Fine mapping of the Co-4 locus of common bean reveals a resistance gene candidate, COK-4, that encodes for a protein kinase

The SAS13 SCAR marker, tightly linked with the CO-4(2) gene segregating in a population of 1018 F-2 individual plants, was used as a starting point for cloning gene sequences associated with the Co-4 locus that conditions resistance to anthracnose caused by the fungal pathogen Colletotrichum lindemuthianum in common bean (Phaseolus vulgaris). A contig developed from genomic clones flanking the marker region revealed a 1110-bp open reading frame, named COK-4. The predicted COK-4 protein contains a serine-threonine kinase domain highly similar to the protein encoded by the Pto gene in tomato, but with a highly hydrophobic membrane-spanning region. COK-4 homologs were cloned and sequenced from different bean cultivars. Single nucleotide polymorphisms were found between the homologous sequences and were confirmed with three restriction enzymes. Restriction patterns among three bean cultivars known to possess different alleles at the Co-4 locus, SEL 1308 (Co-4(2)), TO (Co-4) and Black Magic (co-4), were polymorphic. Absolute co-segregation between COK-4 restriction patterns and the disease phenotype was observed in 96 F-3 families. More than one copy of the COK-4 gene homolog exists in the bean genome as demonstrated by Southern analysis. These results suggest that COK-4 is part of the Co-4 locus conditioning resistance to C. lindemuthianum in bean.