr), one of the major necrotrophic fungal pathogens that affects wheat crops globally. Extensive research has shown that the necrotrophic fungal effectors ToxA, ToxB, and ToxC underlie the genetic interactions of Ptr race classification. ToxA and ToxB are both small proteins secreted during infection; however, the structure of ToxC remains unknown. In line with the recent discovery of the ToxC1 gene that is involved in ToxC production, a subset of 68 isolates collected from the Australian wheat cropping regions were assessed for the presence of all three effectors by pathotyping against four tan spot wheat differential lines and PCR amplification of ToxA, ToxB, and ToxC1. Based on the disease phenotypes, the 68 isolates were grouped into two races with 63 classified as race 1 and five as race 2. A representative selection of each race was tested against eight Australian commercial wheat cultivars and showed no distinction between the virulence levels. Sequencing of ToxA showed that both races had identical gene sequences of haplotype PtrA1. All the race 1 isolates possessed ToxC1 but three race 2 isolates also contained ToxC1 despite being unable to induce a spreading chlorotic symptom on the ToxC differential line. Quantitative trait loci mapping confirmed the absence of the ToxC-Tsc1 association in disease response caused by the ToxC1-containing race 2 isolate; however, ToxC1 expression was detected during plant infection. Altogether, these results suggest that there is a complex regulatory process involved in the production of ToxC within the Australian race 2 isolates.

Genetic characterization of adult-plant resistance to tan spot (syn, yellow spot) in wheat.

KEY MESSAGE: QTL mapping identified key genomic regions associated with adult-plant resistance to tan spot, which are effective even in the presence of the sensitivity gene Tsn1, thus serving as a new genetic solution to develop disease-resistant wheat cultivars. Improving resistance to tan spot (Pyrenophora tritici-repentis; Ptr) in wheat by eliminating race-specific susceptibility genes is a common breeding approach worldwide. The potential to exploit variation in quantitative forms of resistance, such as adult-plant resistance (APR), offers an alternative approach that could lead to broad-spectrum protection. We previously identified wheat landraces in the Vavilov diversity panel that exhibited high levels of APR despite carrying the sensitivity gene Tsn1. In this study, we characterised the genetic control of APR by developing a recombinant inbred line population fixed for Tsn1, but segregating for the APR trait. Linkage mapping using DArTseq markers and disease response phenotypes identified a QTL associated with APR to Ptr race 1 (producing Ptr ToxA- and Ptr ToxC) on chromosome 2B (Qts.313-2B), which was consistently detected in multiple adult-plant experiments. Additional loci were also detected on chromosomes 2A, 3D, 5A, 5D, 6A, 6B and 7A at the seedling stage, and on chromosomes 1A and 5B at the adult stage. We demonstrate that Qts.313-2B can be combined with other adult-plant QTL (i.e. Qts.313-1A and Qts.313-5B) to strengthen resistance levels. The APR QTL reported in this study provide a new genetic solution to tan spot in Australia and could be deployed in wheat cultivars, even in the presence of Tsn1, to decrease production losses and reduce the application of fungicides.