Unusual features of NB-LRR sequences in common bean genome
M. Richard (1), A. Gratias (1), V. Thareau (1), M. Aubert (1), K. Kim (2), S. Jackson (2), V. Geffroy (1) (1) Institute of Plant Sciences Paris-Saclay IPS2, CNRS, INRA, Univ Paris Sud, Univ Evry, Univ Paris-Diderot, Orsay, France; (2) Center for Applied Genetic Technologies, University of Georgia, Athens, GA 30602, U.S.A.

In common bean (Phaseolus vulgaris) genome ~400 NB-LRR (NL) genes have been annotated. Most are organized in complex cluster of genes 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 methylation status using a whole-genome cytosine methylation map at the single-nucleotide resolution. In plant genomes, DNA methylation is present in the symmetrical CG and CHG contexts as well as in the asymmetrical CHH context. Contrasting pattern of DNA methylation have been identified for transposable elements (TE) and genes. If plant genes are occasionally methylated in CG in their exons, TE are usually methylated in the three contexts. In common bean, NL genes displayed an unusual body methylation pattern since half of them are methylated in the three different contexts, reminiscent of the methylation pattern of repeated sequences. This was not observed for two other large multigene families (pentatricopeptide repeat and homeobox transcription factor family) suggesting that this unusual methylation pattern is not ubiquitous to multigenic families. Moreover 90 NL were also abundantly targeted by 24nt siRNA, with 90% corresponding to NL genes methylated in the three different contexts. This suggests the existence of a transcriptional gene silencing mechanism of NL through the RdDM (RNA-directed DNA methylation) pathway in common bean which has not been described in other plant species.

Abstract Number: C2-1
Session Type: Concurrent