Molecular mapping of the stem rust resistance gene rpg8 in barley

Barley can be attacked by both the wheat stem rust pathogen (Puccinia graminis f. sp. tritici) and the rye stem rust pathogen (P. graminis f. sp. secalis) in the northern Great Plains region. The stem rust resistance of the landrace Black Hull-less was first described in response to an isolate of P. graminis f. sp. secalis. This resistance is conferred by a single recessive gene (designated as rpgBH), whose chromosomal position has not been determined. The objective of this investigation was to map this resistance gene on the barley genome and determine the resistance spectrum of Black Hull-less to a diverse panel of both domestic and foreign P. graminis isolates. To map rpgBH, a cross was made between Black Hull-less and the susceptible cultivar Steptoe. Phenotypic evaluations of the resulting F2 population to P. graminis f. sp. secalis isolate A20MN04-2 confirmed the recessive gene action of rpgBH. Genotyping data based on a 3K genotyping platform positioned the gene to the short arm of chromosome 6H. Since rpgBH maps to a genomic position different from all previously described stem rust resistance genes in barley, it was given the new locus designation of rpg8. In seedling assays made against a diverse panel of P. graminis isolates, Black Hull-less exhibited low infection types (from 1 to 23-) to P. graminis f. sp. tritici races MCCFC, QCCJB, HKHJC, TTKSK, TTKTT, TTRTF, and JRCQC and P. graminis f. sp. secalis isolates A20MN04-2 and 92-MN-90, but mostly high infection types (3-2 to 3-3) to P. graminis f. sp. tritici races TTTTF, TKTTF, and TTRTF. Although further evaluations should be made to confirm the adult plant resistance of Black Hull-less in the field, rpg8 may enhance and broaden stem rust resistance in barley.