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To safeguard bread wheat against pests and diseases, breeders have introduced over 200 resistance genes into its genome, thus nearly doubling the number of designated resistance genes in the wheat gene pool1. Isolating these genes facilitates their fast-tracking in breeding programs and incorporation into polygene stacks for more durable resistance. We cloned the stem rust resistance gene Sr43, which was crossed into bread wheat from the wild grass Thinopyrum elongatum2,3. Sr43 encodes an active protein kinase fused to two domains of unknown function. The gene, which is unique to the Triticeae, appears to have arisen through a gene fusion event 6.7 to 11.6 million years ago. Transgenic expression of Sr43 in wheat conferred high levels of resistance to a wide range of isolates of the pathogen causing stem rust, highlighting the potential value of Sr43 in resistance breeding and engineering.

Wheat (Triticum aestivum L.) stem rust caused by Puccinia graminis f. sp. tritici (Pgt) was generally insignificant in Europe from the 1960s until 2016, where a new race (TTRTF) caused damage on huge areas of durum wheat and bread wheat in Sicily (Bhattacharya, 2017). During the following five years, TTRTF was detected in eight additional countries in south- and central Europe (Patpour et al. 2022). In July 2021, seven wheat stem rust samples collected from spring wheat and one from barley in Svalov (55° 54′ 10.8″ N, 13° 6′ 54″ E) and Alnarp (55° 39′ 39.6″ N, 13° 4′ 40.8″ E), Sweden. Both cereal fields had a total disease incidence of 50% or higher. The samples were sent to the Global Rust Reference Center (GRRC, Denmark). Urediniospores of each sample were recovered on two susceptible cultivars, Line E and Morocco, which were used as susceptible controls in all experiments. Single pustular isolates were extracted and race typing was generally repeated two to three times based on Patpour et al. (2022) using 20 North American stem rust differential lines. Seedling infection types (IT) were scored on the first and second leaf 17 days post-inoculation using a 0–4 scale (Stakman et al. 1962; McIntosh et al. 1995). Isolates conferring ‘low’ ITs (i.e., 0, 0; 1, 1+, 2, and 2+), or combinations thereof, were considered ‘avirulent’ (incompatible), whereas ITs of 3-, 3, 3+, and 4 were considered ‘high’ (i.e., compatible, ‘virulent’). Race nomenclature was based on a modified letter code proposed by Jin et al. (2008). We conducted DNA extraction and molecular genotyping using seventeen Simple Sequence Repeat (SSR) primer pairs derived from Stoxen (2012) and applied at large scale by Patpour et al. (2022). Based on the results from pathotyping and genotyping, two samples from wheat showed Pgt race TKTTF (Clade IV-B), three from wheat TKKTF (Clade IV- F), and three samples from wheat and barley were identified as TTRTF (Clade III-B). This is the first report of race TTRTF in northern Europe, particularly, Sweden, which significantly extends the known distribution of this race. The TTRTF race is a serious threat to wheat productivity and evaluation of resistance of commercial European wheat varieties to the TTRTF race confirmed that 70% of the cultivars were susceptible (Patpour et al. 2022). Therefore, if the conditions are suitable for the establishment and development of stem rust, the disease can cause significant damage to the wheat crop in these countries. Susceptibility of European wheat varieties stress an urgent need to initiate new breeding efforts to identify effective sources of resistance to wheat stem rust in breeding programs. Funding: The work is funded by the European Union's Horizon 2020 research and innovation programme under grant agreement No 773311 (RustWatch). References: Bhattacharya, S. 2017. Nature 542, 145-146. Doi: 10.1038/nature.2017.21424 Jin, Y., et al. 2008. Plant Dis. 92:923. 10.1094/PDIS-92-6-0923 McIntosh, R. A. et al. 1995. Catalogue of wheat symbols for wheat. 1993, Beijing, China, 1333–1451. Patpour, M., et al. 2022. Front. Plant Sci., 02 June 2022 | https://doi.org/10.3389/fpls.2022.882440. Stakman, E. C., et al. 1962. ARS E-617. USDA, Washington, DC. USA. Stoxen, S. 2012. Master’s Thesis, University of Minnesota, Minneapolis, MN USA.

To safeguard bread wheat against pests and diseases, breeders have introduced over 200 resistance genes into its genome, thus nearly doubling the number of designated resistance genes in the wheat gene pool. Isolating these genes facilitates their fast-tracking in breeding programs and incorporation into polygene stacks for more durable resistance. We cloned the stem rust resistance gene Sr43, which was crossed into bread wheat from the wild grass Thinopyrum elongatum. Sr43 encodes a protein kinase fused to two domains of unknown function. The gene, which is unique to the Triticeae, appears to have arisen through a gene fusion event 6.7 to 11.6 million years ago. Transgenic expression of Sr43 in wheat conferred high levels of resistance to a wide range of isolates of the pathogen causing stem rust, highlighting the potential value of Sr43 in resistance breeding and engineering.

Key message: Stem rust resistance genes, SrRL5271 and Sr672.1 as well as SrCPI110651, from Aegilops tauschii, the diploid D genome progenitor of wheat, are sequence variants of Sr46 differing by 1–2 nucleotides leading to non-synonymous amino acid substitutions. Abstract: The Aegilops tauschii (wheat D-genome progenitor) accessions RL 5271 and CPI110672 were identified as resistant to multiple races (including the Ug99) of the wheat stem rust pathogen Puccinia graminis f. sp. tritici (Pgt). This study was conducted to identify the stem rust resistance (Sr) gene(s) in both accessions. Genetic analysis of the resistance in RL 5271 identified a single dominant allele (SrRL5271) controlling resistance, whereas resistance segregated at two loci (SR672.1 and SR672.2) for a cross of CPI110672. Bulked segregant analysis placed SrRL5271 and Sr672.1 in a region on chromosome arm 2DS that encodes Sr46. Molecular marker screening, mapping and genomic sequence analysis demonstrated SrRL5271 and Sr672.1 are alleles of Sr46. The amino acid sequence of SrRL5271 and Sr672.1 is identical but differs from Sr46 (hereafter referred to as Sr46_h1 by following the gene nomenclature in wheat) by a single amino acid (N763K) and is thus designated Sr46_h2. Screening of a panel of Ae. tauschii accessions identified an additional allelic variant that differed from Sr46_h2 by a different amino acid (A648V) and was designated Sr46_h3. By contrast, the protein encoded by the susceptible allele of Ae. tauschii accession AL8/78 differed from these resistance proteins by 54 amino acid substitutions (94% nucleotide sequence gene identity). Cloning and complementation tests of the three resistance haplotypes confirmed their resistance to Pgt race 98-1,2,3,5,6 and partial resistance to Pgt race TTRTF in bread wheat. The three Sr46 haplotypes, with no virulent races detected yet, represent a valuable source for improving stem resistance in wheat.

The wild relatives and progenitors of wheat have been widely used as sources of disease resistance (R) genes. Molecular identification and characterization of these R genes facilitates their manipulation and tracking in breeding programmes. We developed a reference-quality genome assembly of the wild diploid wheat relative Aegilops sharonensis and used positional mapping, mutagenesis, RNA-Seq and transgenesis to identify the stem rust resistance gene Sr62, which was also transferred to common wheat. This gene encodes a tandem kinase, homologues of which exist across multiple taxa in the plant kingdom. Stable Sr62 transgenic wheat lines showed high levels of resistance against diverse isolates of the stem rust pathogen, highlighting the utility of Sr62 for deployment as part of a polygenic stack to maximize the durability of stem rust resistance.

Barberry (Berberis spp.) may serve as alternate host of several Puccinia species including Puccinia graminis and P. striiformis causing stem and yellow rust on cereals and grasses, respectively. In order to study the importance of barberry in the epidemiology of Puccinia species in the CWANA region a rust survey was initiated. The aim was to 1) develop a surveillance protocol 2) develop molecular diagnostic tools for identifying Puccinia spp. from aecial samples, and 3) develop a data management and display system of results as part of the Wheat Rust ToolBox (http://wheatrust.org/international-services/service/). Rust infected leaves representing 120 barberry plants were collected in 2011 in Tajikistan and in 2012 in Iran, Azerbaijan, Uzbekistan and Georgia. Dried leaf samples were received at Global Rust Reference Center and DNA was extracted from up to six individual aecial clusters from each barberry plant. Due to variable quality of aecial samples DNA extraction was not successful for 40% of the samples. Sequences of EF1α, β-tubulin or ITS were analysed and compared to reference sequences of rust fungi infecting cereals and grasses. The analysis supported the presence of P. graminis s.l., P. arrhenatheri and P. striiformoides on barberry species. Survey and DNA sample maps with species designation were displayed in the Wheat Rust ToolBox. The future aim is to integrate barberry rust survey data based on molecular diagnostics and infection assays from research groups world-wide in order to gain insights into the role of barberry in the epidemiology of Puccinia spp. infecting cereals and grasses.