Accelerated breeding of black rot resistant brassicas for the benefit of east African smallholders

Plant disease resistance genes can protect crops against pathogens that have the corresponding avirulence (avr) genes. In many cases, the protection conferred by single, generally dominant, disease resistance (R) genes is short lived as the pathogen strains can evolve through mutation and selection, favouring strains with altered or missing avr genes that are therefore able to cause disease. Broad spectrum disease resistance that is quantitative, involving several genes, is generally expected to be more durable. However, this kind of resistance is more difficult to exploit in breeding programmes. The basis of this plant defence strategy is currently unknown. We have chosen quantitative resistance to Xanthomonas campestris pv campestris (Xcc) in Brassica rapa as a model for investigation. Xcc causes black rot of crucifers. This seed-borne disease is one of the most important diseases of Brassicas worldwide. In previous Brassica research at WHRI we have identified good sources of resistance and mapped the location of the major genetic components. In this project we will exploit the exciting new developments in Brassica rapa genomics and emerging information on the close relatedness of the Brassica and Arabidopsis genomes to characterise the resistance and identify the genes involved. The combination of the above resources in the proposed project will allow us to deliver novel and exciting information on the fundamental basis of quantitative disease resistance in a crop species. Key practical outcomes of this work will be resistant plant material with tightly linked molecular markers and information on the extent of natural variation for resistance in Brassica genomes that could be exploited in plant breeding to provide durable resistance to a very significant pathogen.

This multidisciplinary project will combine expertise in genetics, breeding, genomics and pathology to generate novel information on quantitative resistance to black rot caused by Xanthomonas campestris pv. campestris, a major disease of Brassicas. Quantitative resistance, involving the cumulative phenotype expressed by numerous genes, is generally expected to be more durable than resistance due to single dominant genes. However, this kind of resistance is more difficult to exploit in breeding programmes. We will dissect the resistance in Brassica rapa into major components and determine their relative contribution. We will utilise new developments in B. rapa genomics and emerging information on synteny between the Brassica and Arabidopsis genomes to fine-map and identify the genes involved. The spectrum of the resistance will be determined against a wide range of pathogen isolates from our culture collection. Representative sets of the Brassica genepool will be screened to identify allelic variation and potential new candidates for resistance breeding. Well defined resistant germplasm with tightly linked molecular markers and information on allelic variation in Brassica genomes will be key practical outcomes. Specific objectives of this study are: 1. Characterisation of the major components of quantitative Xanthomonas campestris pv. campestris (Xcc) race 1 and race 4 resistance from Brassica rapa; 2. Fine-scale mapping to identify closely linked markers and candidate genes using synteny between the Arabidopsis and Brassica genomes. 3. Clone candidate genes and transform into B. oleracea for functional testing in a C genome recipient. 4. Assess the broad spectrum potential of the B. rapa resistance for protection against a range of Xcc isolates. 5. Survey allelic variation for resistance loci in other Brassica genomes using Brassica Differential Fixed Foundation Sets (DFFS)