Nutrient uptake in rust fungi: How sweet is parasitic life?
Ralf T. Voegele
Fachgebiet Phytopathologie, Institut für Phytomedizin, Fakultät Agrarwissenschaften, Germany
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A better understanding of the fundamental principles of host-pathogen interactions should enable us to develop new strategies to control disease and to eliminate or at least manage their causative agents. This is especially true for obligate biotrophic parasites like the rust fungi. One vital aspect in the field of obligate biotrophic host-pathogen interactions is the mobilization, acquisition and metabolism of nutrients by the pathogen. This includes transporters necessary for the uptake of nutrients as well as enzymes necessary for their mobilization and metabolism. In a broader sense effector molecules reprogramming the host or triggering the infected cell into metabolic shifts favorable for the pathogen also play an important role in pathogen alimentation.
Role of Berberis spp. as alternate hosts in generating new races of Puccinia graminis and P. striiformis
USDA-ARS, Cereal Disease Laboratory
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The common barberry and several other Berberis spp. serve as the alternate hosts to two important rust pathogens of small grains and grasses, Puccinia graminis and P. striiformis. Barberry eradication has been practiced for centuries as a means to control stem rust. Diverse virulence variations have been observed in populations of P. graminis f. sp. tritici that were associated with susceptible barberries in North America. Barberry likely has played a role in generating new races of P. striiformis f. sp. tritici in some regions in the world. Several North American stem rust races, namely races 56, 15B and QCC, initially originated from barberry, were subsequently responsible for generating large-scale epidemics. Thus, sexual cycles on Berberis spp. may generate virulence combinations that could have serious consequences to cereal crop production.
Characterization of two new wheat stem rust races within the Ug99 lineage in South Africa
Department of Plant Sciences, University of the Free State, South Africa
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Two new races of the wheat (Triticum aestivum L.) stem rust pathogen, representing the fifth and sixth variants described within the Ug99 lineage, were detected in South Africa. Races TTKSP and PTKST (North American notation) were detected in 2007 and 2009, respectively. Except for Sr24 virulence, race TTKSP is phenotypically identical to TTKSF, a commonly detected race of Puccinia graminis f. sp. tritici (Pgt) in South Africa. PTKST is similar to TTKSP except that it produces a lower infection type on the Sr21 differential and has virulence for Sr31. Simple sequence repeat (SSR) analysis confirmed the genetic relationship amongst TTKSF, TTKSP, PTKST and TTKSK (Ug99). TTKSK, PTKST and TTKSF grouped together with 99% similarity, while sharing 88% genetic resemblance with TTKSP. These four races in turn shared only 31% similarity with other South African races. It is proposed that TTKSP arose locally as a single step mutation from race TTKSF, whereas PTKST probably represents an exotic introduction of Pgt to South Africa.
Surveillance and race analysis of stem rust in Kenya for the years 2008 and 2009
Limited but targeted stem rust race characterization was undertaken in Kenya in 2004 and 2005 which led to the detection of Ug99 present in Kenya and designation of Ug99 as race TTKS (based on North American stem rust race nomenclature system). Further surveillance in 2006 and 2007 detected variants of TTKS with virulence on Sr24 (TTKST) and Sr36 (TTTSK), respectively. Stem rust surveillance was undertaken at an extended level in 2008 and 2009 within predominant wheat growing regions of Kenya. Three hundred and sixty farms were surveyed from regional districts of Naivasha, Narok, Nakuru, Laikipia, Meru, Uasin-Gishu, Nandi, Elgeyo and Trans-Nzioa, during 2008 main season (May to September and December). The information from farmers indicated that more than 95% of these farms were sprayed with fungicides. Despite the use of fungicides, stem rust was detected in 67% of the surveyed farms. Stem rust ranged from trace amount -100% in severity with minimum infection in Naivasha district (40%) and maximum in Narok district (90%). Yellow rust was detected in 22% of the farms. Out of one hundred and twenty-six stem rust samples collected, 37 and 39 (a total of 76 ) samples were sent to Cereal Disease Laboratory (CDL) Minnesota, USA and Cereal Research Laboratory of Agriculture and Agri-Food Canada respectively, for race typing using the respective differentials used by these labs. From the 39 collections sent to Canada, 17 (43%) survived, of which majority were typed to TTKST (65%) followed by TTKSK (18%), PTKST (12%) and mixture of TTKST and TTKSK (5%). The CDL typed vast majority of pathotypes as TTKSK (84%) followed by TTKST and TTTSK (7% each). The combined results of two labs indicated that predominant frequency in Kenya in 2008 was TTKSK (51%) followed by TTKST (31%), PTKST (6%) and TTTSK (6%). The frequency of TTKST significantly increased in 2008 compared to 2007 which is not surprising, given that Sr24 carrying wheat cultivar KS Mwamba is cultivated on large acreage in Kenya. In 2009, 262 farms were surveyed from regional districts of Narok, Laikipia, Nyandarua, Meru, Uasin-Gishu, Nandi, Elgeyo and Trans-Nzioa. The 2009 season experienced heavy drought in many areas. Nevertheless, stem rust was detected in 79% of the farms with disease severity ranging from trace to 100%. Yellow rust was detected in 15% of the farms. Stem rust infection ranged from 0 to 100% with minimum infection in Nyandarua (18%), Laikipia (42%) and maximum in Uasin-Gishu and Elgeyo (100% each). Out of seventy-four stem rust samples collected, 55 samples were sent to Canada for race typing. Only 20% of the samples survived, of which majority were typed to TTKST (50%), PTKST (34%) and PTKSK (16%). Borlaug Global Rust Initiative 2010 Technical Workshop / Poster Abstracts 7 The 2009 results did not depict real situation of predominance of pathogenic variability because of small sample size, however it provided fair indication that race TTKST is still the most prevalent. This information generated on the distribution of stem rust races, and the incidence of stem rust is important for anticipatory breeding and release of cultivars with effective sources of resistance in Kenya, and at same time mitigating global threat of stem rust by reducing intensity of stem rust inoculum in East Africa.
Association mapping of rust resistance in pre-green revolution wheat accessions
The University of Sydney, Plant Breeding Institute, Australia
Association mapping detects correlations between genotypes and phenotypes in a sample of individuals based on the linkage disequilibrium and can be used to uncover new genetic variation among germplasm collections. Two hundred and five landraces collected by the English botanist A. Watkins in the 1920s were screened for rust response variation under field conditions during three crop seasons. An integrated map of 350 polymorphic DArT markers was developed. Association mapping identified the involvement of several genomic regions in controlling resistance to three rust diseases. Seven, eight and nine genomic regions, respectively, appeared to carry yet uncharacterized leaf rust, stripe rust and stem rust resistance. Three dimensional analyses indicated genetic association of leaf rust and stripe rust resistance in some accessions, whereas no such association was observed between stem rust resistance and resistance to either of the other two rust diseases. A new stripe rust resistance locus, Yr47, has been named.
Genetic map of stem rust resistant gene Sr35 in T. monococcum
Department of Plant Sciences, University of California-Davis, USA
With the TTKS family of races virulent on most genes currently providing protection against stem rust worldwide, identifying, mapping, and deploying resistance genes effective against these races has become critical. We present here a genetic map of Sr35. Both parents of our diploid mapping population (DV92/G3116, 142 SSD lines) are resistant to TTKSK, but the population segregates for resistance to TRTTF (Yemen) and RKQQC (US). Race analysis suggests that G3116 carries Sr21 and DV92 both Sr21 and Sr35. Resistance to TRTTF and RKQQC was mapped to a 6 cM interval on chromosome 3AmL between markers BF483299 and CJ656351. This interval corresponds to a 178-kb region in Brachypodium which contains only 16 annotated genes and exhibits a small inversion (including 2 genes) and a putative insertion (2 genes) relative to rice and sorghum. This map contains closely-linked markers to Sr35 and provides the initial step for this gene's positional cloning.
Cytogenetic manipulation to enhance the utility of alien resistance genes
Department of Crop and Soil Sciences, Washington State University, USA
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Although many wild relatives in the Triticeae tribe have been exploited to transfer stem rust resistance genes to wheat, the derived germplasms have often not been immediately useful in wheat breeding programs. Too frequently, large chromosome segments surrounding desirable genes also harbor deleterious genes that result in unacceptable yield or quality. Recombination between chromosomes of wheat and chromosomes of distant relatives is very rare due to genetic restrictions on chromosome pairing in polyploid wheat. However, chromosome pairing can be manipulated by utilizing mutant stocks that relax this tight genetic control. The ph1b mutant produced by E.R. Sears over 30 years ago is an invaluable chromosome engineering tool, readily employed in the age of high-throughput molecular genetics. Shortened translocations have already been produced for stem rust resistance genes Sr26 and SrR using ph1b-induced homoeologous recombination. We are currently using induced-homoeologous recombination to reduce the sizes of alien chromosome segments surrounding TTKSK-effective genes Sr32, Sr37, Sr39, Sr40, Sr43, Sr47, SrTt3, Sr2S#1 and SrAeg5 to eliminate linkage drag putatively associated with these genes. Additional TTKSK-effective genes Sr44, SrHv6, SrAsp5, and SrAse3 were first targeted for development of compensating translocation stocks and then for shortening the size of each alien segment. Population development is also underway to characterize several potentially new sources of resistance.
Cloned rust resistance genes and gene based molecular markers in wheat: Current status and future prospects
CSIRO Plant Industry, Australia
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Two broad categories of resistance genes in wheat have been described. One group represents the so called seedling resistance or the ‘gene for gene’ class that often provides strong resistance to some but not all strains of a rust species. The other category referred to as adult plant resistance provide partial resistance that is expressed in adult plants during the critical grain filling stage of wheat development. A few seedling rust resistance genes have been cloned in wheat and other cereals and are predominantly from the nucleotide binding site/leucine rich repeat class which is associated with localized cell death at the pathogen entry site. Until recently, the molecular basis of race non-specific, partial and slow rusting adult plant resistance genes were unknown. Gene products that differ from known plant resistance genes were revealed from the recent cloning of the Yr18, Yr36 and Lr34 adult plant genes in wheat. The available range of diverse resistance gene sequences provide entry points for developing genebased markers and will facilitate selection of germplasm containing unique resistance gene combinations.
Molecular-genetic dissection of rice nonhost resistance to wheat stem rust
CSIRO Plant Industry, Australia
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Rust diseases remain a significant threat to the production of most cereals including wheat. New sources of resistance are continually sought by breeders to combat the emergence of new pathogen races. Rice is atypical in that it is an intensively grown cereal with no known rust pathogen. The resistance of rice to cereal rust diseases is referred to as nonhost resistance (NHR), a resistance mechanism that has only recently become genetically tractable. In this report, the mechanisms of rice NHR to wheat stem rust and other cereal rust diseases are explored and the potential for transferring this durable disease resistance to wheat is considered. Approaches being undertaken for the molecular-genetic dissection of rice NHR to rust are described.
Screening for stem rust resistance in East Africa
The University of Sydney, Plant Breeding Institute, Australia
View dsingh_2009.pdf (264.88 KB)
The East Africa program of the Borlaug Global Rust Initiative (BGRI) was launched to reduce the scale and scope of wheat stem rust epidemics in Kenya and Ethiopia, and to mitigate the global threat of virulent and dangerous rust races originating from this region. Since the launch in 2005, the screening facilities in Kenya and Ethiopia have helped to determine the extent of the world’s vulnerability to stem rust race Ug99 and its variants, identify diverse sources of resistance including adult plant resistance based on minor genes, and catalyze a comprehensive global response, leading to expanded awareness, expanded research and breeding activities, and resource mobilization. This paper reviews the role and achievements of the eastern African screening facilities along with the opportunities and challenges faced by the facilities during the ongoing global response to the emergence of Ug99 and its variants.