Molecular Genetics and Biochemistry of Quantitative Resistance against Pyrenopeziza brassicae in Brassica napus

Abstract

The aim of this project was to better understand the molecular genetics and the biochemical components involved in quantitative resistance against Pyrenopeziza brassicae in Brassica napus. In addition, pathogenicity variations in P. brassicae populations and isolates from Aberdeen, Scotland and Hertfordshire, England were assessed to enhance knowledge about the regional sub-populations. Furthermore, resistance of B. napus accessions against P. brassicae was phenotyped, quantitative disease resistance (QDR) genes against P. brassicae were identified by qPCR and potential biochemical components of resistance such as glucosinolates (GSL), cuticular wax and cutin were quantified. To identify novel QDR genes, the resistance phenotyping of 72 B. napus accessions done during this study helped to provide complementary data to that from the original test panel, which included 195 B. napus accessions. This is the first study which reveals the gene expression patterns of QDR genes associated with resistance and/or susceptibility against P. brassicae in B. napus. The results from this study showed that there is a variation in pathogenicity between Aberdeen and Hertfordshire P. brassicae populations/isolates. It will be beneficial to analyse sequence variations between isolates from different regions to enhance the current knowledge about the pathogenicity factors in P. brassicae. Furthermore, a gene knockdown of the mating type genes of P. brassicae can reveal if they are involved in spore size dimorphism and pathogenicity. The current study helped to provide information about the QDR genes, their potential signalling pathways and the proteins encoded to enhance resistance and or susceptibility against P. brassicae in B. napus. An early induction of the vesicle trafficking protein, β-adaptin and universal stress protein in resistant lines was seen to potentially limit the germination and penetration of P. brassicae. Furthermore, Pathogenesis related protein (PR1) and Cinnamate 4 hydroxylase genes were upregulated to reduce branching and colonisation respectively. In addition, the association of HXXXD-type acyl transferase with enhanced pathogenicity of P. brassicae in susceptible lines of B. napus such as Cabriolet was shown in this study. This research showed a significantly greater glucosinolate (GSL) concentration in resistant B. napus lines/cultivars (Cubs Root, POSH and Dwarf Essex) than in susceptible lines (Cabriolet, Sansibar and Laser). Additionally, concentrations of aliphatic compounds (7-methyl sulfinyl heptyl: 7msh, 3-butenyl: 3but, 4-pentenyl: 4pent, 6-methyl sulfinyl hexyl: 6msh and 5 methylthio propyl:5mtp); indolic compounds (4-methoxy-indolyl-3-methyl: 4moi3m, indolyl-3-methyl: i3m) and an aromatic compound (2pe) were significantly greater in resistant B. napus lines/cultivars than in susceptible lines/cultivars. Epicuticular wax load, its components and their structural variation and cutin monomers enhanced the host resistance against P. brassicae. The alkane forming pathway of wax products is negatively correlated with LLS disease severity. This was confirmed by alternate alcohol forming pathway product (10 alcohol) content remaining constant in B. rapa and B. napus resistant lines/cultivars after inoculating with P. brassicae. The results indicate a potential involvement of the minor component C30 aldehyde in the signalling defence response to limit the sporulation by the P. brassicae. Furthermore, a possible association of cutin monomers with P. brassicae pathogenicity was identified. This study revealed many likely genetic resources in B. napus, such as lines POSH, Laser, Moana, Dwarf Essex, SWU Chinese 1 and Cubs Root, which can be used to breed B. napus cultivars resistant against P. brassicae. All these B. napus lines/cultivars showed significantly less LLS disease severity with greater GSL content/higher induction of QDR genes/higher wax and cutin contents. Furthermore, this study confirms the possibility of enhancing the narrow genetic diversity of B. napus available to breed for cultivars resistant against P. brassicae by introgressing resistance from B. rapa.