Genomic and epigenomic immunity in common bean: the unusual features of NB-LRR gene family.

In plants, a key class of genes comprising most of disease resistance (R) genes encodes Nucleotide-binding leucine-rich repeat (NL) proteins. Access to common bean (Phaseolus vulgaris) genome sequence provides unparalleled insight into the organization and evolution of this large gene family (∼400 NL) in this important crop. As observed in other plant species, most common bean NL are organized in cluster of genes. However, a particularity of common bean is that these clusters are often located in subtelomeric regions close to terminal knobs containing the satellite DNA khipu. Phylogenetically related NL are spread between different chromosome ends, suggesting frequent exchanges between non-homologous chromosomes. NL peculiar location, in proximity to heterochromatic regions, led us to study their DNA methylation status using a whole-genome cytosine methylation map. In common bean, NL genes displayed an unusual body methylation pattern since half of them are methylated in the three contexts, reminiscent of the DNA methylation pattern of repeated sequences. Moreover, 90 NL were also abundantly targeted by 24 nt siRNA, with 90% corresponding to methylated NL genes. This suggests the existence of a transcriptional gene silencing mechanism of NL through the RdDM (RNA-directed DNA methylation) pathway in common bean that has not been described in other plant species.