The growing threat of stripe rust worldwide
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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.
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.
Putting Ug99 on the map: An update on current and future monitoring
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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
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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
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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.
Risk assessment of aerial transport of rust pathogens to the Western Hemisphere and within North America
Departments of Plant Pathology and Meteorology, Pennsylvania State University, USA
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The risk of aerial long-distance transport of rust pathogens from potential source locations in the Eastern Hemisphere to the Western Hemisphere and from subtropical to continental interior regions within North America is investigated. Simulations of longdistance transport of rust spores using the Integrated Aerobiology Modeling System indicate that the frequency of transport and deposition in the Western Hemisphere of viable rust spores originating from potential sources in tropical Africa, at high latitudes in Europe, and throughout eastern Asia is low. However, the frequency of trans-oceanic transport and deposition of viable rust spores in the Western Hemisphere is high for potential African source locations poleward of the tropics. The relatively short distance between Western Africa and northeastern South America coupled with the presence of persistent Northeasterly Trade Winds create an active pathway for spore transport. Western Hemisphere regions that are influenced by the Intertropical Convergence Zone have the highest likelihood of receiving viable rust spores from the Eastern Hemisphere. The risk of aerial transport of viable rust spores to U.S. regions from potential Eastern Hemisphere source regions is low. Analysis of wind streamline maps for North America indicate that strong low-level advection of air northward from the subtropics is prevalent east of the Rocky Mountains from early April to mid-May providing opportunities for long-distance transport of rust pathogens into the continental interior. After mid-June, the number of days with strong lowlevel advection of air from south to north across these regions and thus opportunities for long-distance spore transport decrease dramatically.
Cracking the codes: genetic basis of nonhost resistance of barley to heterologous rust fungi
Rients E. Niks
Wageningen University, Laboratory of Plant Breeding
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Full nonhost resistance can be defined as immunity, displayed by an entire plant species against all genotypes of a plant pathogen. The genetic basis of (non)host-status of plants is hard to study, since identification of the responsible genes would require interspecific crosses that suffer from sterility and abnormal segregation. There are some plant/potential pathogen combinations where only 10% or less of the accessions are at most moderately susceptible. These may be regarded as marginal host or near-nonhost, and can provide insights into the genes that determine whether a plant species is a host or a nonhost to a would-be pathogen. Barley (Hordeum vulgare L.) is a near-nonhost to several rust pathogens (Puccinia) of cereals and grasses. By crossing and selection we developed an experimental line, SusPtrit, with high susceptibility to at least nine different heterologous rust taxa such as the wheat and Agropyron leaf rusts (caused by P. triticina and P. persistens, respectively). On the basis of SusPtrit and several regular, fully resistant barley accessions, we developed mapping populations. We established that the near-nonhost resistance to heterologous rusts inherits polygenically (QTLs). The QTLs have different and overlapping specificities. In addition, an occasional R-gene is involved. In each population, different sets of loci were implicated in resistance. Very few resistance genes were common between the populations, suggesting a high redundancy in barley for resistance factors. Selected QTLs have been introduced into near-isogenic lines to be fine-mapped. Our results show that the barley- Puccinia system is ideal to investigate the genetics of host-status to specialized plant pathogens.
Rpg1-mediated durable stem rust resistance: mechanisms of action
Department of Crop and Soil Sciences, Washington State University, USA
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Stem rust, caused by Puccinia graminis f. sp. tritici, is a devastating disease on wheat and barley. A single barley gene, Rpg1, has provided durable resistance since its commercial introduction in the 1940s. The cloned Rpg1 gene encodes a protein with two tandem protein kinase domains, one an active kinase (pK2) and one a pseudokinase (pK1). Function of both domains is required for resistance. The gene is constitutively expressed in all tissues with elevated levels in the epidermis. It is mostly cytoplasmic with small, but significant amount associated with the cell membrane. We have been studying this gene and protein to try to understand how it works and why it has been so durable. Here we report our most recent results showing that RPG1 is phosphorylated within 5 min after urediniospores from avirulent, but not virulent, races land on the leaf surface. Two effector proteins were isolated from the ungerminated spores and shown to work cooperatively to induce RPG1 phosphorylation and eventual degradation. The proteins were identified as a hypothetical protein (PGTG10537.2) with a fibronectin type III and BRCA1 C-terminal domains and vacuolar protein sortingassociated protein 9 (PGTG_16791). The rapidity of the effector function and the nature of the two protein effectors indicate that a unique mechanism for effector entry and signaling in the host cell is involved. This hypothetical mechanism may be similar to what is observed in animal cells where fibronectin proteins with an RGD-binding domain act to mediate communications between the extracellular matrix and plasma membrane.
Towards an understanding of the molecular mechanisms of durable and non-durable resistance to stripe rust in wheat
USDA-ARS Wheat Genetics, Quality, Physiology and Disease Research Unit, Pullman, WA
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Stripe rust of wheat, caused by Puccinia striiformis f. sp. tritici, continues to cause severe damage worldwide. Durable resistance is a key for sustainable control of the disease. High-temperature adult-plant (HTAP) resistance, which expresses when the weather becomes warm and plants grow old, has been demonstrated to be durable. We have conducted numerous of studies for understanding molecular mechanisms of different types of stripe rust resistance using a transcriptomics approach. Through comparing gene expression patterns with racespecific, all-stage resistance controlled by various genes, we found that a greater diversity of genes is involved in HTAP resistance. The genes involved in HTAP resistance are induced more slowly and their expression induction is less dramatic than genes involved in all-stage resistance. The high diversity of genes and less dramatic expression induction may explain the durability and incomplete level of HTAP resistance. Identification of transcripts may be helpful in identifying resistance controlled by different genes and in selecting better combinations of genes for pyramiding to achieve adequate and more durable resistance.
Unraveling the entry mechanism of oomycete and fungal effector proteins into host cells
Shiv D. Kale
Virginia Bioinformatics Institute, Virginia Tech University, USA
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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.