AUTHOR=Sharma Poudel Roshan , Al-Hashel Abdullah F. , Gross Thomas , Gross Patrick , Brueggeman Robert 

TITLE=Pyramiding rpg4- and Rpg1-Mediated Stem Rust Resistance in Barley Requires the Rrr1 Gene for Both to Function

JOURNAL=Frontiers in Plant Science

VOLUME=9

YEAR=2018

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2018.01789

DOI=10.3389/fpls.2018.01789

ISSN=1664-462X

ABSTRACT=<p>Stem rust, caused by <italic>Puccinia graminis</italic> f. sp. <italic>tritici</italic> (<italic>Pgt</italic>) is an economically important disease of wheat and barley. <italic>Rpg1</italic> is the only resistance gene deployed in Midwestern US barley varieties and provides remarkable resistance to most North American races, except <italic>Pgt</italic> race QCCJB. <italic>Rpg1</italic> is also ineffective against <italic>Pgt</italic> race TTKSK and its lineage that originated in Africa. The barley <italic>rpg4</italic>-mediated resistance locus (RMRL) conferring resistance to <italic>Pgt</italic> races QCCJB and TTKSK was isolated from line Q21861, which is resistant to all known <italic>Pgt</italic> races due to <italic>Rpg1</italic> and RMRL. To develop elite barley varieties RMRL was pyramided into the varieties, Pinnacle and Conlon (both contain <italic>Rpg1</italic>), producing the near isogenic lines (NILs), Pinnacle RMRL-NIL (PRN) and Conlon RMRL-NIL (CRN). The CRN was resistant to <italic>Pgt</italic> races QCCJB (RMRL specific) and HKHJC (<italic>Rpg1</italic> specific) at the seedling stage and <italic>Pgt</italic> race TTKSK (RMRL specific) at the adult stage. In contrast, PRN was susceptible to QCCJB and HKHJC at the seedling stage and TTKSK at the adult stage. Interestingly, PRN’s susceptibility to QCCJB and HKHJC showed that RMRL was non-functional in the Pinnacle background but its presence also suppressed <italic>Rpg1</italic>-mediated resistance. Thus, in the absence of a gene/s found in the Q21861 background, <italic>Rpg1</italic> becomes non-functional if RMRL is present, suggesting that another polymorphic gene, that we designated <italic>Rrr1</italic> (<underline>r</underline>equired for <italic>rpg4</italic>-mediated <underline>r</underline>esistance <underline>1</underline>), is required for RMRL resistance and <italic>Rpg1</italic>-mediated resistance in the presence of RMRL. Utilizing a PRN/Q21861 derived recombinant inbred line (RIL) population, <italic>Rrr1</italic> was delimited to a ∼0.5 MB physical region, slightly proximal (∼1.8 MB) of RMRL on barley chromosome 5H. A second gene, designated <underline>r</underline>equired for <italic><underline>R</underline>pg1</italic>-mediated <underline>r</underline>esistance 2 (<italic>Rrr2</italic>), with duplicate gene action to <italic>Rrr1</italic> in <italic>Rpg1</italic>-mediated resistance function, was genetically delimited to a physical region of ∼0.7 MB, slightly distal (∼3.1 MB) to <italic>Rpg1</italic> on the short arm of barley chromosome 7H. Thus, <italic>Rrr1</italic> is required for RMRL resistance and <italic>Rrr1</italic> or <italic>Rrr2</italic> is required for functional <italic>Rpg1</italic>-mediated resistance in the presence of the RMRL introgression. Candidate <italic>Rrr1</italic> and <italic>Rrr2</italic> genes were identified that need to be considered when pyramiding <italic>Rpg1</italic> and RMRL in barley.</p>