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.
Primary Author: Loewen, National Research Council of Canada, Canada
High quality molecular markers that are closely linked, codominant, and high throughput are critical for developing varieties with durable rust resistance. We are using a combination of microsatellite, sequence tagged site, and Diversity Array Technology markers for haplotyping, pyramiding, and mapping stem rust resistance genes. The primary goal of our research team is to identify and optimize markers for previously characterized and novel stem rust resistance genes in wheat. The specific objectives are to: 1) optimize markers for previously characterized stem rust resistance genes to maximize efficiency of the breeding programs, 2) haplotype uncharacterized rust resistant genotypes to infer novelty and to plan new mapping experiments, 3) pyramid novel sources of rust resistance, and 4) map novel sources of rust resistance, including adult plant resistance. To date, we have evaluated 58 markers associated with 21 stem rust resistance genes and used 20 for haplotyping 318 wheat lines and varieties for 15 Ug99 effective resistance genes. This germplasm panel is also being DArT genotyped. For tetraploids, the pyramiding includes Sr2, Sr13 and Sr25 in the breeding line UC1113 which is a high yielding semi-dwarf durum variety with the high-grain protein content gene Gpc-B1 and the non-race specific stripe rust resistance gene Yr36. The Australian group is developing markers for the stem rust resistance genes Sr33 and Sr45 that come from Aegilops tauschii and are located on wheat chromosomes 1DS. Diagnostic, codominant markers for Sr25 and Sr26 have been developed and are being pyramided into CIMMYT breeding lines. Three new sources of race-specific resistance in CIMMYT-derived spring wheat have been mapped and are designated SrA, SrB, and SrC. SrA mapped on 3DL, SrB on 3BS and SrC on 5DL. These genes provided moderate levels of resistance to stem rust at the seedling stage and acceptable to moderate levels at the adult plant stage.
Primary Author: Long-Xi Yu, Department of Plant Breeding, Cornell University
Erratic weather patterns associated with climate change pose unique challenges for wheat breeders playing a key part in the fight to ensure global food security. Within the rainfed winter wheat areas of Turkey and Iran this erratic weather patterns may prevent attaining maximum potential increases in winter wheat genetic gains. This is primarily related with the fact that the ranking of tested varieties may greatly change from one year to the other. Erratic weather patterns may interfere with breeders decision on the ideotype(s) they should aim for during selection. To support breeding decisions, this study aimed at optimizing major traits through modelling different combinations of environments and defining probabilities of the range of variation of traits (phenology and pant height) that maximized grain yields. Optimal phenology was found to be highly related with the temperatures at which the winter wheat varieties were exposed at around heading time (20 days before and after heading). Specifically later winter wheat varieties were exposed to higher temperature both before and after heading and this exposure had a negative effect on grain filling duration and final grain yield. Finally, the use of at least five different wheat varieties in one production field (with different phenology and plant height) was compared to a field with monoculture to test for improved resilience. It was concluded that by selecting one best wheat variety in a wide range of environments it was possible to maximize grain yield and that using a set of diverse varieties was not beneficial.
Stem rust caused by Puccinia graminis f. sp. tritici (Pgt) is a destructive disease on bread and durum wheat. Following the identification and distribution of Ug99, major national and international efforts have been made to detect additional spread and emergence of new Pgt races. Since 2011, GRRC has accepted to receive live samples of stem rust year round, and up to 2014, a total of 428 dried samples of Pgt infected wheat tissue were received from 15 African and Asian countries, i.e., Azerbaijan, Egypt, Ethiopia, Iran, Iraq, Kenya, Lebanon, Nepal, Rwanda, Sudan, Tanzania, Turkey, Uganda, Yemen and Zimbabwe. Additional samples were received from Germany, Sweden and Denmark, where wheat stem rust re-emerged in 2013-2014. Recovery procedures using susceptible seedlings of cv. Morocco was done upon arrival and a total of 269 samples were successfully recovered, multiplied and stored in liquid nitrogen until further use. To date, 140 Pgt isolates have been pathotyped based on the method of Jin et al. (2008). Subsets of isolates were selected for molecular characterization including SNP genotyping and shipped to USDA-ARS, Cereal Disease Lab (CDL). The Pgt race TKTTF was widely distributed and found in ten countries including Egypt, Ethiopia, Iran, Iraq, Lebanon, Sudan, Turkey and the three European countries. Races of the Ug99 lineage were frequently observed in Africa. Clear indication of a new race in the Ug99 race group with additional virulence for SrTmp, TTKTK, was observed in samples from four African countries in 2014. PCR diagnostics developed by CDL confirmed the new race being member of the Ug99-lineage. The experimental work was supported by the DRRW project and new research facilities were funded by Aarhus University.
Primary Author: M. Patpour, Aarhus University, Denmark
The Himalayan region of Pakistan has been shown to be the centre of diversity of wheat yellow rust pathogen Puccinia striiformis f.sp. tritici (Pst) with a probable role of sexual reproduction in the population temporal maintenance. However, the populations of southern part of Pakistani Himalayan region remains unexplored, where wheat yellow rust is an important disease on rainfed wheat. The current study was thus carried out to assess the disease status and population structure of Pst prevalent in the southern part of Pakistani Himalayan region, mainly the districts of Kohat, Karak, Bannu, Lakki-Marwat and DI-Khan. A high disease pressure was observed during wheat season in 2013 in the region, where the level of severity ranged from 5% to 100% depending upon the variety tested. Microsatellite genotyping of 102 isolates with 18 SSR markers revealed a high diversity ranging from 0.86 (for DI Khan) to 1.00 (for Karak). The recombination signature was lower compared to the Himalayan populations. Analyses of the population subdivision revealed no clear evidence of spatial structure, with the maximum FST value of only 0.081. The overall diversity was higher in the region as compared to European clonal population, though it was still lower than the recombinant Himalayan populations, which could be attributed to their distance from Berberis spp. plantation zone.
Primary Author: M.R. Khan, The University of Agriculture, Peshawar, Pakistan
In the past decade Pgt race Ug99 and its variants have been a challenge to wheat production in Kenya. Towards identifying suitable varieties, 37 lines selected from rust screening nurseries and 3 checks were tested for yield and adult plant reaction to natural stem rust epidemics across 11 diverse Kenyan environments in 2013 and 2014. Trial locations were chosen to mainly represent key wheat growing areas as well as three new sites. Evaluations based on the AMMI linear-bilinear model indicated significant (P≤0.01) genotype (G), environment (E), and GE interactions with the first three principal components (PC) explaining ~70% of the observed variation. With a contribution of over 90% to total sum of squares, environment was the predominant source of variation and the genotypic effect was approximately twofold higher than the GE effect. Based on biplot projections, clusters of lines were most closely associated with specific environments. Biplots also pointed to at least five environments, clearly those in traditional wheat growing areas that were highly correlated and associated with positive PC suggesting a similar ability to discriminate genotypes. Each non-traditional testing environment was associated with negative PC and was uncorrelated in its discriminatory ability. Combined yield and stability results achieved through classifying genotypes based on Shukla’s stability variance and Kang’s stability rating, revealed four genotypes (R1357, R1362, R1372, and R1374) as desirable candidates. The hitherto popular variety Robin, used as the ‘best check’ for yield, posted an at least 10% lower yield relative to the highest yielding genotype (R1357). Moreover, Robin which was released as a high yielding variety with adult plant resistance in 2009, was not stable in performance across environments, perhaps due its current susceptibility to a new Pgt race (TTKTT) within the race Ug99 group, that is virulent to the SrTmp-based resistance.
Primary Author: Macharia, Kenya Agricultural and Livestock Research Organization, Kenya
Emergence of Pgt race Ug99 and rapid proliferation of lineal highly virulent races imminently threaten Kenyan wheat. Devastating epidemics have led to huge losses among smallholder farmers who invariably are unable to spray appropriately and in situations where susceptible varieties are grown. To combat stem rust, the Kenya wheat improvement program seeks to release high yielding stable genotypes with suitable levels of disease resistance. Moreover, detection of genotypes that are adapted to rain-fed environments is an overarching objective. Six hundred and seventeen genotypes from various CIMMYT nurseries (PCBW, EPCBW, PCHPLUS, and 9th SRRSN) were selected based on plant type and reaction to stem rust at Njoro. The reconstituted nursery-KSRON, was sown in the main season of 2016 at Njoro and Timau for further evaluation. Forty red grained lines depicting R-MR infection types, severity of 30% or less, and average Thousand Kernel Weight of >40g were then selected to constitute a yield trial. At each of eight diverse environments, trials also comprising four commercial varieties as checks, were designed in RCBD, three replicates laid out in contiguous array of 8 rows x 10 m plots. Genotype (G), Environment (E) and GE interactions effects were estimated by fitting the AMMI model to yield data, supported by a biplot visualization of the results. Analysis revealed significant (P ?0.01) genotype (G), environment (E), and GE interactions. The first three principal components (PC) explained ~78% of the observed variation. Environment was the predominant source contributing over 85% to total sum of squares. The biplot pointed to at least four environments that were highly correlated. By classifying genotypes based on Shukla's stability variance and Kang's stability rating, six genotypes (R1402, R1411, R1424, R1481, R1484, and R1486) were deemed high yielding and stable, and thus suitable candidates for further testing through the release pipeline.
Primary Author: Macharia, Kenya Agricultural and Livestock Research Organization
The physical environment and farming system in Chile are conducive to high yields from winter/alternate wheat cultivars. The national average yields for 2012-2014 were 6.5 t/ha for pasta wheat and 5.3 t/ha for bread wheat grown on 19,000 and 239,000 ha, respectively. The most efficient farmers obtain averages of 8-9 t/ha, and experimental plots at southern INIA sites are as high as 14 t/ha. The most important diseases are Septoria leaf blotch, stripe rust, powdery mildew, and BYDV. Recent increases of leaf rust on winter cultivars from near non-existence to the level of a major threat are a concern. Wheat cultivars such as Bicentenario INIA showed yield increases of 31.7% to reach 12.4 t/ha yield when sprayed twice with a mixture of strobilurin and triazol compared to 9.4 t/ha for the unsprayed control. Susceptible winter cultivars being introduced by private companies require complete chemical protection. In order to understand the virulences present in the pathogen population the Thatcher NILs were grown in 2014/15 under non-inoculated conditions in central [Chillan] and southern [Osorno] Chile. The Morocco check showed 100S, Thatcher 60S, TcLr1 40S, TcLr2b 30S, TcLr2c 40S, TcLr3a 30S, TcLr3ka 20S, TcLr3bg 30S, TcLr9 20S, TcLr10 60S, TcLr11 80S, TcLr12 60S, TcLr13 70S, TcLr14a 70S, TcLr15 50S, TcLr16 60S, TcLr17a 30S, TcLr18 20MR, TcLr19 0, TcLr20 30S, TcLr21 0, TcLr22a 0, TcLr23 70S, TcLr24 60S, TcLr25 0, TcLr26 60S, Lr27+31 80S, TcLr28 10MR, TcLr29 40MS, TcLr30 60S, TcLr32 70S, TcLr33 60S, TcLr34 70S, TcLr35 10MR–MS, Lr36 0, and TcLr37 0. The most significant differences (>40%) in response between the two locations were for TcLr2b, TcLr2c, TcLr11 and TcLr33. The Cereal Disease Laboratory (U.S.A.) tested 68 isolates from 55 samples from 2012/13 and identified 14 races, including one Triticum turgidum race (BBBQJ 26%). Significant breeding efforts are currently underway to address the leaf rust problem in Chile.
Primary Author: Madariaga, National Institute of Agricultural Research [INIA], Chile
Stripe rust of wheat (yellow rust) is a recurring production constraint in the majority of wheat growing areas of the world. The transboundary nature of the pathogen coupled with its current virulence capabilities, favorable environmental conditions, sometimes overlapping and/or continuous cultivation of susceptible varieties in stripe rust-prone zones, and genetic uniformity of certain recent ‘mega-cultivars’ were major driving forces in stripe rust epidemics worldwide. Breeding for resistance must continue be the central pillar of stripe rust control, and for this to be effective there must be adequate pathogen monitoring combined with commitment to identify and incorporate diverse sources of resistance, preferably of the durable type. Deployment of resistance will only be successful if it is combined with high yield and appropriate end-use quality to meet the needs of farmers and consumers. Suitable seed systems need to be in place for timely distribution of varieties. This paper deals with the historical impacts and current status of stripe rust epidemics and highlights the need for regional and global collaboration in mitigating the global impact of this disease.
Rusts are one of major threats to reduce wheat production and productivity in Nepal. Rust fungi are obligate parasite survival during off-season either on voluntary wheat plants or other grass or timber plant species is not yet confirmed in Nepal. High-inputs, suitable hosts and existence of warm humid and cool high lands in different parts of country promote carryover of inoculums of rust fungi. Nepal could be potential sources of yellow rust and leaf rust epidemic for itself and for Indian sub-continent. Surveillance is one of important steps to know status of wheat diseases especially rusts occurrence in country. The SAARC rust tool box is systematic and regular monitoring activity of wheat and barley diseases conducted at various locations in Nepal. Altogether, 183 and 180 locations were surveyed in different parts of Nepal were put in global rust tool box server and validated in fiscal years 2014/15 and 2015/16. Wheat rusts disease scenario has been observed differently, it could be due to climate change and different virulent spectrum of races/pathotypes of rusts fungi and deployment of different wheat varieties. Yellow rust was widely occurred throughout mid hills in Nepal. Higher severity of yellow rust was observed in Kathmandu valley (80S -100S). Leaf rust was moderate to high (10MS-100S) in plain and hills. There was higher score of leaf rust observed in plain as well as in mid hills on susceptible wheat cultivar. Regular monitoring and surveillance at different locations in Nepal has been found helpful in digging out actual problems of wheat crop. Monitoring races of all three rusts occurring in Nepal is necessary for successful planning to manage rusts by deploying effective genes. Rust tool box is important to keep vigilance of new emerging rust races in country. This in turn could increase production and productivity of wheat in Nepal.
Primary Author: Mahto, Nepal Agricultural Research Council (NARC)