New approaches to rust resistance in wheat

Ug99 is new race of Puccinia graminis that is virulent on most of the widely deployed stem rust resistance genes from wheat, posing a serious threat to global wheat production. Sr35, a resistance gene from Triticum monococcum, confers resistance to Ug99 and all related Ug99-derived stem rust races identified so far. Using large segregating populations we mapped Sr35 to a 0.1 cM interval in the distal part of chromosome arm 3AL. Using Sr35 flanking markers we completed physical maps from two resistant T. monococcum accessions (~300 kb). We sequenced these contigs and identified three related CC-NBS-LRR genes completely linked to Sr35. Re-sequencing of these candidate genes in a panel of resistant (Sr35+) and susceptible (Sr35-) T. monococcum accessions suggested that one of them, designated NL9, was the best candidate for Sr35. This was further confirmed in a screen of an EMS mutant population from a resistant accession. Sequencing of a susceptible mutant identified a premature stop codon in NL9 and no changes in the other linked genes confirming that this CC-NBS-LRR is the most likely candidate for Sr35. NL9 transgenic wheat plants are being developed to complete the validation process. We are currently using NL9 to screen a yeast-two-hybrid library of P. graminis infected wheat leaves to identify either the targeted P. graminis effector or the wheat protein guarded by NL9. We are complementing this study with the re-sequencing of a set of Sr35 virulent and avirulent P. graminis races to search for Sr35-associated effectors.