Transfer to Wheat of Potentially New Stem Rust Resistance Genes from Aegilops speltoides
Abstract Stem rust resistance genes have been found in four different sources of Aegilops speltoides. These include diploid accessions AEG357-4 and AEG874-60 and the amphiploids Chinese Spring/Ae. speltoides TA8026 and TS01. Stem rust resistance was mapped to the 2S chromosomes derived from each of these lines. The previously reported 2B-2S#3 translocation derived from AEG357-4 was found to carry two stem rust resistance genes, here temporarily named SrAes2t and SrAes3t. The resistance genes found on the 2S chromosomes each derived from TA8026, TS01 and AEG874-60 are named SrAes4t, SrAes5t and SrAes6t, respectively. Lines carrying genes SrAes2t and SrAes3t are being distributed to wheat breeding programs around the world.
Keywords Aegilops speltoides • Resistance genes • Stem rust
The fungal disease stem rust, caused by Puccinia graminis f. sp. tritici, has long been a scourge of wheat crops since the beginnings of agriculture. Uredia have even been found on ancient wheat spikelets estimated at 3,300 years old (Kislev 1982).
Recently, the appearance and spread of mutants of the Ug99 stem rust lineage (summarized in Mago et al. 2013) has raised the awareness of the need to search for new resistance genes for use in wheat cultivars. Most of the currently named stem rust genes in wheat originate from related species (e.g., Secale cereale, Triticum monococcum, T. turgidum, T. timopheevii, Thinopyrum ponticum, Aegilops speltoides, Ae. tauschii and Ae. ventricosum). Many of these genes (e.g., Sr2, Sr22, Sr24, Sr26, Sr36 and Sr38) are widely used in agriculture.
The diploid species Ae. speltoides Tausch. (2n = 2x = 14) has provided several resistance genes against stem rust, namely Sr32, Sr39, Sr47, SrAes1t and SrAes7t (summarized in Mago et al. 2013), and leaf rust (caused by P. triticina), namely Lr28, Lr35, Lr36, Lr47, Lr51 and Lr66 (summarized in McIntosh et al. 2013). To date, none of these genes are deployed in agriculture. This work describes the discovery of potentially new stem rust resistance genes in four different sources of Ae. speltoides.
Materials and Methods
Diploid accessions of Aegilops speltoides AEG357-4 and AEG874-60 were provided courtesy of the The Harold and Adele Lieberman Germplasm Bank, Tel Aviv University, Israel. Amphidiploids of Chinese Spring/Ae. speltoides TS01 and TA8026 were provided courtesy of the Weizmann Institute, Rehovot, Israel and Wheat Genetics Resource Center, Kansas State University, USA, respectively.
Diploid Ae. speltoides accessions were crossed with cultivar Angas (from Dr Hugh Wallwork, Urrbrae, Australia) using wheat as the female parent. F1 seedlings were treated with 0.07 % colchicine (described in Dundas et al. 2008). Hybrids were crossed as females with Angas and later with cv. Westonia up to BC5. The amphiploid accessions were crossed with Angas then backcrossed with Westonia as the female parent to BC5.
Identification of backcrossed plants carrying each of the Ae. speltoides S-genome chromosomes using RFLP markers has been described in Dundas et al. (2008). PCR markers for 2S chromosomes were obtained from Mago et al. (2009) (Sr39#22r) and Seyfarth et al. (1999) (35R2/ BCD260F1). Backcrossed lines carrying S-genome chromosomes were tested for stem rust resistance at the University of Sydney (Pgt 34-1,2,3,4,5,6,7) and at Urrbrae (Pgt 343-1,2,3,5,6). Selected lines were screened against Ug99 races TTKSK, TTKST (+Sr24) and TTTSK (+Sr36) at the USDA Cereal Disease Laboratory, St. Paul, USA. Genomic in situ hybridization (GISH) was conducted on mitotic chromosome spreads using the procedure described in Mago et al. (2013).