RP Singh, J Huerta-Espino, K Ammar
Variants of Puccinia triticina race BBG/BN, separately overcoming three resistance genes, were identified from durum wheat (Triticum turgidum ssp. durum) fields in northwestern Mexico since its introduction in 2001. Major genes available for use in breeding programs are limited and an alternative strategy is required. Previous studies indicated that slow rusting resistance in eight CIMMYT durums was determined by 2 to 3 minor genes with additive effects. Twenty-eight 4-way crosses were made between these lines with the aim of developing new germplasm with enhanced levels of resistance through pyramiding diverse minor genes. Plants in F1 (4-way) through F3 generations were selected for slow rusting under high leaf rust pressure at the Cd. Obregon and El Batan field sites in Mexico and spikes from selected plants were harvested as bulks. Plants in the F4 generation were individually harvested and1,843 advanced lines obtained, among which 106 lines with enhanced resistance, and desirable agronomic and grain characteristics were selected for non-replicated yield and leaf rust evaluation trials at Obregon during the 2007-2008 season. The best 19 lines, exhibiting near-immunity but with the presence of a few susceptible type pustules, parents and susceptible checks were evaluated for leaf rust resistance under very high disease pressure in replicated trials sown on two dates (16 May and 6 June) at El Batan during 2008. Spreader rows of susceptible cultivar ‘Banamichi C2004’, sown as border and as hills on one side of each plot, were inoculated with P. triticina race BBG/BP. Leaf rust severities, and host responses to infection were determined from weekly readings, and area under the disease progress curves (AUDPC) were calculated. Several lines were identified with significantly lower final leaf rust severity responses and AUDPC values than the most resistant parent in each cross. Our results show that enhanced levels of slow rusting can be generated by pyramiding diverse genes present in different parents. The trial is being repeated during the 2008-2009 season at Obregon to validate the results. In addition these lines are being used for transferring slow rusting resistance into high yielding, superior quality adapted backgrounds using the single-backcross approach.
Agriculture and Agri-Food Canada, Cereal Research Centre, Canada
T. Fetch, C.W. Hiebert, and B. McCallum
Stem rust, caused by Puccinia graminis f. sp. tritici, is a highly destructive fungal disease of wheat. This pathogen has been effectively controlled in western Canada through resistance since the 1950s. In 1999, a new highly virulent race of stem rust was identified in Uganda. The new strain, named “Ug99”, was given the North American race designation TTKSK. In situ screening has demonstrated that approximately 75% of Canadian wheat cultivars are susceptible to this new race of stem rust. Fortunately, two cultivars, Peace and AC Cadillac, were highly resistant to Ug99. A doubled haploid population was generated from the cross: RL6071/Peace, where RL6071 was the stem rust susceptible parent. In 2008, 189 DH lines from this population were evaluated for response to Ug99 in Kenya. RL6071 and Peace were rated: 80 S and 5 R, respectively. Disease ratings of the DH lines, ranged from 80 S to 1 R. Mendelian evaluation of the stem rust scores indicated a two-gene model (X2=5.51; 0.25<P<0.10; d.f.=3) of inheritance. Peace has the positive allele for the diagnostic Lr34 DNA marker (csLVMS1) published by Spielmeyer et al. (2008). It is believed that Peace carries Lr34 and that this gene may be one of the genes responsible for Ug99 resistance in this cultivar. Molecular mapping of the Ug99 resistance in cultivar Peace is underway.
1International Programs in the College of Agriculture and Life Sciences, and Plant Breeding and Genetics Section in the School of Integrative Plant Science, Cornell University, USA, and CIMMYT, Mexico
Stem rust is a globally important wheat disease that can cause severe yield loss. Breeding for quantitative stem rust resistance (QSRR) is important for developing cultivars with durable resistance. Genomic selection (GS) could increase rates of genetic gain for quantitative traits, but few experiments comparing GS and phenotypic selection (PS) have been conducted. Our objectives were to compare realized gain from GS based on markers only with that of PS for QSRR in spring wheat using equal selection intensities; determine if gains agree with theoretical expectations; and compare the impact of GS and PS on inbreeding, genetic variance, and correlated response for pseudo-black chaff (PBC), a correlated and likely pleiotropic trait. Over two years, two cycles of GS were performed in parallel with one cycle of PS, with each method replicated twice. For GS, markers were generated using genotyping-by-sequencing, the prediction model was initially trained using historical data, and the model was updated before the second GS cycle. Overall, GS and PS led to a 31±11 and 42±12% increase in QSRR and a 138±22 and 180±70% increase in PBC, respectively. Genetic gains were not significantly different, but were in agreement with expectations. Per year, gains from GS and PS were equal, but GS led to significantly lower genetic variance. This shows that while GS and PS can lead to equal rates of short-term gains, GS can reduce genetic variance more rapidly. Further work to develop efficient GS implementation strategies in spring wheat is warranted.
Department of Animal, Plant and Soil Sciences, La Trobe University, Australia
Climatic changes permit the spread of plant diseases to new areas. To ensure grain production meets the needs of the growing world population, wheat breeding must combine multiple disease resistances in single cultivars in order to maintain current yield potential. Over the last three decades, classical mapping and association studies have identified disease resistance loci for individual diseases, but few studies have investigated loci that confer resistance to multiple diseases. To address this limitation, we extensively surveyed the literature to identify wheat genomic regions harboring resistance to multiple diseases. We identified 174 trait-linked markers distributed across all wheat chromosomes, except chromosome 4A, and the numbers of disease resistance loci in each region ranged from two to ten. Our survey suggests that some regions of the genome contain multiple disease resistance genes, or genes with pleiotropic effects. We are using the Chinese Spring flow sorted chromosome survey contigs to investigate the genic contents of genomic regions containing multiple disease trait loci to address this question.
Department of Plant Pathology, University of Minnesota, USA
During the past 15 years, significant efforts have been directed to develop the grass species Brachypodium distachyon as a genetically tractable model for monocot plants, especially economically valuable cereals such as wheat, barley and oat. Such efforts have led to an increasing availability of genomic, genetic and bioinformatics tools designed to bypass the experimental challenges faced when addressing important biological questions in complex systems. Moreover, such advances may translate in the use of other valuable species of Brachypodium (e.g., B. hybridum), which are not nearly as well characterized as B. distachyon. Given the 2050 global food demands and needs to increase grain production we seek to develop innovative and sustainable approaches to decrease crop yield losses due to rust fungi. One possible strategy is the use of transgenic plants harboring non-host resistance-related genes from closely related species. B. distachyon and B. hybridum can serve as potential sources to engineer plant resistance against highly destructive rust fungi, such as Puccinia graminis and P. coronata. Advancing our understanding of non-host resistance in monocot species has been a slow process. However, the amenability of Brachypodium as a model system offers a means to accelerate scientific discovery of factors controlling non-host pathogen interactions involving stem and crown rust fungi. In a multi-pronged approach, we are leveraging genetic and genomic tools, as well as generating new resources to provide foundational knowledge in order to support plant genetic engineering programs.
State Key Laboratory of Crop Stress Biology for Arid Areas and College of Agronomy, Northwest A&F University, P. R. China
Rust outbreaks cause severe yield lossespose a serious threat to global food security. As biotrophic pathogens, rust fungi produce effectors to suppress host immunity. In this study, we used a systemic approach to identify and characterize effectors in Puccinia striiformis f. sp. tritici. Among secreted proteins encoded by the Pst genome, we identified 150 putative effectors that are Cys-rich or up-regulated during hostinfection. A systematic screen showed that 14 of them suppressed programmed cell death (PCD) triggered by BAX and INF1 in Nicotiana benthamiana, and all have high intra- and inter-species polymorphisms at the protein level. Although these 14 effectors individually made only minor contributions to Pst virulence, delivery of them into wheat plants via a bacterial type-III secretion system efficiently suppressed PTI and ETI. Interestingly, three of them, Pst_4941, Pst_4884, and Pst_5578, triggered HR-like PCD in the AvSYr1NIL, but only Pst_4941 also caused PCD in the AvSYr7 NIL, suggesting that they may function as avirulence factors. This study suggests that Pst effectors function to suppress PTI and ETI.
Kulumsa Agricutural Research Center, Ethiopian Institute of Agricultural Research, Ethiopia
Pgt race TKTTF, virulent for the SrTmp gene present in Ethiopian cv. Digalu and first detected in 2012, caused significant yield losses in Digalu during the 2013 and 2014 seasons. No suitable replacement varieties with significant seed volume were available, and alternate solutions were sought. EIAR, with the support of the DRRW project through CIMMYT-Ethiopia, introduced 5 tonnes of adult plant, rust resistant wheat cv. Kingbird from Kenya. Kingbird was evaluated for agronomic performance at seven locations vs. three checks in 2014, and was also evaluated for stem rust reaction in single-race nurseries (TKTTF, TTKSK, TRTTF and JRCQC). With support from USAID/CIMMYT, seed was concurrently multiplied on 37 ha producing 80 tonnes of seed that was distributed to farmers in 2015. Mean grain yield over locations was 2.76 t ha-1. Mean performance of Kingbird was 3.00 t ha-1 compared to 2.79 t ha-1 for Ogolcho, 2.83 t ha-1 for Biqa and 2.42 t ha-1 for Kakaba. Thus Kingbird gave yield advantages of 5 to 22% over the check varieties. Stem rust severities on Kingbird in the single race nurseries ranged from Tr to 15% and reactions ranged from TMR to SMS. The check varieties rated up to 45% severity with S type reactions. Thus Kingbird was superior in terms of yield potential and stem rust resistance as measured in these trials vs. the check varieties. Stem rust resistance of Kingbird is based on Sr2 and Sr57 and is hypothesized to have at least three additional APR loci. Seedling reactions of Kingbird to races TKTTF and Ug99 are characterized as susceptible. Sr57 is pleiotropic and confers partial resistance to all three rusts, powdery mildew, spot blotch, and BYDV. Based on early maturity, yield performance, and stem rust resistance, Kingbird is recommended for low- to mid-altitude wheat-growing areas of Ethiopia.
The University of Sydney, Plant Breeding Institute, Australia
Landraces and wild relatives of wheat are rich repositories of new rust resistance genes. Landraces are preferred over wild relatives for the absence of deleterious effects associated with large alien segments. A common wheat landrace, AUS 27969 (ex Portugal), from the Watkins Collection was resistant under field conditions and produced seedling infection type (IT) 2C against the widely virulent Australian Puccinia striiformis f. sp. tritici (Pst) pathotype 134 E16 A+ Yr17+ Yr27+. AUS 27969 was crossed with the susceptible genotype Avocet S (AvS) and the distribution of F3 lines conformed to monogenic segregation [40 non-segregating resistant (NSR), 93 segregating (Seg), and 37 non-segregating susceptible (NSS); ?2 = 1.61, P2d.f. >0.05] when tested with the same pathotype at the seedling stage. The population is currently being selfed to F6. DNA from NSR and NSS lines will be sent for high throughput analysis to identify the genomic region carrying the resistance gene. Resistance-linked SNPs will be mapped on the F6 RIL population. The resistance gene will be backcrossed into modern Australian wheat backgrounds.
The University of Sydney Plant Breeding Institute, Australia
Stripe rust of wheat was estimated to cause losses of A$127 m annually in Australia. Although stripe rust can be controlled through the use of chemicals, breeding for resistance is considered to be the best means of control. Identification and characterization of diverse sources of resistance is essential to achieve durable stripe rust control. A common wheat landrace AWCC618 showed resistance (IT 1CN) to Australian Puccinia striiformis f. sp. tritici (Pst) pathotypes. AWCC618 was crossed with the susceptible genotype Avocet S (AvS) to determine the genetic basis of resistance. Seedling tests on 123 AWCC618/AvS F3 families using Australian Pst pathotype 134 E16 A+ 17+ 27+ indicated monogenic inheritance of resistance (22HR:68SEG:33HS; χ21:2:1=3.34, non-significant at P=0.05 and 2 d.f.). The resistance locus was temporarily named YrAW3. Selective genotyping of eight homozygous resistant (HR) and eight homozygous susceptible (HS) F3 families using the 90K SNP Infinium assay tentatively located YrAW3 on chromosome 6A. The AWCC618/AvS population was advanced to F6 for detailed mapping of the target region. YrAW3 appears to be a new locus. AWCC618 was crossed with three current Australian cultivars to transfer YrAW3 to modern wheat backgrounds. Backcross-derivatives will also be useful for validation of linked markers.
Department of Plant Sciences, University of the Free State, South Africa
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Leaf rust is a common wheat disease in South Africa. Annual surveys conducted by the Agricultural Research Council - Small Grain Institute (ARC-SGI) during the last 35 years used infection type (IT) data on a defined differential set to identify individual field isolates. Results from these surveys confirmed that the South African Pt population is affected by both local evolution and foreign introductions. A good correlation was found between avirulence/virulence phenotypes and simple sequence repeat (SSR) genotypes in the South African Pt population. We therefore evaluated whether identification of field isolates by SSR analysis would complement the traditional IT analysis using 47 field isolates collected during the 2013 growing season. Of the 39 phenotyped isolates, 35 were correctly genotyped while three were incorrectly genotyped only because the corresponding race was not included as a control. Five isolates that could not be phenotyped due to non-viable spores were successfully genotyped. The dominant race 3SA145 (North American race annotation CCPS) was represented by nine different genotypes sharing 82% genetic similarity. The SSR data further showed that the field isolates formed part of two distinct lineages with little admixture between them. This study confirmed the supporting value of SSR genotyping to traditional race analysis in monitoring the South African Pt population.