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
(
r
equired for
rpg4
-mediated
r
esistance
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
r
equired for
R
pg1
-mediated
r
esistance 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.