Three previously characterized resistances to yellow rust are encoded by a single locus Wtk1

Abstract

The wild emmer wheat (Triticum turgidum ssp. dicoccoides; WEW) yellow (stripe) rust resistance genes Yr15, YrG303, and YrH52 were discovered in natural populations from different geographic locations. They all localize to chromosome 1B but were thought to be non-allelic based on differences in resistance response. We recently cloned Yr15 as a Wheat Tandem Kinase 1 (WTK1) and show here that these three resistance loci co-segregate in fine-mapping populations and share an identical full-length genomic sequence of functional Wtk1. Independent ethyl methanesulfonate (EMS)-mutagenized susceptible yrG303 and yrH52 lines carried single nucleotide mutations in Wtk1 that disrupted function. A comparison of the mutations for yr15, yrG303, and yrH52 mutants showed that while key conserved residues were intact, other conserved regions in critical kinase subdomains were frequently affected. Thus, we concluded that Yr15-, YrG303-, and YrH52-mediated resistances to yellow rust are encoded by a single locus, Wtk1. Introgression of Wtk1 into multiple genetic backgrounds resulted in variable phenotypic responses, confirming that Wtk1-mediated resistance is part of a complex immune response network. WEW natural populations subjected to natural selection and adaptation have potential to serve as a good source for evolutionary studies of different traits and multifaceted gene networks.

Keywords: Wtk1; Yr15; YrG303; YrH52; EMS mutants; phenotypic response; positional cloning; tandem kinase domains; wild emmer wheat; yellow rust.

Fig. 1.

Differences in seedling phenotypic responses of WEW accessions donors of YrH52, Yr15, and YrG303, and the susceptible control PI428074 accession, at 14 dpi with Pst isolate #5006. Left panel: binocular microscopic observation of hypersensitive response and fungal sporulation (scale bar=1 cm). Right panel: sizes of fungal colonies and uredinia bags visualized by the fluorescent dye WGA–FITC (scale bar=100 µm). Note that G303 plants exhibited variable levels of resistance responses upon inoculation at the seedling stage (IT1–IT5) and only predominant IT5 is presented here. (This figure is available in color at JXB online.)

Fig. 2.

Comparisons of seedling and adult responses of four recombinant lines from the YrG303 segregating mapping population (G303×D447) to inoculation with Pst isolate #5006 at 14 and 21 dpi, respectively. SD, seedling inoculation under controlled growth chamber conditions; A, adult plant inoculation under field conditions. RILs 2-1-58-1, 2-4-88-4, and 1-1-66-4 harbor the functional Wtk1 allele, while 1-1-82-1 harbors the non-functional wtk1 allele, and was used as a susceptible control. (This figure is available in color at JXB online.)

Fig. 3.

Susceptible reaction of yrG303 and yrH52 mutants to Pst inoculation at 14 dpi with Pst isolate #5006. 2298 and Ariel-YrH52 are wild-type YrG303 and YrH52 hexaploid introgression lines, respectively, used to develop mutants. (This figure is available in color at JXB online.)

Fig. 4.

Genetic and physical maps of Yr15, YrG303, and YrH52 showing the same position for all three genes that correspond to WTK1. The consensus physical map represents three reference genomes, based on 1BS pseudomolecules of WEW Zavitan, T. durum Svevo, and T. aestivum Chinese Spring. The consensus physical map contains only collinear markers between the genetic and the physical maps. (This figure is available in color at JXB online.)

Fig. 5.

Primary and secondary structures of WTK1 kinase and pseudokinase domains alongside positions of knockout EMS mutations in yr15, yrG303, and yrH52 susceptible mutants. The diagram of WTK1 domain architecture highlights eight key conserved residues in the kinase domain (with numbers that correspond to their positions in cAPK (Hanks et al., 1988)) and the absence of five of them in the pseudokinase domain. Vertical lines indicate EMS mutations that block resistance. KinI, kinase domain; KinII, pseudokinase domain. (This figure is available in color at JXB online.)