Isolation and fine mapping of Rps6: an intermediate host resistance gene in barley to wheat stripe rust

Theor Appl Genet (2016) 129:831–843 DOI 10.1007/s00122-015-2659-x ORIGINAL ARTICLE Isolation and fine mapping of Rps6: an intermediate host resistance gene in barley to wheat stripe rust Andrew M. Dawson1 · John N. Ferguson1,2 · Matthew Gardiner1 · Phon Green1 · Amelia Hubbard3 · Matthew J. Moscou1 Received: 10 August 2015 / Accepted: 14 December 2015 / Published online: 11 January 2016 © The Author(s) 2016. This article is published with open access at Springerlink.com Abstract Key message We uncouple host and nonhost resistance in barley to Puccinia striiformis ff. spp. hordei and tritici. We isolate, fine map, and physically anchor Rps6 to chromosome 7H in barley. Abstract A plant may be considered a nonhost of a pathogen if all known genotypes of a plant species are resistant to all known isolates of a pathogen species. However, if a small number of genotypes are susceptible to some known isolates of a pathogen species this plant may be considered an intermediate host. Barley (Hordeum vulgare) is an intermediate host for Puccinia striiformis f. sp. tritici (Pst), the causal agent of wheat stripe rust. We wanted to understand the genetic architecture underlying resistance to Pst and to determine whether any overlap exists with resistance to the host pathogen, Puccinia striiformis f. sp. hordei (Psh). We mapped Pst resistance to chromosome 7H and show that host and intermediate host resistance is genetically uncoupled. Therefore, we designate this resistance locus Rps6. We used phenotypic and genotypic selection on Communicated by K. Smith. Electronic supplementary material The online version of this article (doi:10.1007/s00122-015-2659-x) contains supplementary material, which is available to authorized users. * Matthew J. Moscou matthew.moscou@sainsbury‑laboratory.ac.uk 1 The Sainsbury Laboratory, Norwich Research Park, Norwich NR4 7UH, UK 2 Present Address: School of Biological Sciences, University of Essex, Colchester CO4 3SQ, UK 3 National Institute of Agricultural Botany, Huntingdon Road, Cambridge CB3 0LE, UK F2:3 families to isolate Rps6 and fine mapped the locus to a 0.1 cM region. Anchoring of the Rps6 locus to the barley physical map placed the region on a single fingerprinted contig spanning a physical region of 267 kb. Efforts are now underway to sequence the minimal tiling path and to delimit the physical region harboring Rps6. This will facilitate additional marker development and permit identification of candidate genes in the region. Introduction Nonhost resistance is often described as the complete resistance of an entire plant species to a specific pathogen (Heath 2000; Mysore and Ryu 2004; Nürnberger and Lipka 2005). In the majority of cases, this definition will hold true, as generally, most plants remain healthy, despite the ubiquity of potentially pathogenic microbes in the environment. However, it is clear that some plant pathogen interactions do not prescribe to the qualitative separation of host and nonhost. Instead, they appear to exist in a transitional phase between the two states, where radial coevolution with microbial species leads to the erosion, or reinforcement, of host status to pathogenic microbes (Niks and Marcel 2009; Schulze-Lefert and Panstruga 2011). This ‘coevolution’ can be considered a short-term interaction relative to the evolutionary time of plant speciation. Under long-term timescales, the preponderance of evidence supports hostshift speciation rather than cospeciation in the evolution of plant and microbial species (de Vienne et al. 2013). In contrast, our understanding of the short-term dynamics of host specialization remains poorly understood. Host specialization is often observed in the interaction of mildew and rust fungi with grasses, particularly the formae speciales divide of cereal rusts (Bushnell and Roelfs 13 832 1984; Eriksson 1894; Niks and Marcel 2009). Eriksson (1894) first proposed the formae speciales to differentiate forms of cereal rusts that were pathogenically specialized to given host genera but were otherwise morphologically indistinguishable. However, it was found that the formae speciales were not exclusively restricted to their host genera (Straib 1937) and the application to plant species outside of the host genera can result in varying degrees of compatibility: ranging from haustoria formation and hyphal colonization continuing through to lifecycle completion and pustule formation (Bettgenhaeuser et al. 2014). Despite the observation of non-exclusivity, the formae speciales division has been maintained. Bettgenhaeuser et al. (2014) proposed that interactions involving inappropriate formae speciales and nonhost plant genera are intermediate host systems that exist in the evolutionary transition between host and nonhost. To date, a number of studies have reported on the genetic architecture of intermediate host systems with the majority reporting evidence for the role of major loci underlying resistance to nonhost formae speciales (Jafary et al. 2006, 2008; Pahalawatta and Chen 2005; Sui et al. 2010; Tosa 1989, 1992). So far, no major locus conditioning intermediate host resistance has been cloned within the Triticeae tribe. However, numerous major loci have been cloned for host pathosystems with the majority coding for intracellular, nucleotide-binding, leucine-rich repeat proteins (NLRs) (Krattinger et al. 2009). Whether the same observations will be made for major loci in intermediate systems is unclear. However, the proposed contribution of NLRs to nonhost immunity is now widely accepted despite the relatively few well-characterized examples (Mysore and Ryu 2004; Schulze-Lefert and Panstruga 2011; Thordal-Christensen 2003). Molecular characterization of two tandemly arranged NLRs, RGA4 (Resistance gene analogue 4) and RGA5 (Resistance gene analogue 5), have been shown to condition Pi-CO39(t) mediated resistance to a nonhost Magnaporthe oryzae (rice blast) isolate in rice (Cesari et al. 2013). Similarly, WRR4 conditions nonhost resistance to Albugo candida, the causal agent of white blister rust, in Arabidopsis thaliana (Borhan et al. 2008). These observations support the molecular evolutionary model proposed by Schulze-Lefert and Panstruga (2011) that implicates NLRs in nonhost resistance. In the model, the authors assert that the contribution of NLR triggered immunity will decrease as a function of evolutionary divergence time from the host. Given the presumed evolutionary infancy of the formae speciales divide, one may hypothesize that major loci governing nonhost resistance in intermediate host systems may be underpinned by NLRs analogous to host systems. However, very little evidence exists to support this notion due to a lack of wellresourced, model pathosystems, with robust phenotypes, 13 Theor Appl Genet (2016) 129:831–843 that permit the elucidation of the underlying molecular mechanisms of resistance. Barley (Hordeum vulgare L.) has many traits that make it an (...truncated)