Resistance offers the best means of control of the cereal rusts, but must be strategically deployed so as to avoid exposure of single major genes, which have faltered so many times in the past. The pyramiding of multiple effective resistance genes is a strategy that has proven effective in a number of wheat production areas around the world. However, the process of incorporating multiple resistance genes into a single cultivar using standard breeding techniques is time consuming, laborious, and hampered by the problem of linkage drag. If a suite of effective resistance genes could be efficiently cloned and transferred into wheat as a cassette, it would accelerate the development of durably resistant varieties without the problem of linkage drag. Toward this end, we have developed a resistance gene cloning technology based on resistance gene enrichment sequencing (RenSeq) of EMS-derived mutant R gene alleles. As a proof of concept test, we successfully ‘re’-cloned the already characterized gene Sr33 and are now targeting the cloning of eight other effective resistance genes. For the identification of susceptible mutants for the cloning of Sr32 from Aegilops speltoides, we screened 1,109 M2 families with race TPMKC — as a surrogate for race TTKSK. Five susceptible M2 mutants were confirmed by progeny testing. These mutants were also susceptible to race TTKSK. For the population involving Sr1644 from Ae. sharonensis, 1,649 M2 families were screened, yielding 33 M2 families that appeared to segregate for susceptibility. Thirteen of 33 families were confirmed as bona fide susceptible mutants by progeny tests in the M3 generation. Identification of susceptible EMS mutants of Sr32 and Sr1644 suggests that the underlying resistance in these lines is conferred by single genes. We will report on progress to clone and characterize these genes using R gene exome capture and sequencing technology (RenSeq).
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Stem rust is a potentially destructive fungal disease of wheat worldwide. In 1998 Pgt pathotype TTKSK virulent to Sr31 was detected in Uganda. The same pathotype was confirmed in Lorestan and Hamedan provinces of Iran in 2007. We used a derivative of race TTKSK to phenotype 62 Iranian wheat landraces (resistant to stripe rust in a previous study) at the seedling stage to this new pathotype (TTSSK). Twenty eight accessions were evaluated for the presence of resistance genes Sr2, Sr22, Sr24, Sr25, Sr26, Sr35, Sr36 and Srweb using SSR markers. None carried Sr2, Sr24 or Sr26, but the presence of Sr22, Sr25, Sr35 and Sr36 was indicated. Some susceptible landraces predicted to carry Sr2 by marker analysis require further investigation. To evaluate defense gene expression in compatible and incompatible stem rust interactions we sampled resistant and susceptible cultivars at 0, 12, 18, 24, 72 hours post-inoculation (hpi). ?-1,3 glucanase expression was studied using qGLU-S and qGLUU-AS primers and a real-time PCR step-one ABI machine, with ?-tubulin and EF1-? genes used as internal controls. In incompatible interactions defense gene expression was increased at 24 hpi, but in compatible interactions the highest level of expression occurred at 12 hpi and was significantly decreased at 18 hpi. The results revealed that expression of defense genes such as ?-1,3 glucanase was earlier in compatible than in incompatible interactions but the expression level was less in incompatible interactions. On the other hand, in susceptible genotypes the expression of defense genes increased immediately after inoculation and declined sharply after infection. In contrast defense gene expression in resistant genotypes began to increase after establishment of the pathogen.
Wild species are sources and donors of many valuable traits for wheat improvement. We studied winter wheat introgression lines for productivity traits, disease resistance, and protein, globulin, gliadin and glutenin contents as well as grain mineral concentrations. Laboratory and field studies allowed selection in populations segregating for resistance to yellow rust and leaf rust. Lines 1718, 1721-9, 1721-4, 1675 and 1727 had the highest yields (6.2 t/ha) and stable leaf rust and stem rust resistances, but were still variable in response to stripe rust (30-80 S). Lines 1718 (Bezostaya 1 x Ae. cylindrica, genomes CCDD) and 1721 (Bezostaya 1 x T. militinae2 - 6, ABG) were resistant to stripe rust in trials at yield levels of 3.7-7.6 t/ha and from 5.7 to 8.2 t/ha, respectively. Line 1675 (Zhetisu x T. kiharae, ABGD) was resistant to all three rusts. Line 1676 (Steklovidnaya 24 x T. timopheevi, ABG) was resistant to LR and SR at a yield level of 8.3 t/ha, and 1671 (Zhetisu x T. militinae, ABG) was resistant to YR and SR at a yield level of 7.5 t/ha. Protein contents of the lines ranged from 13.6 to 18.4%, and grain mineral contents were above average.
Stem rust is a major threat to wheat production in Georgia. Breeding for resistance to the rusts is a major strategy for wheat improvement programs. Our objective was determination of the stem rust resistance levels in entries of the 4nd International Winter Wheat Stem Rust Resistance Nursery. Responses of 85 varieties/lines were evaluated in an inoculated field nursery. A coefficient of infection (CI) and area under the disease progress curve (AUDPC) were estimated for each entry. Fifteen entries (T03/17, TAM-107/T21, SD92107-2/SD99W042, KS95U522/TX95VA0011/F1/JAGGER, AR800-1-3-1/NW97S320, FL9547/NC00-14622, FL9547/TX00D1626, TAM302/KS93U450, MCCORMİCK/TREGO, NC00-14622/2137, TAM200/KAUZ//GOLDMARK/3/BETTY, KS920709-B-5-1-1/BURBOT-4, AFINA SOMNEZ, TAM200/KAUZ/4/BEZ/NAD//KZM(ES85.24/3/F900K) were resistant; 39 showed moderate resistance; 15 were scored MR-MS and 17 were moderately susceptible. Most of entries had very low CI (0.2 - 0.5) and AUPDC (less than 10.0); the best including T07/08, T07/09, T08/02, T08/01, T08/02, T08/04, CAKET/PEHLIVAN, ID800994.W/VEE//PIOPIO/3/MNCH/4/FDL4/KAUZ, PBI1013.13.3/3233.35 /3/STAR//KAUZ/STAR, DULGER-1//VORONA/BAU, ZANDER-17/3/YE2453/KA//1D13.1/MLT, 55-1744/7C//SU/RDL/3/CROW/4/MILAN/5/ITOR, 1D13.1/MLT//TUI/3/S?NMEZ/4/ATAY/GALVEZ87, TAM107//ATAY/GALVEZ87, HBF0290/X84W063-9-39-2//ARH/3/LE2301, STAR/BWD/3/PRL/VEE#6//CLMS, FRTL//AGRI/NAC/3/KALYOZ-17, CV. RODİNA/AE.SPELTOİDES10 KR, TAM 107//ATAY/ GALVEZ87, and 06393GP1. The severities for susceptible entries varied from 20 to 70%, with checks Morocco and Bezostaya 1 at 80% and 60%, respectively. However, the majority of entries (94%) had average CI of 0.2 - 20 and five entries with CI 21 - 40 had high to moderate levels of resistance.
Wheat rusts can be controlled by host resistance or chemicals. Ethiopian farmers are not widely experienced with chemicals. Sixty seven (49 bread wheat (BW) and 18 durum (DW)) non-replicated varieties were planted on 0.4 m2 plots at 22 rust hotspot locations in 2014. Kubsa and Digalu were used as susceptible checks for YR and SR, respectively. Rust severities were scored according to the modified Cobb scale. Ten YR and 12 SR hotspot locations with mean rust severities of ≥40% were used in data analyses. Kubsa had a mean 59% YR severity and Digalu, a 73% SR severity. Rust severity levels were divided into three categories, viz. low (≤35%), moderate (36-40%) and high (>40%), across locations for both diseases. The frequency of varieties with low YR severities in the BW group was 26.5%, medium 18.4% and high 55.1% compared to DW varieties at 61%, 28% and 11%, respectively. In the case of SR, both BW and DW had large proportions of entries in the high severity category at 69.4% and 72.2%, respectively. The medium and low SR severity groups were represented by 20.4% and 10.2% for BW, and 11.1% and 16.7% for DW, respectively. In summary, the top 10 widely cultivated BW and a few DW varieties categorised in the medium and high severity groups for both YR and SR, would definitely require fungicides in rust-prone areas for optimum disease control. Many cultivars released after 1974 are still cultivated indicating that susceptible varieties are only slowly replaced. Hence, development and distribution of resistant cultivars, replacement of susceptible cultivars, and training industry workers and farmers on effective field scouting and fungicide use will be paramount for sustainable wheat production in Ethiopia.
This study reports the inheritance and genetic mapping of YrA seedling resistance to stripe rust in a resistant selection of the Australian spring wheat variety Avocet (AUS20601). Genetic analysis was performed on F2 and F3 generation families derived from crosses between wheat genotypes previously reported to carry the YrA resistance and lines that lack the YrA resistance phenotype. Seedling tests with two Pst pathotypes (104 E137 A- and 108 E141 A-) avirulent with respect to YrA confirmed that the resistance was inherited as two complementary dominant genes. Ninety-two doubled haploid (DH) lines derived from a cross between the Australian cv. Teal (seedling-susceptible) and Avocet R were used to confirm the mode of inheritance of YrA and to develop a DArT-Seq genetic map to locate the components of the YrA resistance. Marker-trait association analysis based on 9,035 DArT-Seq loci mapped the two genes to chromosomes 3DL and 5BL. F2 populations derived from intercrosses of seedling susceptible DH lines that carried each gene (based on marker genotype) reproduced the YrA phenotype and specificity, confirming the complementary resistance gene model. The YrA resistance component loci were designated Yr73 (3DL) and Yr74 (5BL). Candidate single gene reference stocks will be permanently accessioned following cytological analysis to avoid a T5B-7B translocation in Teal relative to Avocet and Chinese Spring.
The appearance and spread of new Pst races are common consequences of the widespread use of single resistance genes in one or more widely grown cultivars, with epidemics occurring some time later. Based on the geographical situation in China, epidemiology of stripe rust can be divided into three major zones, namely autumn sources of inocula, spring sources of inocula, and the spring epidemic areas. About 67 stripe rust resistance genes (Yr1 – Yr67) and some temporarily designated genes have been catalogued in cultivated wheat varieties. Many of the genes have unique linked markers that enable their transfer by marker assisted selection (MAS). We recommend firstly that wheat breeders, rust geneticists and pathologists work in together in evaluating the effectiveness of resistance in multi-pathotype seedling tests in the greenhouse and in field trials at hot-spot locations to identify the genes conferring stable resistance across environments; and secondly to apportion the available resistance genes to the different epidemiological regions. We expect that such regional diversity of resistance genes will provide strong barriers to seasonal spread between regions.
Leaf rust is the most widely occurring disease of wheat worldwide. Resistance is the most practical and effective way to control the disease. Most leaf rust resistance genes are race-specific (“R”, qualitative resistance) and a relatively few are adult plant resistance genes, some of which have been described as slow rusting (“APR”, quantitative resistance). Due to limited knowledge, most resistance genes have been deployed in cultivars by an inefficient “blind” approach. This results in the well known “boom and bust cycle” (resistance followed by susceptibility) because the pathogen evolves rapidly and migrates over long distances. Therefore, a breeding-by-design approach is needed to achieve durable resistance. Pyramiding multiple R, APR or APR+R genes has been used successfully over many years to achieve durable resistance to leaf rust in Canada and some other countries. To further enhance this strategy we seek to understand the molecular mechanisms underlying key resistance genes. To identify the molecular mechanisms underlying rust resistance conferred by major R and APR genes, we performed an integrated systemic transcriptome analysis via RNA-seq on the Thatcher NILs with Lr16, Lr22a, Lr21, Lr34, Lr34+Lr16, and Lr67 challenged with Pt race BBBD. Sampling was conducted over a time series during the infection process of both seedlings and adult plants. Through RNA-seq we were able to capture the dynamic interactome of host-pathogen interactions conferred by these R and APR genes. Preliminary results revealed that resistance reactions conferred by R gene Lr21 and APR gene Lr67 were significantly different compared to other R and APR genes. Significantly, the Thatcher NIL line with Lr34+Lr16 showed the combines defense reactions of Lr16 and Lr34.
Stripe rust and leaf rust have been major constraints to wheat production in Nepal since the 1960s. Several rust epidemics causing hardship for Nepalese wheat growers were due to race changes. Breeding for rust resistance was initiated with establishment of the National Wheat Research Program in 1972, but concerted searches for durable resistance came later with the introduction of wheat genetic resources from CIMMYT, Mexico. The early wheat varieties Nepal 297, Siddhartha, Vinayak, BL1473, BL 1022 and Annapurna series with leaf rust and stripe rust resistance genes Lr13, Lr23, Lr26 and Yr9, and Yr27 in the 1970s and 1980s succumbed to new races within a few years of release. However, Bhrikuti (CMT/COC75/3/PLO/FURY/ANA) with both major and minor gene combinations (Lr10, Lr14a, Lr26/Yr9/Sr31+ and Lr34/Yr18) and released in 1994 was unaffected by Yr9 virulence in 1997 and Yr27 virulence in 2004. This variety with >20 years of leaf rust and stripe rust protection continues to be the most popular wheat variety in Nepal. Three other varieties, Gautam (Siddhartha/Ning8319//Nepal 297) released in 2004, WK 1204 (SW89-3064/Star) released in 2007, and Pasang Lhamu (PGO/SERI) released in 1997 with Lr16, Lr26/Yr9/Sr31, Lr34/Yr18, Lr46/Yr29, Yr7, and Sr2 also remain resistant. The Ug99 resistant varieties Vijay (NL748/NL837), Danphe(KIRITATI//2*PBW65/2*SERI.1B) and Tilottama (Francolin#1 = Waxwing*2/Vivitsi) also possesses APR to the three rusts. Nepalese wheat researchers work closely with the CIMMYT Global Wheat Program and DRRW/BGRI to utilize knowledge and APR germplasm. Strong networks for participatory varietal selection involving women farmers in both the hills and terai help in faster adoption and in establishing varietal diversity. In summary, Nepalese wheat breeders have successfully used APR in protecting wheat crops.
Wild relatives are rich sources of genetic diversity for wheat improvement. Our research focuses on characterizing stem rust resistance in Aegilops, a genus whose 23 species are part of the secondary genepool of wheat. In a previous study, we evaluated nine Aegilops species (885 total accessions) from Israel for reaction to Pgt race TTKSK and found the frequency of resistance ranged from 14% for Ae. searsii to 100% for Ae. speltoides. To extend this investigation, we evaluated 231 additional Aegilops accessions from five of the same species, plus 165 accessions from seven uncharacterized species. All of these accessions were collected from countries other than Israel or were of unknown provenance. The frequencies of resistant accessions in Ae. speltoides (94% in this study vs. 100% previously), Ae. bicornis (93% vs. 79%), Ae. geniculata (48% vs. 45%), Ae. peregrina (50% vs. 57%), and Ae. searsii (10% vs. 14%) were very similar to those for the Israel cohort in the previous study with the exception of Ae. bicornis. Of the latter accessions, the highest frequencies of resistance were in Ae. cylindrica (88%) and Ae. columnaris (85%) followed by Ae. binuncialis (37%) and Ae. ventricosa (13%). Accessions resistant to race TTKSK were not found in Ae. crassa, Ae. juvenalis, or Ae. vavilovii. These data show that certain Aegilops species are particularly rich sources of resistance to TTKSK. Yet other species carry no resistance. Research is underway to characterize the genetics of resistance in several select accessions.