Quantitative trait loci for adult plant resistance to stem rust in bread wheat cv. ‘Akuri’
Department of Plant Breeding and Biotechnology, University of Eldoret, Kenya
Resistance is the most economically viable approach to curb the threat of rusts in wheat. The defeat of Sr31 and vulnerability of other resistance genes to the highly virulent Pgt race Ug99 and variants led to renewed efforts to discover and deploy resistance genes/QTLs in new durably resistant varieties. Akuri is a CIMMYT-developed bread wheat line exhibiting adult plant resistance (APR) in field trials in Kenya despite susceptibility to many races at the seedling stage. This study was designed to identify genomic regions contributing APR to stem rust in Akuri. One hundred and forty one RILs and parents of an F2:5 Akuri x PBW343 population were evaluated in Njoro for APR to stem rust over three seasons. Composite interval mapping was implemented on Windows QTL Cartographer to detect QTLs at a LOD threshold of 2.5 utilizing 910 high quality SNPs previously typed on the DArT-GBS platform. Preliminary QTL analyses revealed loci on chromosomes 1B, 2B and 3B consistently contributing to stem rust resistance. These QTL respectively explained ~7, 9, and 8% of the phenotypic variation. A comparison with the recently reported QTL consensus map revealed that the QTL herein discovered are probably novel. Work is underway to saturate the identified genomic regions with microsatellite markers to identify candidate, linked markers for use in marker assisted selection (MAS)
In vitro differentiation of the transport specificities of naturally occurring recombinant Lr34 variants
National Research Council of Canada, Canada
Plant breeders use naturally occurring resistance genes to fight plant diseases. However, new fungal strains rapidly emerge and defeat these genes. For almost a century, the wheat Lr34 gene has conferred a degree of stable resistance to the wheat rusts, making it one of the most important resistance genes. While sequence homology of the cloned Lr34 gene predicted that it encodes a putative ATP binding cassette (ABC) transporter protein belonging to the ABC G subfamily (also known as Pleiotropic Drug Resistance or PDR), its target transport substrate and mechanism of action remains enigmatic. In an effort to understand this transporter we designed several DNA constructs of the Lr34 gene and expressed them in yeast (Saccharomyces cerevisiae). Here we report the successful expression and purification of functional recombinant Lr34 protein. In vitro proteoliposome translocation assays identified the transport substrate of the Lr34Sus protein and demonstrated that the LR34Res protein has the same transport specificity. We also report the identification of related metabolites from flag leaves of Lr34-expressing wheat plants and discuss the functional relevance of these metabolites to the disease resistance and leaf tip necrosis (LTN) phenotypes caused by expression of Lr34Res.
Genomic selection for wheat traits and trait stability
Horticulture and Crop Science, the Ohio State University, Ohio Agricultural Research and Development Center, USA
Trait and trait stability are important for wheat breeding. Our objectives were to assess the relative efficiency of genomic selection (GS) for various wheat traits and trait stability using four different models. Genotyping was conducted with a 90K SNP chip panel. SNP tagging was used to obtain a subset of 3,919 relatively independent markers for downstream analysis. Phenotyping was conducted on a population consisting of 273 lines, from seven different soft red winter wheat breeding programs in the U.S.A. Eberhart and Russells’ regression and additive main effects and multiplicative interaction (AMMI) models were used to assess trait stability. GS accuracy (r) was assessed for ridge regression best linear unbiased prediction (rr-BLUP), Bayesian ridge regression (BRR), reproducing kernel hilbert space (RKHS), and elastic net (EN). Across all models, GS produced a wide range of accuracies for trait stability (0.1 to 0.65) that varied by trait and stability method. Accuracy was 0.35 for yield and 0.44 for yield stability using AMMI, indicating the viability of GS in selecting lines with both high and stable yield. Our findings lay the foundation for wheat breeding programs in northeastern U.S.A. to implement GS. It also provides useful information for wheat rust researchers: as phenotypic selection for rust resistance can be both expensive and time consuming and rapid evolution of rust pathogens require an emphasis on durable resistance controlled by multiple genes, the GS approach applicable for complex traits thus has potential to achieve higher gains per unit time than traditional breeding for rust resistance.
The status of wheat diseases in Ethiopia in the 2014/2015 main crop season
Ambo Plant Protection Research Center, Ethiopia
During the 2014/15 main crop season 831 wheat fields and experimental plots were assessed for diseases; 66.2% were in the Oromiya, 20.3% in Tigray and 13.4% in SNNP regions. The major diseases encountered include rusts, septoria leaf blotches, foot and root diseases, Fusarium head blight and smuts. The overall mean prevalence of stem rust was 61%, yellow rust 22%, leaf rust 18.8% and Septoria leaf blotches 52.9%. The incidence of the important diseases was highest in SNNP region. The mean incidence of yellow rust ranged from 5.7% in Oromiya to 39.2% in SNNP. The lowest incidence of stem rust, 15.9%, was noted in Tigray and the highest, 64.4%, in SNNP. The minimum mean Septoria incidence was 23% in Oromiya, and maximum was 66.7% in SNNP. Stem rust severities varied from 7 MR-MSS in Tigray to 36 MR-MSS in SNNP. Furthermore, the widely grown varieties Digelu and Danda’a were resistant to yellow rust, but susceptible to stem rust. Varieties Meda Wolbu, Hogona and Ogolcho were not affected by any of the rusts in Oromiya. Variety Hidase was susceptible in SNNP. Out of 115 Pgt cultures established from stem rust samples collected from the three regions, races TKTTF, RRTTF, TTKSK, TRTTF and JRCQC were identified. Stem rust samples were collected from varieties Digelu, Danda’a, Kakaba, and Hidase. Race TKTTF, virulent on Digalu and first identified in Oromiya region in 2013, is now present in all areas. Adapted varieties with durable stem rust resistance in Ethiopia remains an urgent requirement.
Wheat rust status and its implications in Pakistan
Crop Diseases Research Institute, Pakistan Agricultural Research Council, Pakistan
Detailed rust surveillance of wheat growing areas in Pakistan was conducted from 2011 to 2014. Information about varietal distribution, growth stage, and rust incidence and severity was collected at 950 locations, and rust samples collected from these locations were subjected to race analysis. Yellow rust showed increasing incidence of high to moderate severity. Commercial cultivars released during 1991 to 2011 showed MS to S reactions. Twenty eight races were identified, most with wide virulence ranges. The frequencies of virulence to Yr1, Yr6, Yr7, Yr8, Yr9, Yr17, Yr27, Yr43, Yr44 and YrExp2 all exceeded 50%. Leaf rust also showed increasing incidence, mainly due to cultivation of the susceptible cv. Sehr-06. Fourteen races were identified. The frequencies of virulence to Lr1, Lr2c, Lr3a, Lr16, Lr26, Lr3c, Lr17a, Lr30, LrB, Lr10, Lr14a, Lr14b and Lr21 exceeded 50%. Virulences to Lr2, Lr9, Lr24, Lr18 and Lr19 were not detected and frequencies of virulence to Lr11 and Lr20 were low. Current and old commercial cultivars showed MS to S reactions. In 2011 and 2014 stem rust incidence was sporadic, but in 2013 it was present at 33 of 232 locations in Sindh, Punjab, Khyber Pakhtunkhwa and Baluchistan. Race RRTTF was identified in all samples.
Emerging new virulence gene combinations in the Mexican Pst population
Campo Experimental Valle de México INIFAP, México
Yellow (stripe) rust continues to be an important disease of wheat in the irrigated EL Bajio region and northwestern Mexico, and in the High Plateau of Central Mexico. Isolate MEX 96.11, virulent to race-specific resistance genes Yr2, Yr3, Yr6, Yr7, Yr9 and Yr27, represented the most prevalent Pst race until 2001 and evolution of new virulences was slow. Several additional R-genes, including a gene in Pollmer triticale, are now defeated. The aggressive Pst race with Yr8 virulence, first detected in the United States in 2000, not only became widespread in Mexico by 2003 but continued to evolve at a more rapid rate with virulence to Yr1 occurring in 2003. However these races did not cause crop losses other than increased levels of head infections in some cultivars. Variants with virulences to Yr17 and Yr31 were detected in 2007 and 2008, respectively. These resistance genes are known to occur in some wheat varieties and breeding materials. 2010 disease data from trap nurseries that included the Avocet isolines and other varieties indicated the existence of virulence for Yr1, 2 (Siete Cerros), 3 (Tatara), 6, 7, 8, 9, 17, 27 and 31 (Rebeca F2000) in different Pst isolates. During the 2014 crop season, an epidemic occurred in farm fields and samples were collected and analyzed in greenhouse tests. Virulence combination V2, 3, 6, 7, 8, 9, 17, 27 and 31 first identified and represented in isolates CEVAMEX14.25 and MEX 14.141, and a similar isolate MEX14.146 virulent to Yr1 were the most frequent. These new virulence combinations caused yield losses in cultivar Nana F2007 grown in the Mexican highlands and Luminaria F2012, released for the irrigated areas of Bajio. Our results indicate a continuing evolution and accumulation of virulences in the aggressive Pst lineage. Determination of the defeated genes in Nana F2007 and Luminaria F2012 is underway.
Pyramiding genes Sr33, Sr35, Sr36, SrCad, and Sr43 for resistance to Ug99
Agriculture and Agri-Food Canada, Eastern Cereal and Oilseed Research Centre, Canada
Most of the current stem rust resistance genes (Sr) in Canadian wheat varieties are ineffective against the Pgt race Ug99 lineage, which pose a major threat to wheat production worldwide. Several stem rust resistance genes, including Sr33, Sr35, Sr36, SrCad/Sr42 and Sr43, are effective against race TTKSK. Although Sr36 is ineffective against Ug99 race TTTSK, it is still potentially useful for pyramiding genes to develop germplasm with durable stem rust resistance. For this purpose, we made crosses among RL5405 (Sr33), RL6099 (Sr35), Lang (Sr36), AC Cadillac (SrCad/Sr42), and RWG34 (Sr43) containing the respective Sr genes. A total of 54 doubled haploid (DH) lines were produced from the F1 from AC Cadillac/Lang//RWG34/RL5405, and 82 DH lines were obtained from RWG34/RL5405//RL6099. The DH progeny were tested at the seedling stage with race TTKSK and susceptible lines were discarded. We putatively developed 12 genotypes with multiple Sr gene combinations, including Sr33+Sr36+SrCad/Sr42+Sr43, Sr33+Sr36+SrCad/Sr42, Sr33+Sr36+Sr43, Sr33+SrCad/Sr42+Sr43, Sr36+SrCad/Sr42+Sr43, Sr35+Sr33+Sr43, Sr33+Sr36, Sr33+Sr43, Sr36+SrCad/Sr42, Sr36+Sr43, Sr35+Sr33, and Sr35+Sr43, based on positive association with linked PCR markers. Another population with 63 DH lines was derived from (Hoffman*2/RL6099)//(Hoffman*2/Lang) to combine the Fusarium head blight (FHB) resistance of Hoffman (Fhb1) with Sr35 and Sr36. We found 17 of 63 DH lines containing both Sr35 and Sr36 based also on linked PCR markers. This indicated that the combination Sr35+Sr36 was pyramided into the Canadian cultivar Hoffman; this derivative will be useful for development of cultivars resistant to Ug99 and FHB in Canada.