Novel and Classical Resistance Genes Provide Nonhost Resistance to Wheat Stripe Rust in Brachypodium distachyon

The plant pathogen Puccinia striiformis f. sp. tritici (PST), commonly known as wheat stripe rust, is an obligate biotroph with a wide host range. In addition to the agronomically important grass species wheat, barley and rye, it can also rarely infect the taxonomically distant Brachypodium distachyon. As a model grass species with a sequenced genome, a high rate of recombination, and advantageous morphological characteristics, B. distachyon is an excellent system to study the genetic basis of PST resistance in an intermediate nonhost species. We found that most B. distachyon accessions were completely resistant to various UK and Australian PST isolates, whereas some accessions showed a range of susceptibility symptoms. Five populations were used to establish the genetic architecture of this resistance, including ABR6 x Bd21, BdTR10H x Tek-4, BdTR13K x Bd21, Luc1 x Jer1, and Foz1 x Luc1. Inoculation with three UK and one Australian PST isolates identified three major QTLs, designated Yrr1 to Yrr3. Interestingly, Yrr1 was narrowed down to a 75 kb gain of function interval on chromosome 4, which does not contain any known resistance gene homologs. However, classical resistance genes encoding nucleotide binding site, leucine-rich repeat domains (NBS-LRRs) are the candidates underlying Yrr2 (chromosome 4) and the 102 kb gain of function interval of Yrr3 (chromosome 2). Strikingly, isolate specificity was observed to Yrr2, highlighting the conservation of typical aspects of host-pathogen interaction in this system. These results suggest the involvement of both novel and classical resistance pathways in intermediate nonhost resistance.