UC Davis

Key message

The common bean locus Co - 4, traditionally referred to as an anthracnose-resistant gene, contains a cluster of predicted receptor-like kinases (COK-4 and CrRLK1-like), and at least two of these kinases are co-regulated with the plant's basal immunity. Genetic resistance to anthracnose, caused by the fungus Colletotrichum lindemuthianum (Sacc. and Magnus) Briosi and Cavara, is conferred by major loci throughout the Phaseolus vulgaris genome, named Co. The complex Co-4 locus was previously reported to have several copies of the COK-4 gene that is predicted to code for a receptor-like kinase (RLK). In general, plant RLKs are involved in pathogen perception and signal transduction; however, the molecular function of COK-4 remains elusive. Using newly identified molecular markers (PvTA25 and PvSNPCOK-4), the SAS13 marker, COK-4 sequences and phylogeny, and the recently released bean genome sequence, we determined the most probable boundaries of the Co-4 locus: a 325-Kbp region on chromosome Pv08. Out of the 49 predicted transcripts in that region, 24 encode for putative RLKs (including 18 COK-4 copies) with high similarity to members of the Catharanthus roseus RLK1-like (CrRLK1L) protein family from different plant species, including the well-described FERONIA (FER) and ANXUR. We also determined that two RLK-coding genes in the Co-4 locus (COK-4-3 and FER-like) are transcriptionally regulated when bean plants are challenged with the flg22 peptide, a commonly used elicitor of plant immunity, or the bacterium Pseudomonas syringae pv. phaseolicola, the causal agent of halo blight. While COK-4-3 is activated during immune response, FER-like is downregulated suggesting that these genes could play a role in plant responses to biotic stress. These results highlight the importance of dissecting the regulation and molecular function of individual genes within each locus, traditionally referred to as resistance gene based on genetic segregation analysis.