The Puccinia graminis f. sp. tritici (Pgt) Ug99 race group is virulent to most stem rust resistance genes currently deployed in wheat and poses a threat to global wheat production. The durum wheat (Triticum turgidum ssp. durum) gene Sr13 confers resistance to Ug99 and other virulent races, and is more effective at high temperatures. Using map-based cloning, we delimited a candidate region including two linked genes encoding coiled-coil nucleotide-binding leucine-rich repeat proteins designated CNL3 and CNL13. Three independent truncation mutations identified in each of these genes demonstrated that only CNL13 was required for Ug99 resistance. Transformation of an 8-kb genomic sequence including CNL13 into the susceptible wheat variety Fielder was sufficient to confer resistance to Ug99, confirming that CNL13 is Sr13. CNL13 transcripts were slightly down-regulated 2–6 days after Pgt inoculation and were not affected by temperature. By contrast, six pathogenesis-related (PR) genes were up-regulated at high temperatures only when both Sr13 and Pgt were present, suggesting that they may contribute to the high temperature resistance mechanism. We identified three Sr13-resistant haplotypes, which were present in one-third of cultivated emmer and durum wheats but absent in most tested common wheats (Triticum aestivum). These results suggest that Sr13 can be used to improve Ug99 resistance in a large proportion of modern wheat cultivars. To accelerate its deployment, we developed a diagnostic marker for Sr13. The identification of Sr13 expands the number of Pgt-resistance genes that can be incorporated into multigene transgenic cassettes to control this devastating disease.
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We calculated the annual genetic gains for grain yield (GY) of wheat (Triticum aestivum L.) achieved over 8 yr of international Elite Spring Wheat Yield Trials (ESWYT), from 2006–2007 (27th ESWYT) to 2014–2015 (34th ESWYT). In total, 426 locations were classified within three main megaenvironments (MEs): ME1 (optimally irrigated environments), ME4 (drought-stressed environments), and ME5 (heat-stressed environments). By fitting a factor analytical structure for modeling the genotype × environment (G × E) interaction, we measured GY gains relative to the widely grown cultivar Attila (GYA) and to the local checks (GYLC). Genetic gains for GYA and GYLC across locations were 1.67 and 0.53% (90.1 and 28.7 kg ha−1 yr−1), respectively. In ME1, genetic gains were 1.63 and 0.72% (102.7 and 46.65 kg ha−1 yr−1) for GYA and GYLC, respectively. In ME4, genetic gains were 2.7 and 0.41% (88 and 15.45 kg ha−1 yr−1) for GYA and GYLC, respectively. In ME5, genetic gains were 0.31 and 1.0% (11.28 and 36.6 kg ha−1 yr−1) for GYA and GYLC, respectively. The high GYA in ME1 and ME4 can be partially attributed to yellow rust races that affect Attila. When G × E interactions were not modeled, genetic gains were lower. Analyses showed that CIMMYT’s location at Ciudad Obregon, Mexico, is highly correlated with locations in other countries in ME1. Lines that were top performers in more than one ME and more than one country were identified. CIMMYT’s breeding program continues to deliver improved and widely adapted germplasm for target environments.
The hexaploid wheat (Triticum aestivum) adult plant resistance gene, Lr34/Yr18/Sr57/Pm38/Ltn1, provides broad-spectrum resistance to wheat leaf rust (Lr34), stripe rust (Yr18), stem rust (Sr57) and powdery mildew (Pm38) pathogens, and has remained effective in wheat crops for many decades. The partial resistance provided by this gene is only apparent in adult plants and not effective in field-grown seedlings. Lr34 also causes leaf tip necrosis (Ltn1) in mature adult plant leaves when grown under field conditions. This D genome-encoded bread wheat gene was transferred to tetraploid durum wheat (T. turgidum) cultivar Stewart by transformation. Transgenic durum lines were produced with elevated gene expression levels when compared with the endogenous hexaploid gene. Unlike nontransgenic hexaploid and durum control lines, these transgenic plants showed robust seedling resistance to pathogens causing wheat leaf rust, stripe rust and powdery mildew disease. The effectiveness of seedling resistance against each pathogen correlated with the level of transgene expression. No evidence of accelerated leaf necrosis or up-regulation of senescence gene markers was apparent in these seedlings, suggesting senescence is not required for Lr34 resistance, although leaf tip necrosis occurred in mature plant flag leaves. Several abiotic stress-response genes were up-regulated in these seedlings in the absence of rust infection as previously observed in adult plant flag leaves of hexaploid wheat. Increasing day length significantly increased Lr34 seedling resistance. These data demonstrate that expression of a highly durable, broad-spectrum adult plant resistance gene can be modified to provide seedling resistance in durum wheat.
Stripe rust, caused by Puccinia striiformis f. sp. tritici, was observed on bread wheat (Triticum aestivum L.) lines previously considered to be resistant. Specifically, near isogenic lines (NIL) carrying the resistance genes Yr10, Yr24, and Yr26 at the CIMMYT-Toluca research station were observed to be infected with stripe rust in 2015. Several leaves exhibiting stripe rust uredinia and urediniospores were collected from field plots containing the Yr24 (MEX16.04) and Yr26 (MEX16.03) NILs. More than 20 pustules were isolated from the diseased leaves and inoculated onto 12-day-old wheat seedlings of cultivars Morocco and Avocet Yr24. Inoculated plants were incubated in a dew chamber with 100% relative humidity for 20 h in the dark, maintained between 7 and 10°C. After incubation, plants were moved to a greenhouse where each pot was isolated in its own compartment. Greenhouse temperature was maintained at 15 to 18°C, supplemented with 10,000 lx of fluorescent light for 8 h per day. Urediniospores representing each of 24 isolates were collected 12 days after inoculation and maintained at 4°C. Virulence spectra of each isolate were determined by inoculating 30 differential lines that contained known Yr resistance genes. Cultivars Moro (Yr10), Chuanmai 42, and Neimai 836 (Yr24) were also included. Infection types were recorded approximately 2 weeks postinoculation using a 0 to 9 scale (McNeal et al. 1971). A change in the infection types from 1 to 9 for wheat lines containing Yr10 and from 3 to 9 for Yr24 and Yr26 indicated that a mutation for virulence to Yr10 and Yr24 (= Yr26) occurred in a recently identified isolate accessioned and maintained at CIMMYT as MEX14.141. MEX14.141, among others, had caused a severe yellow rust epidemic during 2014 on Mexican cultivars Nana F2007 and Luminaria F2014 (Solis et al. 2016). One of the 20 isolates mentioned, designated as MEX16.04, has the following avirulence/virulence formula: Yr1, 5, 15, Sp/Yr2, 3, 6, 7, 8, 9, 10, (17), 24, 26, 27, 28, 31, 32. MEX16.04 is avirulent onto the most susceptible barley cultivar Kaputar, which allowed us to classify the isolate as P. striiformis f. sp. tritici instead of P. striiformis f. sp. hordei. MEX16.04 belongs to the aggressive race group first identified in North America in 2000 and which became predominant in subsequent years (Milus et al. 2009). Seedling tests using the above mentioned methodology on 12-day-old seedlings of 167 bread wheat, 508 durum, and 460 member set composed of synthetic hexaploid wheats with their respective durum parents from CIMMYT indicated that MEX16.04 does not represent a major threat since a majority of the lines remained resistant to this isolate. Moreover, I is not known to be present in CIMMYT bread wheat germplasm and I occurs in low frequency; however, I is present in combination with other effective gene(s) in durum wheat. Purified urediniospores of MEX16.03 and MEX16.04 isolates are stored in the CIMMYT and INIFAP yellow rust collections.
Puccinia graminis f. sp. tritici (Pgt) race TKTTF was reported as the dominant race in the wheat stem rust epidemics in Ethiopia during 2014–15 (Olivera et al. 2015). The race and variants hereof have also been recorded elsewhere in Africa, the Middle East, and Europe (www.wheatrust.org/stem-rust-tools-maps-and-charts/race-frequency-map). Here, we report the presence of additional virulence to Sr25 in the TKTTF population, a resistance gene transferred to several Australian and CIMMYT wheat genotypes. At the seedling stage, Sr25 confers infection type (IT) 2 or lower for isolates in the Ug99 race group and up to IT 2+ toward race TKTTF (Newcomb et al. 2016; Olivera et al. 2015). Our results are based on Pgt isolates of the TKTTF race from Ethiopia (2012, 2013, 2015), Egypt (2014), Azerbaijan (2014), Iran (2009, 2011, 2014), Iraq (2014), Lebanon, Sudan, and Turkey (2012), Denmark and Germany (2013), and Sweden (2014). Race typing was carried out at the Global Rust Reference Center according to Jin et al. (2008), except that we scored IT on both leaf 1 and 2; additional single pustule isolates of each sample were raised and stored in liquid nitrogen (–196°C). Sr25 response was assayed using seedling leaves and stems of adult plants of Misr1 (Oasis/Skauz//4*BCN/3/2*Pastor) and Agatha/9*LMPG (Sr25 carriers) along with two reference lines, Triumph 64 (SrTmp) and NA101/MqSr7a (Sr7a), and Morocco as a control. Seedling ITs were scored 17 days post-inoculation at 18 ± 2°C using a 0 to 4 scale (McIntosh et al. 1995). Isolates showing ITs of 33+ to 4 on Misr1, Agatha/9*LMPG, and susceptible check were considered Sr25 virulent, and clearly different from ITs conferred by Sr25 avirulent isolates. Results were confirmed for each isolate by race typing additional single-pustule isolates derived from cultivars Misr1 and/or Agatha, along with avirulent reference isolates. Virulence for Sr25 was observed in race TKTTF isolates from Azerbaijan, Egypt, Ethiopia, Iran, Iraq, and Sweden, collected in 2014 or 2015, but not in any sample collected earlier than 2014. The results were confirmed on adult plants of Misr1 and Agatha/9*LMPG by Sr25 virulent and avirulent isolates of TKTTF, TTKSK, and TTKST, respectively. Spore suspensions of ∼0.5 ml at concentration of ∼3 × 105 spores/ml were injected into the stem internodes at Zadoks 45. The adult plant and seedling tests were carried out concurrently using the environmental conditions described above. The plants containing Sr25 were susceptible to the Sr25 virulent isolate and moderately resistant to moderately susceptible to the Sr25-avirulent isolates of TKTTF, TTKSK, and TTKST. The experiments were repeated two times with three replicates, using cv. Morocco as a susceptible check. Emergence of virulence to Sr25 in the race TKTTF is considered significant due to its spread into new areas and the potential loss of a significant source of resistance against Ug99.