Tuesday, November 3, 2009
Convention Center, Exhibit Hall BC, Second Floor
The largest class of race-specific plant disease resistance (R) genes encodes proteins containing nucleotide binding site (NBS) and leucine rich repeat (LRR) domains. Nearly 800 NBS-LRR encoding resistance gene candidates (RGCs) have been identified from partial cDNA or genomic DNA sequences in sunflower (Helianthus annuus L.). Nearly one-fourth have been genetically mapped and several are tightly linked to downy mildew R genes found on linkage groups 1 (Pl14) and 13 (Pl8) and a rust R gene found on linkage group 13 (RAdv) 8 cM upstream of Pl8. The phylogenetic identities of many of the NBS-LRR loci found on linkage groups 1 and 13 are unknown, and RAdv, Pl8, and Pl14 have not been cloned. To lay the groundwork for positionally cloning RAdv, Pl8, and Pl14, we isolated, genetically and physically mapped, and phylogenetically classified several NBS-LRR encoding genes tightly linked to Pl8, Pl14, and RAdv. Using overgo probes targeting candidate NBS-LRR sequences, numerous bacterial artifical chromosome (BAC) clones were isolated, end-sequenced, fingerprinted, assembled into contigs, and genetically mapped using DNA markers developed from BAC end sequences. Contigs cosegregated or were very tightly linked to the NBS-LRR sequences used for BAC isolation. Selected BAC clones from each of the mapped contigs were fully sequenced and structurally and functionally annotated. Several NBS-LRR encoding genes were identified in contigs tightly linked to RAdv, Pl8, and Pl14 and were phylogenetically classified as members of a single highly duplicated NBS-LRR encoding gene family found in large clusters on linkage groups 1 and 13__the former spanning at least 12 cM and the latter spanning at least 26 cM and each harboring several family members. DNA markers for NBS-LRR loci from clusters on linkage groups 1 and 13 mapped within 0.3-0.6 cM of RAdv, Pl8, and Pl14 and supply resources for marker-assisted breeding.