A single amino acid change can alter the specificity of the multi-allelic wheat stem rust resistance locus SR9

Abstract Most resistance genes thus far isolated from wheat have a very limited number of functional alleles, with the exception of the powdery mildew PM3 resistance locus. Here we report the isolation of most of the alleles at wheat stem rust resistance gene locus SR9, representing the largest multi-allelic rust resistance locus in common wheat. The seven previously reported resistance alleles (Sr9a, Sr9b, Sr9d, Sr9e, Sr9f, Sr9g, and Sr9h) at the locus were characterised using a synergistic strategy. Loss-of-function mutants and/or transgenic complementation were used to confirm Sr9b, two haplotypes of Sr9e (Sr9e_h1, Sr9e_h2), Sr9g, and Sr9h. Each allele encodes a highly related nucleotide-binding site leucine-rich repeat (NB-LRR) type immune receptor, containing a previously unreported motif at their N termini and an unusual long LRR domain, that confers resistance to a unique spectrum of isolates of the wheat stem rust pathogen. The only SR9 protein effective against stem rust pathogen race TTKSK (Ug99), SR9H, differed from SR9B by a single amino acid. SR9B and SR9G resistance proteins were also distinguished by only a single amino acid. The SR9 allelic series found in the B subgenome are orthologs of wheat stem rust resistance gene Sr21 located in the A subgenome with around 85% identity in protein sequences.