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Chromosomal Location and Inheritance of Stem Rust Resistance Transferred from Hordeum bulbosum into Cultivated Barley (H. vulgare)

Stem rust, caused by Puccinia graminis f. sp. tritici, is an important disease on barley (Hordeum vulgare). Host resistance has effectively controlled stem rust, primarily through use of gene Rpg1. However, virulence to Rpg1 is present in North America, and a new race (TTKSK, or Ug99) from eastern Africa threatens barley production. A search for novel resistance was previously conducted, and an interspecific barley breeding line (212Y1) with introgressed chromatin from H. bulbosum was identified as carrying resistance to races MCCF and QCCJ. This study evaluated the inheritance of resistance in 212Y1 using populations from crosses to Morex (Rpg1 donor) and Q21861 (rpg4 donor) and the pathogen races MCCF (avirulent on Rpg1 and rpg4) and QCCJ (virulent on Rpg1 and avirulent on rpg4), and determined the chromosomal position of the introgression using genomic in situ hybridization (GISH) and chromosome-specific polymerase chain reaction (PCR)-based markers. Progeny from the 212Y1/Q21861 F₂ population segregated for resistant and susceptible plants, indicating different gene loci. Genetic analyses of Morex/212Y1 F₃ families fit a 7 homozygous resistant (HR):8 segregating:1 homozygous susceptible (HS) family segregation ratio to race MCCF, indicating that two genes controlled resistance. Plants in segregating families were in 3R:1S (Rpg1), 13R:3S (Rpg1+212Y1), and 1R:3S (212Y1) ratios. Genetic analyses of the same F₃ families fit a 1HR:2 segregating:1HS family segregation ratio to race QCCJ, indicating monogenic inheritance. Plants in segregating families were in a 1R: 3S ratio, indicating recessive inheritance in 212Y1. The introgression from H. bulbosum into H. vulgare was positioned on chromosome 6HS based on GISH and the PCR-based markers. No known stem rust resistance gene has previously been mapped to that region. Thus, it is proposed to name this novel gene from H. bulbosum as rpg6.