Characterisation of major genes (Stb)-mediated resistance to Septoria tritici blotch disease in wheat

Wheat is one of the most important staple food crops providing 20% of total daily calories consumed by humans worldwide. Septoria tritici blotch (STB) is a devastating disease encountered in most wheat growing areas of the world and is the primary foliar disease of wheat in the UK responsible for year-on-year wheat losses of 5-10% despite fungicide treatments. The causal agent of STB is the fungus Zymoseptoria tritici. Emergence and spread of fungicide resistance in fungal populations seriously threatens wheat production and compromises food security, therefore resistance to STB is an important target in wheat breeding.
To date, 21 major (Stb) genes and several quantitative trait loci (QTL) that make smaller contributions to the resistance phenotype have been identified and mapped genetically. Only one of these genes has so far been cloned and mechanisms of resistance remain poorly understood. Owing to a lack of well-defined QTLs with additive effects and near absence of diagnostic markers, the current STB resistance breeding strategies rely mostly on phenotypic evaluation of breeding materials rather than targeted genotyping based selection. Consequently, over the past decades, there has been only a modest improvement of STB resistance in the commercial wheat germplasm and the majority of current AHDB recommended UK wheat cultivars show only moderate resistance to STB.
Published genetic maps at Stb loci are mostly low resolution and often involve early generation markers (eg RFLP, SSR), which are considered impractical in modern marker-assisted breeding. It is not known whether any of the Stb genes (apart from Stb6) may be present in the UK commercial wheat, or how effective these may be in controlling STB under UK conditions. The proposed PhD project, aims to fill this significant void in wheat breeding research by enhancing our understanding of genetics and mechanisms of STB resistance controlled by known Stb genes through addressing the following objectives:
1.Identify frequency of virulence/avirulence in the current UK field Z. tritici populations towards each Stb gene, and whether any of these genes confer broad-spectrum resistance
2.Assess whether resistance operates at seedling and/or adult plant stage
3.Fine-map the most interesting and useful resistance gene(s), and develop the diagnostic second generation genetic markers (KASP) for use in breeding
4.Gain insight into how the Stb gene(s) are conferring resistance
The Stb genes will be transferred by crossing from foreign and exotic genotypes into a highly susceptible UK wheat background. Development of mapping populations nearly-isogenic lines is already in progress. These materials will then be phenotyped with current UK field Z. tritici isolates under glasshouse conditions followed by high-density genotyping. We have recently cloned Stb6 and shown that it encodes a Wall-associated receptor-like kinase (WAK) protein that defines a novel class of disease resistance proteins (Nat Genet, accepted). Using the latest IWGSC assembly of wheat genome, we have annotated and manually curated all WAK encoding genes and hypothesised that at least some of these may correspond to Stb genes. Indeed, bioinformatics analyses have indicated that several Stb loci correspond to physical intervals containing one or more WAK genes. The student will further test the above hypothesis by assessing the function of candidate WAK genes using Virus-induced gene silencing (VIGS) approach well established in our laboratory (Plant Physiol 160: 582-90). Mechanisms resistance conferred by the Stb genes will be investigated using a combination of molecular biology and bioimaging approaches, in particular determining whether disease resistance is associated or not with cell death, what step in the fungal lifecycle is being targeted, and what defence pathways are activated.
The knowledge gained will aid breeding for STB resistance and therefore help to enhance crop productivity.