How has Lr34/Yr18 conferred effective rust resistance in wheat for so long?
Institute of Plant Biology, University of Zurich, Switzerland
View keller_2012.pdf (270.05 KB)
The Lr34/Yr18 gene has been used in agriculture for more than 100 years. In contrast to many other resistance sources against leaf rust and stripe rust, it has remained effective and no virulence has been reported. This makes Lr34 a unique and highly valuable resource for rust resistance breeding. The pleiotropic nature of the gene conferring partial resistance to different pathogen species, the associated leaf tip necrosis and its durability suggest a molecular mechanism that is different from major gene resistance. This is supported by the molecular nature of Lr34 which was recently found to encode an ABC transporter. Interestingly, all tested wheat lines contain an allele of the Lr34 gene on chromosome 7DS. In its susceptible form, the gene does not confer resistance. The difference between the encoded resistant and susceptible LR34 isoforms consists of only two amino acid changes, whereas the rest of the proteins are identical. These two changes must change the biochemical properties of the resistant LR34 transporter in such a way that the plant becomes resistant. We speculate that there is a slight conformational change in the resistant form of the protein, resulting either in modified specificity or kinetics of the transported molecule, or that the binding properties to an unknown second protein interacting with LR34 are changed, resulting in altered function. While the molecular nature of the molecule(s) transported by the LR34 protein remains unclear, it is likely that a physiological change related to Lr34 activity is at the basis of resistance. We are currently establishing transgenic approaches in heterologous grass species to further investigate the molecular activity of Lr34 and to better understand a physiological mechanisms resulting in disease resistance.
Development and characterization of wheat lines carrying stem rust resistance gene Sr43 derived from tall wheatgrass
USDA-ARS, Northern Crop Science Laboratory
Stem rust resistance gene Sr43, derived from tall wheatgrass (Thinopyrum ponticum), is effective against Ug99 lineage Pgt races. Previous studies indicated that Sr43 was located on large Th. ponticum 7el2 chromosome segments in 7D/7el2 translocation stocks KS10-2 and KS24-1. In the present work, we applied a recently-established chromosome engineering procedure to reduce the size of the alien chromosome carrying Sr43. KS10-2 was crossed and backcrossed to the Chinese Spring (CS) ph1b mutant. BC1F1 plants were screened for stem rust response and Ph1- associated molecular markers. Resistant BC1F1 plants homozygous ph1bph1b were further backcrossed to CS. The resulting population of 706 BC2F1 plants was screened for stem rust response and with six co-dominant SSR markers. Wheat lines RWG33 and RWG34 carry Sr43 on shortened alien segments that are about 15% of that in KS10-2. Two molecular markers closely linked to Sr43 were identified; one was an SSR marker and the other a STS marker based on sequences of deletion bin-mapped expressed sequenced tags in wheat. The two new wheat lines with Sr43 and closely-linked markers may provide new resources for combating the threat of race Ug99 and derivatives.
Unraveling the entry mechanism of oomycete and fungal effector proteins into host cells
Shiv D. Kale
Virginia Bioinformatics Institute, Virginia Tech University, USA
View kale_2011.pdf (224.58 KB)
Oomycetes and fungi facilitate pathogenesis via secretion of effector proteins that have apoplastic and intracellular localizations. These effector proteins have a diverse array of functions that aid in pathogenesis, including modification of defense responses. In the oomycetes, well characterized effector proteins that can translocate into the host cells share a pair of conserved N-terminal motifs known as RXLR and dEER. The RXLR motif has been shown to mediate translocation of the oomycete avirulence proteins Avr1b and Avr3a into host cells. Detailed mutagenesis of the RXLR motif of Avr1b revealed that the motif is tolerant to several amino acid substitutions while retaining functional translocation activity, resulting in the definition of a broadened RXLR-like motif, [R,K,H] X[L/M/I/F/Y/W]X. This motif has been used to identify functional translocation motifs in several fungal effector proteins, AvrL567, Avr2, and AvrLm6. Effectors with both RXLR and RXLR-like motifs bind phosphatidylinositol- 3-phosphate (PI-3-P) to mediate translocation via lipid raft mediated endocytosis. Mutations in RXLR or RXLRlike motifs result in loss of phospholipid binding and translocation by effectors. Effector entry into plant cells can be blocked by proteins and inositides that disrupt binding to PI-3-P, suggesting effector-blocking technologies that could be used in agriculturally important plant species.
Investigating rust resistance with the model grass Brachypodium
USDA-ARS Plant Science Research Unit and Department of Agronomy and Plant Genetics, University of Minnesota, USA
View garvin_2011.pdf (116.24 KB)
The model plant Arabidopsis thaliana has provided unique opportunities to explore and unravel many key biological features of plant biology including disease resistance. However, the inability of rust fungi of the genus Puccinia to infect Arabidopsis has prevented its use in exploring grass-rust interactions. The model plant Brachypodium distachyon is a member of the same grass subfamily as the principal cool-season grain crops, and can be infected with various Puccinia species. We have focused our efforts on establishing Brachypodium as a model for exploring grass - Puccinia graminis interactions. Brachypodium can be successfully infected by different formae speciales of the stem rust pathogen, including P. graminis f. sp. tritici. A wide range of response to stem rust occurs in Brachypodium and efforts are underway to decipher the genetic basis for this variation using recombinant inbred populations from parents with differing levels of response. Similarly, induced mutants with compromised stem rust resistance have been identified and are now being employed within a program to understand the molecular biology of stem rust resistance and susceptibility. Our results to date suggest that Brachypodium holds promise as a model plant for advancing our understanding of stem rust resistance.
New tools for wheat genetics and breeding: Genome-wide analysis of SNP variation
Department of Plant Pathology, Kansas State University, USA
View akhunov_2011.pdf (124.21 KB)
Single nucleotide polymorphism (SNP) is one of the most broadly distributed types of molecular variation in a genome which, along with the availability of costand labor-effective genotyping platforms, make it the marker of choice for many crops. Our work is aimed at the development of a dense set of genetically mapped SNP markers for low-cost high-throughput genotyping of wheat germplasm. Next generation sequencing of normalized cDNA libraries was used for developing gene-associated SNPs in polyploid wheat. A total of 7.5 million 454 reads were generated from cDNA libraries of 10 wheat cultivars from US and Australia and processed for discovering SNPs using a bioinformatical pipeline specifically designed for variant discovery in polyploid transcriptomes. A total of 25,000 high-quality SNPs distributed among 14,500 EST contigs were identified. All these SNPs were validated by comparison with RNAseq data generated from an additional set of 17 U.S. and Australian cultivars. A total of 9,000 genome-wide common SNPs were selected for designing an Illumina iSelect assay. Preliminary testing showed that more than 95% of SNPs produce high-quality genotype calls with up to 70% being polymorphic in a diverse sample of U.S. and Australian cultivars with a minor allele frequency >0.05. The assay is currently being used for studying patterns of genetic diversity in a worldwide collection of wheat cultivars and for developing a high-density SNP map. A long term goal of this initiative is to advance wheat research and breeding by developing genetic and genomic tools for efficient analysis of agronomic traits using high-resolution linkage and association mapping and deploying SNP markers in breeding programs
High yielding CIMMYT spring wheats with resistance to Ug99 and other rusts developed through targeted breeding
View rsingh_2011.pdf (216.09 KB)
Targeted breeding to develop high yielding wheat germplasm resistant to Ug99 and other rusts initiated at CIMMYT in 2006. Ug99 resistant materials, especially those with adult plant resistance (APR), were used in crossing. F3 and F4 populations from simple, BC1 and top crosses were grown for two generations under high rust pressures at Njoro, Kenya in a Mexico-Kenya shuttle breeding scheme. Parallel populations were also grown in Mexico for comparison. Approximately 5,000 advanced lines were tested for grain yield performance at Ciudad Obregon, Mexico in 2009/10 season, and phenotyped for resistance to Ug99 and other rusts. The 728 retained lines were evaluated for grain yield performance in five environments during the 2010/11 season in Mexico. About 68% of the 728 lines had nearimmune (16.5% entries) to adequate APR to Ug99. An additional 13.6% lines carried one of the six (Sr25, Sr26, SrTmp, SrHuw234, SrSha7, and an unidentified gene) race-specific resistance genes often in combination with APR gene Sr2. About 80% entries were highly resistant to yellow rust in Kenya and Mexico, and 90% entries to leaf rust in Mexico. Yield distribution of lines derived from Mexico-Kenya shuttle breeding was similar to lines selected only in Mexico. Sufficient lines with >5% superior yields than the Mexican checks varieties in 2 years testing were identified. Our results indicate that targeted crossing and shuttle breeding are powerful tools for a simultaneous improvement of grain yield potential and resistance to rusts.
Mapping of durable adult plant stem rust resistance in six CIMMYT wheats to Ug99 group of races
View bhavani_2011.pdf (343.8 KB)
Durable resistance to wheat stem rust fungus can Be achieved by developing and deploying varieties that have race-nonspecific, adult plant resistance (APR) conferred by multiple minor, slow rusting genes. Wheat lines ‘Kingbird, ‘Kiritati’, ‘Huirivis#1’, ‘Juchi’, ‘Muu’ and ‘Pavon 76’ showed high levels of APR to Ug99 races of stem rust fungus when tested in Kenya. The F5 and F6 generation recombinant inbred line (RIL) populations developed from the crosses of moderately susceptible ‘PBW343’ with five resistant parents were used in mapping. The non-Sr26 fraction of the ‘Avocet’ x Pavon 76 RIL population, developed earlier for leaf rust and stripe rust resistance studies, was also included. Field phenotyping of the parents and RILs were conducted at Njoro, Kenya for at least two years with Ug99+Sr24 (TTKST) race under high stem rust pressures. The continuous variation of APR in each RIL population and genetic analyses indicated quantitative nature of resistance that was likely governed by 3 or 4 minor genes. Single and joint year analyses by Inclusive Composite Interval Mapping (ICIM) using informative DArT and/or SSR markers identified consistent APR QTLs on chromosomes 1AL, 3BS, 5BL, 7A and 7DS in Kingbird; 2D, 3BS, 5BL and 7DS in Kiritati; 2B, 3BS, 4A, 5BL and 6B in Juchi; 2B, 3BS, 7B in Huirivis#1; 2B, 3BS and 5BL in Muu; and 1BL, 3BS, 5A and 6B in Pavon 76. QTLs on each genomic regions explained 10- 46% of the phenotypic variation for APR. Pseudo-black chaff phenotype associated with APR gene Sr2 on chromosome 3BS in all six resistant parents and identification of an APR QTL in the same region in all mapping populations confirmed the role of Sr2 in reducing stem rust severity. The 1BL QTL in Pavon 76 was in the same region where pleiotropic APR gene Lr46/Yr29/Pm39 is located. Similarly a 7DS QTL in Kingbird and Huirivis#1 was in the chromosomal region where pleiotropic APR gene Lr34/Yr18/Pm38 is located. These results indicate that the above two pleiotropic resistance genes confer APR to stem rust in addition to leaf rust, yellow rust and powdery mildew. Further studies are underway to saturate the genomic regions harboring new APR QTLs with additional molecular markers.
Putting Ug99 on the map: An update on current and future monitoring
View hodson_2011.pdf (456.74 KB)
Detection of stem rust race TTKSK (Ug99) from Uganda in 1998/99 highlighted not only the extremely high vulnerability of the global wheat crop to stem rust but also a lack of adequate global systems to monitor such a threat. Progress in the development and expansion of the Global Cereal Rust Monitoring System (GCRMS) is described. The current situation regarding the Ug99 lineage of races is outlined and the potential for expansion into important wheat areas is considered. The GCRMS has successfully tracked the spread and changes that are occurring within the Ug99 lineage and is now well positioned to detect and monitor future changes. The distribution of Ug99 variants possessing combined virulence to Sr31 and Sr24 is expanding rapidly and future spread outside of Africa is highly likely. Efficient and effective data management is now being achieved via the Wheat Rust Toolbox platform, with an expanding range of dynamic information products being delivered to endusers. Application of new technologies may increase the efficiency of the GCRMS, with mobile devices, molecular diagnostics and remote sensing all seen to have potential application in the medium to longterm. Expansion of the global capacity for race analysis is seen to be critical and integration of the Global Rust Reference Centre into the stem rust monitoring network is seen as a positive development. The current acute situation with severe epidemics of stripe rust in many countries indicates a clear need for more effective global monitoring systems and early warning for this pathogen. The existing GCRMS for stem rust is seen as a good foundation for this to occur.
Challenges in controlling leaf rust in the Southern Cone region of South America
National Institute of Agricultural Research [INIA], Uruguay
View german_2011.pdf (162 KB)
Leaf rust (caused by Puccinia triticina) continues to be the most important and widespread foliar disease of wheat in the Southern Cone. The P. triticina population of the region is extremely dynamic, leading to short-lived resistance in commercial cultivars. Some high yielding materials susceptible to leaf rust have been released and their increasing cultivation relies on fungicide applications to control leaf rust. The most important challenge of breeding programs in the Southern Cone is to incorporate durable leaf rust resistance in high yielding cultivars. These cultivars must also combine resistance to other relevant diseases and meet industrial quality standards demanded by the market. Leaf rust resistance in wheat varieties and lines lies mostly in combinations of seedling resistance genes or combinations of these with adult plant resistance (APR), including Lr34. Few recently released cultivars carry APR to leaf rust that might be expected to be durable. Since efforts to introduce slow rusting into high yielding adapted germplasm are increasing in most countries, more cultivars carrying this type of resistance will likely be released. If major genes are used, the introduction of effective genes not present in the regional germplasm will increase the diversity of resistance. Molecular markers are used in breeding in Argentina and are starting to be implemented in Brazil and Uruguay. Increased use of molecular tools could improve genetic progress in breeding programs, allow identification of APR genes present in current regional germplasm, and facilitate identification of new resistance genes.
Yellow rust in CWANA in 2010 & 2011: The situation and measures taken to control it
View nazari_2011.pdf (77.7 KB)
Asia and North Africa (CWANA). The total acreage in CWANA is approximately 53 million hectares. Wheat stripe (yellow) rust caused by Puccinia striiformis f. sp. tritici (Pst) continuously poses a serious threat to wheat production in CWANA. Several factors have contributed to the current severe epidemics of stripe rust, including; the rapid shift of virulence in the pathogen population, genetic uniformitity of mega-cultivars, favorability of environmental conditions, and an overlapping/ continuous crop calendar. During 1985-1997 the widespread appearance of Yr9 virulent pathotypes in CWANA, and eventually in the Indian sub-continent, resulted in several epidemics that caused a series of severe crop losses in popular cultivars known to be protected by the Yr9 resistance gene. Following the Yr9 virulence epidemics, susceptible cultivars were extensively replaced with CIMMYT-derived germplasm such as Kauz, Atilla, Opata, Nacozari, Bucbuc and Crow. The resistance of many of the replacement cultivars, including the mega-cultivars in India (PBW343), Pakistan (Inquilab-91, Bakhtwar), Iran (Chamran, Shiroudi), Ethiopia (Kubsa), and Syria (Cham 8) was based on Yr27. Breakdown of Yr27 resistance in PBW343, Inquilab 91 and Chamran, in India, Pakistan, and Iran, respectively, was reported between 2002-2004. Although occasional stripe rust outbreaks appeared in some areas, unfavorable environmental conditions presumably restricted the increase of the Yr27 Pst population until 2009, when conducive environmental conditions resulted in severe epidemics in several CWANA countries e.g., Morocco, Algeria, Uzbekistan, Turkey, Iran, Azerbaijan, Georgia, and Afghanistan. Environmental conditions favouring rust development continued into 2010, with mild winters and adequate rainfall in several CWANA countries resulting in early outbreaks of stripe rust. The 2010 stripe rust outbreaks occurred throughout the major wheat growing areas in the CWANA and Caucasus countries, causing severe yield losses particularly in Syria where Cham 8 (with Yr27) occupied more than 70% of the wheat areas. Inspite of favorable environmental conditions in many areas in CWANA in 2011, similar severe stripe rust epidemics have not been reported to date. Climate change now appears to be playing a major role in Pst population dynamics in CWANA. Direct, multiple affects of climatic changes on epidemiology of rust pathogens are expected, including the survival of primary inoculum, the rate of disease development, duration of rust epidemics, and development and distribution of rust populations. Emergence of stripe rust in non-traditional areas, changes in the frequency of new race evolution, early infection of stripe rust, shifts in predicted pathways of rust migrations, and finally wide spread epidemics of stripe rust in warmer areas as a potential indicator of adaptation to high temperatures are considered as possible consequences of climatic changes. Regional pathogen surveys indicated the widespread distribution of aggressive Pst pathoype (s) with adaptation to higher temperature. In the absence of resistant varieties, fungicide application remains the only practical measure to control stripe rust. Effective disease surveillance and monitoring systems, coupled to timely application of fungicides has effectively controlled stripe rust epidemics in Iran, Turkey, and Syria during 2010-11. Regional monitoring of pathogen variability and disease development must be undertaken as a matter of high priority, and timely chemical control measures will continue to play a major role for control of stripe rust in CWANA in the short-term. In the medium to long-term, existing resistant varieties and advanced breeding lines need to be promoted and susceptible varieties have to be urgently replaced.