A highly differentiated region of wheat chromosome 7AL encodes Pm1a immune receptor that recognises its cognate AvrPm1a effector from powdery mildew

Pm1a, the first powdery mildew resistance gene described in wheat, is part of a complex resistance (R) gene cluster that resides in a distal region of chromosome 7A with suppressed genetic recombination due to unknown causes. A nucleotide-binding leucine-rich repeat (NLR) immune receptor-encoding gene was isolated using mutagenesis and R gene enrichment sequencing (MutRenSeq). Single chromosome enrichment sequencing (ChromSeq) was also performed to investigate the suppressed recombination. A high-density map from the progeny of Blumeria graminis pathogen segregating for avirulence/virulence to Pm1a together with genome resequencing and RNAseq delineated AvrPm1a effector gene candidates.Mutant sequencing, genetic segregation and stable transformation demonstrated the isolated NLR was indeed Pm1a, which could independently induce hypersensitive cell death in transformed susceptible plants. Co-expression of Pm1a and Avr candidates identified a single effector candidate containing N terminal Y/FxC motif that exhibited specific and strong hypersensitive response in the heterologous Nicotiana benthamiana system. Flow-sorting and assembly of chromosome 7A indicated the region of suppressed recombination was due to rearrangements likely involving chromosomes 7B and 7D.The cloning of Pm1 and sequencing of highly differentiated chromosome 7A is a key step in deciphering this complex resistance cluster and understanding its unique evolution relating to chromosomal rearrangements.