Vulnerability of Barley to African Pathotypes of Puccinia graminis f. sp. tritici and Sources of Resistance

Abstract

The emergence of widely virulent pathotypes (e.g., TTKSK in the Ug99 race group) of the stem rust pathogen (Puccinia graminis f. sp. tritici) in Africa threatens wheat production on a global scale. Although intensive research efforts have been advanced to address this threat in wheat, few studies have been conducted on barley, even though pathotypes such as TTKSK are known to attack the crop. The main objectives of this study were to assess the vulnerability of barley to pathotype TTKSK and identify possible sources of resistance. From seedling evaluations of more than 1,924 diverse cultivated barley accessions to pathotype TTKSK, more than 95% (1,844) were found susceptible. A similar high frequency (910 of 934 = 97.4%) of susceptibility was found for the wild progenitor (Hordeum vulgare subsp. spontaneum) of cultivated barley. Additionally, 55 barley lines with characterized or putative introgressions from various wild Hordeum spp. were also tested against pathotype TTKSK but none was found resistant. In total, more than 96% of the 2,913 Hordeum accessions tested were susceptible as seedlings, indicating the extreme vulnerability of the crop to the African pathotypes of P. graminis f. sp. tritici. In total, 32 (1.7% of accessions evaluated) and 13 (1.4%) cultivated and wild barley accessions, respectively, exhibited consistently highly resistant to moderately resistant reactions across all experiments. Molecular assays were conducted on these resistant accessions to determine whether they carried rpg4/Rpg5, the only gene complex known to be highly effective against pathotype TTKSK in barley. Twelve of the 32 (37.5%) resistant cultivated accessions and 11 of the 13 (84.6%) resistant wild barley accessions tested positive for a functional Rpg5 gene, highlighting the narrow genetic base of resistance in Hordeum spp. Other resistant accessions lacking the rpg4/Rpg5 complex were discovered in the evaluated germplasm and may possess useful resistance genes. Combining rpg4/Rpg5 with resistance genes from these other sources should provide more durable resistance against the array of different virulence types in the Ug99 race group.

Genome-Wide Association Mapping of Stem Rust Resistance inHordeum vulgaresubsp.spontaneum

“…In addition to the WBDC, the resistant controls of Chevron, carrying the stem rust resistance gene Rpg1 ; Q21861, carrying genes Rpg1 and rpg4/Rpg5 ; and Q/SM20, a doubled haploid line derived from Q21861 that carries only rpg4 / Rpg5 (Steffenson et al 1995) were included. Rpg1 and rpg4/Rpg5 are the most common stem rust resistance genes being used in commercial barley breeding programs and are readily detected by seedling phenotyping to selected Pgt pathotypes and also by genotyping (Steffenson et al 2017). Hiproly (PI 60693), a barley accession that lacks stem rust resistance, was used as the susceptible control in all stem rust experiments.…”

“…The rpg4/Rpg5 gene complex is very effective against pathotype TTKSK (and also QCCJB, MCCFC, and 92-MN-90); thus, accessions exhibiting resistance to this pathotype may carry this gene complex. To corroborate this possibility, TTKSK-resistant accessions were subjected to a molecular assay for a functional Rpg5 gene, described in detail by Steffenson et al (2017). Resistance to pathotype HKHJC may indicate the presence of Rpg1 in barley.…”

“…To break possible residual dormancy and encourage uniform germination and emergence, seeds were incubated at 4° for 1 wk and then transferred to a growth chamber at 20–22° with 14 hr photoperiod (provided by 160-W fluorescent and 60-W incandescent lamps emitting 150–250 μmol photons s −1 m −2 ). For the evaluations to pathotype TTKSK, plants were grown, inoculated, and incubated inside the Minnesota Department of Agriculture/Minnesota Agricultural Experiment Station Biosafety Level-3 (BSL-3) Containment Facility on the University of Minnesota St. Paul campus under a temperature regime of 19–22° and 14- to 16-hr photoperiod (400-W high-pressure sodium lamps emitting a minimum of 300 μmol photons s −1 m −2 ) (Steffenson et al 2017). Postinoculation conditions for the WBDC varied depending on the rust culture used and the resistance genes targeted for detection.…”

“…Pathotype TTKSK is a major threat to wheat production because 90–95% of the global crop hectarage is susceptible (Singh et al 2006, 2011). Similarly, in an extensive evaluation of over 1900 cultivated barley accessions from around the world, over 95% were found susceptible (Steffenson et al 2017). The large percentage of cultivated wheat and barley germplasm susceptible to a single pathotype of Pgt is both exceptional and extremely dangerous with respect to food security.…”

“…Several investigations of cultivated barley germplasm revealed that stem rust resistance to pathotypes QCCJB and TTKSK is very rare (Jin et al 1994; Zhou et al 2014; Steffenson et al 2013, 2017; Case 2017). One readily accessible source of genetic diversity for barley breeding is the wild progenitor H. vulgare subsp.…”