Dissection of quantitative resistance to leaf rust (Puccinia brachypodii) in Brachypodium distachyon, the model plant for Triticeae

The model plant Brachypodium distachyon (L.) Beauv. has been employed to dissect its quantitative resistance to leaf rust. An F2 mapping population generated between the two B. distachyon diploid inbred lines Bd1-1 and Bd3-1 was used to develop a molecular marker linkage map. The map was initially populated with 192 AFLP marker loci. SSRs and conserved orthologous sequence (COS) markers have also been added to the AFLP framework to provide anchor points for comparative genomics studies with other Brachypodium and Triticeae maps. To locate quantitative resistance loci on the map, the F2 plants were evaluated for their reaction to the leaf rust Puccinia brachypodii. To improve and validate the dissection of the trait, F2-derived F3 families were tested for resistance to leaf rust in two additional independent experiments. Disease evaluations showed continuous, quantitative and transgressive segregation. Interval mapping and MQM mapping were performed on the data of the different experiments by using the software MapQTL 5.0 and then QTL positions were compared. Two major genomic regions involved in resistance to leaf rust were detected and are here discussed according to recent increase of genomic knowledge in Brachypodium. Together they accounted for about 40-50% of the observed phenotypic variation. Our results suggest that leaf rust resistance in B. distachyon is a polygenic trait influenced by few major genes with large effect as observed in the Triticeae. A search for candidates has been started through the Brachypodium distachyon 8x released genomic sequence, to obtain and map candidate gene-derived markers in the QTL intervals.