Exploitation of resistance genes from oilseed rape for control of light leaf spot (PhD)
The aim of this project was to use novel genetic and genomic information to improve understanding of resistance against Pyrenopeziza brassicae (light leaf spot) in oilseed rape.
A major gene locus for resistance on Brassica napus chrA1 was further studied using flanking marker information. Approximate physical location of this marker at the bottom of chrA1 was confirmed by using sequence homology and the synteny between B. napus and B. rapa genomes. KASP markers were developed for the single nucleotide polymorphisms (SNPs) identified between the flanking marker and the telomere of chrA1. Genotyping and linkage mapping of newly developed KASP markers led to the identification of markers linked to the south of the resistance locus, providing a defined chromosomal region. Six candidate resistance genes were identified, based on the functional annotations for the gene content in the corresponding chromosomal region available from the Darmor-bzh and Z11 genome sequences of B. napus and the Brassica pan-transcriptome. These included four receptor-like kinases (RLKs) and two nucleotide-binding site leucine rich repeat genes (NLRs).
Segregation for resistance against P. brassicae has been identified in a doubled haploid B. napus population (Q DH) developed by incorporating genetic diversity from ancestral Brassica species, B. rapa oleifera and B. oleracea atlantica. Resistance against P. brassicae segregating in this population was studied with controlled environment, glasshouse and winter oilseed rape field experiments. Composite interval mapping analysis had identified 17 QTL across 10 chromosomes, using data for the percentage leaf area covered with P. brassicae sporulation and five QTL (with LOD ≥ 3.2) across five chromosomes for the P. brassicae DNA data. There were some QTL hotspots on chromosomes C1, C3, C6 and C9, where QTL from different experiments and/or traits were co-located.
This work also reports the identification of specific interactions between B. napus and P. brassicae using controlled environment and glasshouse experiments. There were two main phenotypes of resistance, formation of black necrotic flecking and limitation of P. brassicae asexual sporulation (acervuli), which often appeared to be correlated. Interestingly, there appeared to be a quantitative nature to the black flecking phenotype. Considering various effects of resistance or susceptibility to P. brassicae (i.e. leaf deformations, necrotic flecking, different amounts of pathogen colonisation and asexual sporulation), it can be suggested that recognition of P. brassicae by resistant lines/cultivars occurs at a late stage of P. brassicae colonisation, possibly during the phase of asexual sporulation.