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Title: QTLs for resistance to the false brome rust Puccinia brachypodii in the model grass Brachypodium distachyon L.

Author
item BARBIERI, M - University Of Modena And Reggio Emilia (UNIMORE)
item MARCEL, T - Wageningen Agricultural University
item NIKS, R - Wageningen Agricultural University
item FRANCIA, E - University Of Modena And Reggio Emilia (UNIMORE)
item PASQUARIELLO, M - University Of Modena And Reggio Emilia (UNIMORE)
item MAZZAMURRO, V - University Of Modena And Reggio Emilia (UNIMORE)
item Garvin, David
item PECCHIONI, N - University Of Modena And Reggio Emilia (UNIMORE)

Submitted to: Genome
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/3/2012
Publication Date: 2/9/2012
Citation: Barbieri, M., Marcel, T., Niks, R., Francia, E., Pasquariello, M., Mazzamurro, V., Garvin, D.F., Pecchioni, N. 2012. QTLs for resistance to the false brome rust Puccinia brachypodii in the model grass Brachypodium distachyon L. Genome. 55(2):152-163.

Interpretive Summary: Rust diseases caused by fungi belonging to the genus Puccinia cause significant cereal crop losses around the world. Although traditional breeding has been reasonably effective at controlling rust-related damage, the recent emergence of forms of rust fungi that overcome resistance genes used by breeders points to a need to search for alternative methods to control rusts that complement traditional breeding approaches. The model grass Brachypodium is infected by the leaf rust fungus Puccinia brachypodii, and genetic variation for resistance is present in Brachypodium. Therefore, it has been proposed as a model system for understanding the intricacies of resistance to rust fungi. This study sought to determine the genetic basis for variation in resistance to P. brachypodii in Brachypodium and to locate the positions of resistance genes detected on Brachypodium chromosomes. Three genes that improved disease resistance in two or three of the disease evaluations were found; further genetic analysis using a new genetic linkage map located them to three different Brachypodium chromosomes. They contribute between 12 and 30% of the variation for P. brachypodii resistance; thus, they are only partial disease resistance genes. Partial resistance genes are of particular interest because they are believed to retain their ability to protect plants from diseases longer than genes with major effects. Genetically dissecting partial leaf rust resistance in Brachypodium opens up avenues for isolating partial resistance genes from wheat and barley because the order of genes is highly conserved between these species. This in turn will offer the opportunity to stack partial resistance genes in these crops, which will provide durable rust resistance. This not only will increase grower profits in the U.S. but also will increase global food security in the face of new highly aggressive forms of rust fungi that have emerged in the last decade.

Technical Abstract: The wild grass Brachypodium distachyon (Brachypodium) is a new model system for temperate cereals, but its potential for studying interactions between grasses and their pathogens remains underexploited. Leaf rust caused by members of the fungal genus Puccinia is a major disease affecting temperate cereals such as wheat and barley, and recently the interaction between Brachypodium and the leaf rust pathogen Puccinia brachypodii was proposed as a model plant pathosystem. The objective of our study was to identify genomic regions in Brachypodium associated with quantitative resistance to P. brachypodii. The inbred lines Bd3-1 and Bd1-1, which differ in their level of resistance to P. brachypodii, were crossed to develop an F2 population of 110 plants. This population was evaluated for reaction to a virulent isolate of P. brachypodii at both the seedling and more advanced vegetative growth stages. To validate the results obtained on the F2 population, resistance levels were quantified in F2-derived F3 families in two independent experiments. Disease evaluations showed continuous, quantitative and transgressive segregation for leaf rust resistance. A new AFLP-based Bachypodium linkage map was developed and anchored to the genome sequence with SSR and SNP markers; the map consisted of 203 loci spanning 812 Kosambi cM. Molecular mapping identified three leaf rust resistance QTLs on chromosomes 2, 3, and 4 that were detected across experiments. This study is, to our knowledge, the first quantitative trait analysis in Brachypodium. We demonstrate that resistance to P. brachypodii in this population is governed by a small number of QTLs, two which act at the seedling stage and one which acts at both seedling and advanced growth stages. The results obtained, coupled with the wide range of genomic resources available for Brachypodium, offers perspectives to elucidate the molecular basis of quantitative resistance to rust fungi.