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

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

doi:10.3389/fpls.2018.01789

Abstract

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

Highlights

The obligate biotrophic fungus Puccinia graminis f. sp. tritici Eriks. and E
In 1942, the barley variety Kindred was released as the first commercial barley containing the stem rust resistance gene Rpg1 (Resistance to P. graminis 1) (Steffenson, 1992) and since barley-breeding programs in the upper Midwest of the United States have fixed the Rpg1 gene in their lines, which originally came from either cv
Conlon gave a clear susceptible reaction with an infection response (IR) ranging from 5MS to 25MSS and a median score of 20MS

Summary

Introduction

The obligate biotrophic fungus Puccinia graminis f. sp. tritici Eriks. and E. Stem rust epidemics caused devastating yield losses of wheat and barley in several parts of the world, including the northern Great Plains of the United States (Roelfs, 1992; Steffenson, 1992). The northern Great Plains is a stem rust prone area that regularly suffered stem rust epidemics in barley prior to the 1940s (Steffenson, 1992). In 1942, the barley variety Kindred was released as the first commercial barley containing the stem rust resistance gene Rpg (Resistance to P. graminis 1) (Steffenson, 1992) and since barley-breeding programs in the upper Midwest of the United States have fixed the Rpg gene in their lines, which originally came from either cv. For nearly 80 years, Rpg has been the only source of durable resistance in barley and has maintained its effectiveness against the majority of isolates making up the North American Pgt populations

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