Understanding the Molecular Basis of Disease Resistance Against Septoria Tritici Blotch in Wheat

Zymoseptoria tritici, a fungal pathogen of wheat and the causal agent of Septoria Tritici Blotch (STB), is a particular challenge to wheat production on a global level, causing crop losses of up to 50% in some cases. Genetic resistance presents an effective way of controlling STB in wheat, with numerous resistance loci having been identified. Stb6, which encodes a wall-associated kinase-like protein and confers resistance to Z. tritici isolates expressing the corresponding AvrStb6 effector, has recently been cloned. However, many questions regarding Stb6-mediated resistance remain unanswered. The global frequency of avirulence AvrStb6 alleles in modern Z. tritici populations remains unknown. The role that AvrStb6 plays in Z. tritici pathogenicity, the nature of its interaction with Stb6 and the mechanism by
which Stb6 confers resistance are also yet to be elucidated.
In the following thesis, I present data indicating a dramatic shift in the frequency of virulence AvrStb6 isoforms in modern Z. tritici populations, relative to previous studies. Transient expression of fluorophore-tagged Stb6 and AvrStb6 in the tobacco species Nicotiana benthamiana also revealed that AvrStb6 localises to the apoplast. Stb6 displays an unusual cellular localisation pattern, being present in both the plasma membrane but also identified in the cell wall and in hectian strands adjoining the two. Expression of Stb6 in N. benthamiana induces a kinase-dependent cell death phenotype, which is independent of the co-receptor BAK1. I also report on the identification of candidate protein interactors for Stb6 and AvrStb6. Both interactors are predicted to be chloroplast-localised. These findings present an intriguing hypothesis as to the possible mechanism of Stb6-mediated resistance. This study therefore represents an important contribution to our understanding of evolutionary genetics in a plant pathogen, provides a putative mechanism for AvrStb6 in pathogenicity and has also potentially identified a completely novel resistance mechanism for an immune receptor protein. Research such as this plays a role in helping develop more productive crop plants with improved genetic resistance to pathogens.