Genetics and genomics of disease resistance in potato
The James Hutton Institute hosts the Commonwealth Potato Collection (CPC), an invaluable germplasm collection of more than 1800 potato accessions which are maintained as true seeds. The CPC represents a snapshot of potato co-evolution with major pathogens including Phytophthora infestans, nematodes, bacteria and viruses, as well as various environmental conditions. Extensive disease resistance screens for multiple pathogens have been conducted. Since 1989, the Institute has conducted a multi-trait (MT) potato breeding programme to combine resistances to late blight, potato cyst nematodes and viruses with other commercially desirable traits. This programme has been supported throughout by industrial sponsorship and has delivered a number of successful potato cultivars including, for example, Lady Balfour and Mayan Gold for various markets.
Diploid Solanum species including S. bulbocastanum, S. capsicibaccatum, S. circaeifolium, S. microdontum, S. okadae, S. polyadenium and S. verrucosum, with novel P. infestans resistances, feature in current, intraspecies, crosses to identify and clone underlying resistance to P. infestans (Rpi) genes. Resistant accessions have been characterised by screens with diverse late blight isolates that, in combination, overcome all cloned Rpi genes to date. We use state-of-the-art effector recognition assays to determine the underlying resistance mechanisms. We have identified approximately 50 universally expressed ‘core effectors’ from P. infestans and concentrate our efforts on identifying resistances that are based upon the recognition of such core effectors and all known allelic variances thereof to identify more durable resistances. 
The recently published genome sequence of potato provides a blueprint for more rapid resistance (R) gene isolation. A novel approach for R gene cloning utilises target enrichment followed by second generation sequencing of resistant and susceptible bulks derived from segregating intraspecies crosses. More than 400 fold enrichment for nucleotide-binding, leucine-rich-repeat (NB-LRR) type R genes has been achieved with R gene specific baits. Individually sequenced resistant and susceptible bulks are compared to identify candidate R genes that segregate with the resistance phenotype within the segregating population and validated in transient and stable transformation assays.
