Pao Theen See

The increasing demand for resilient, high-yielding crops has intensified the need for effective strategies to combat biotic stresses and reduce reliance on costly and environmentally detrimental chemical controls. Mutagenesis, through physical, chemical, and site-directed approaches, complements conventional breeding by accelerating the discovery of genes and their functional characterization. This review examines the application of mutagenesis in major crop species, with particular emphasis on disease resistance research and the dissection of plant–pathogen interactions. Although relatively few mutagenized disease resistance alleles have been directly deployed to date, we highlight how mutagenesis has advanced the identification and validation of major genes controlling plant immunity, particularly susceptibility genes against a wide range of diseases, from viruses and bacteria to fungi with diverse lifestyles. A prominent example is the development of durable mlo -mediated resistance to powdery mildew, from its discovery through X-ray mutagenesis in barley to targeted editing of mlo homologs in polyploid crops. The range of plant immunity genes identified by researchers and growing regulatory approval of gene editing as an alternative to GMOs suggest accelerating adoption of the technology. Copyright © 2026 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Septoria nodorum blotch (SNB) and tan spot (TS) wheat diseases are caused by necrotrophic fungal pathogens Parastagonospora nodorum (Pn) and Pyrenophora tritici-repentis (Ptr), respectively. Although recognised as premier model pathosystems for our understanding of necrotrophic effectors, no resistance mechanism has been reported in both diseases. Here, two SNB and TS resistance wheat lines (‘56:ZWB11’ and ‘105:ZIF14’) derived from the Australian national germplasm evaluation programme (CAIGE) were used to develop a double haploid mapping population. Two Pn and Ptr isolates of different pathotypes, their respective culture filtrates and effector SnTox267 were evaluated on the population. Genetic analysis of Ptr conidial inoculation of race 1 and race 2 identified a major resistance quantitative trait locus (QTL) ( QTs.cur –1B) on chromosome 1B, while resistance to SNB was explained by several minor QTL. SnTox267 sensitivity was mapped to six locations (2A2, 2A3, 2B1, 2D3, 5B and 7B1) with only one QTL co-localized to known corresponding gene Snn7 . Sensitivity loci 5B and 7B1 also conferred SNB resistance at seedling and adult stages. Two QTL on chromosome 2D1 and 7B2 were common in both SNB and TS, associated with disease at seedling stage and culture filtrate bioactivity, respectively. Resistance responses of ‘56:ZWB11’ and ‘105:ZIF14’ were confirmed cytologically, however, distinct responses were observed on wounded leaves. The defence responses were more effective against Ptr, while resistance to Pn infection was likely a combination of lack of susceptibility and effective physical barriers. Overall results demonstrated the distinction between the underlying resistance mechanisms to TS and SNB.

Population genetics are routinely studied over vast geographical areas to address questions of origin, routes of migration and epidemiology. However, genetic diversity of certain microorganisms such as plant pathogenic fungi are difficult to study even on a small geographical scale due to their mixed reproduction systems, variable population sizes, clonality and versatility to host adaptation. The genetic variation of an economically important wheat fungal pathogen, Pyrenophora tritici-repentis (Ptr), was studied with isolates collected from a naturally infected wheat field located in Western Australia, using microsatellite (SSR) markers. Out of 142 Ptr isolates, 89 individuals showed unique multilocus haplotypes. On single leaves, individual disease lesions mostly consisted of genetically variable Ptr isolates, with very few clonal isolates. Distribution analysis of these indicated that wind-dispersal of Ptr conidia occurred over a small scale within individual leaves as well as longer dispersions of up to 45 m. Race evaluation of the population via effector gene PCR assays defined 136 isolates as race 1 and six as race 2, and there was no indication of genetic groupings based on micro-locations or race. Population structure analysis suggests that the population consists of mixed migrants from long wind-dispersal or field management activities. Nucleotide sequence analysis of the homothallic mating-type genes, MAT1-1 and MAT1-2 revealed sequence conservation within the population. Altogether, the microscale evaluation of the single wheat field isolates demonstrated that the population is suitable for disease screening as it is representative of the wider Ptr population demographic in Australia.

Tan spot, caused by the fungus Pyrenophora tritici-repentis ( Ptr ), is one of the major diseases of spring wheat ( Triticum aestivum L.) in the Nordic countries. However, little is known about the role of Ptr necrotrophic effectors (NEs), ToxA, ToxB and ToxC, behind tan spot epidemics in the region. A total of 217 Nordic Ptr isolates were screened for ToxA and ToxB genes using PCR. ToxA was found to be prevalent (64.5%), whereas ToxB was absent from the population. A subset of 25 Ptr isolates was further tested on a set of wheat differentials. The majority ( n = 21) of these isolates induced susceptible reactions on ToxC sensitive genotype ‘6B365’, indicating the presence of the ToxC effector. The symptoms observed on ToxA and ToxB differentials were not completely in line with the PCR results, suggesting the presence of ‘atypical’ Ptr isolates. In a collection of 197 spring wheat genotypes, ToxA and ToxB sensitivities were determined and found in 46.2% and 23.9% of the lines, respectively. Additionally, 179 of the wheat accessions were tested in the field over three years at two locations in Finland, and NE sensitivities were found to explain very little of the variation in tan spot susceptibility. Interestingly, ToxB sensitivity was found to have a counter-intuitive effect in European spring wheat germplasm, as ToxB-sensitive genotypes were less susceptible to tan spot disease compared to the insensitive ones. The results presented here provide the first comprehensive overview of the importance of Ptr NEs in tan spot epidemics in the Nordic countries.

The Dothideomycete fungal pathogen Pyrenophora tritici-repentis (Ptr) is the causal agent of the tan spot disease of wheat. The proteinaceous necrotrophic effectors ToxA and ToxB are well characterized. A nonproteinaceous effector called ToxC has also been partially characterized. Ptr produces a number of other small molecular weight compounds, but these remain poorly characterized. In this study, two novel compounds, designated ToxE1 and ToxE2, capable of inducing chlorotic symptoms on wheat leaves in a cultivar-specific manner, were purified from Ptr liquid cultures. There is no evidence that these compounds correspond to ToxC. Most isolates produced ToxE1, ToxE2, or both, and both compounds were detected in infected wheat leaves. The structures of both analogues were elucidated by NMR spectroscopy and comprise a phthalide core structure with an amide moiety. We postulate that these compounds have a general phytotoxic effect and may have an ancillary role in disease development.

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, Tox...

Tan spot disease of wheat (also known as yellow spot) is caused by the necrotrophic fungal pathogen Pyrenophora tritici‐repentis (Ptr). Initially described as a grass pathogen, Ptr has become a wheat pathogen of global importance. In Australia, tan spot was first recorded in 1952 as a minor wheat disease. However, by the 1970s tan spot had reached epidemic levels in Australia with yield losses of up to 49% reported in the north‐eastern region. A national survey carried out in 2007/2008 placed tan spot as the most economically important wheat disease with an estimated yield loss of 6%, corresponding to nearly a quarter of all disease losses. The incidence of tan spot in Australia has now markedly reduced with some wheat breeding programmes no longer considering tan spot disease resistance to be a priority trait. The disease can be effectively managed with integrated control strategies such as crop rotation, timely application of fungicides and removal of surface stubble. However, the key to the success of controlling tan spot disease in Australia has been achieved through wheat breeding programmes. The development of tan spot resistance resources suitable for the Australian climate was a concerted effort between national and international research organizations and Australian wheat breeders, supported by the Australian Grains Research Development Corporation (GRDC). While traditional breeding was essential to combat the disease, this article highlights the value of effector biology in assisting the development of resistant cultivars.

Pyrenophora tritici-repentis (tan spot) is a destructive foliar pathogen of wheat with global impact. This ascomycete fungus possesses a highly plastic open pangenome shaped by the gain and loss of effector genes. This study investigated the allelic variations in the chlorosis-encoding gene ToxB across 422 isolates representing all identified pathotypes and worldwide origins. To gain better insights into ToxB evolution, we examined its presence and variability in other Pyrenophora spp. A ToxB haplotype network was constructed, revealing the evolutionary relationships of this gene (20 haplotypes) across four Pyrenophora species. Notably, toxb, the homolog of ToxB, was detected for the first time in the barley pathogen Pyrenophora teres. The ToxB/ toxb genes display evidence of selection that is characterized by loss of function, duplication, and diverse mutations. Within the ToxB/ toxb open reading frame, 72 mutations were identified, including 14 synonymous, 55 nonsynonymous, and 3 indel mutations. Remarkably, a, ∼5.6-kb Copia-like retrotransposon, named Copia-1_Ptr, was found inserted in the toxb gene of a race 3 isolate. This insert disrupted the ToxB gene's function, a first case of effector gene disruption by a transposable element in P. tritici-repentis. Additionally, a microsatellite with 25 nucleotide repeats (0 to 10) in the upstream region of ToxB suggested a potential mechanism influencing ToxB expression and regulation. Exploring ToxB-like protein distribution in other ascomycetes revealed the presence of ToxB-like proteins in 19 additional species, including the Leotiomycetes class for the first time. The presence/absence pattern of ToxB-like proteins defied species relatedness compared with a phylogenetic tree, suggesting a past horizontal gene transfer event during the evolution of the ToxB gene. [ ] Copyright © 2024 His Majesty the King in Right of Canada, as represented by the Minister of Agriculture and Agri-Food. This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

ToxA is one of the most studied proteinaceous necrotrophic effectors produced by plant pathogens. It has been identified in four pathogens ( Pyrenophora tritici-repentis, Parastagonospora nodorum, Parastagonospora pseudonodorum [formerly Parastagonospora avenaria f. sp. tritici], and Bipolaris sorokiniana) causing leaf spot diseases on cereals worldwide. To date, 24 different ToxA haplotypes have been identified. Some P. tritici-repentis and related species also express ToxB, another small protein necrotrophic effector. We present here a revised and standardized nomenclature for these effectors, which could be extended to other poly-haplotypic genes found across multiple species.

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.