The concept of durable resistance was introduced by Dr Roy Johnson about 40 years ago, following a breakdown in the slow rusting or adult plant resistance of several English winter wheats to stripe rust, including Joss Cambier, and continued effectiveness of resistance in several other cultivars including Cappelle Desprez and Hybrid de Bersee. The resistance in the latter was referred to as durable, and durable resistance defined as “resistance that remains effective when a cultivar is grown widely in environments favouring disease development”. Durable resistance is a descriptive term; it does not provide any explanation of the causes underlying long lasting resistance. It does, however, contain two conceptual elements, one being that there may be any of several underlying causes for durable resistance and the other that resistance that has remained effective for a long period of widespread use may not necessarily continue to do so in the future. This paper will discuss the role of durable resistance in achieving sustained control of cereal rust diseases. In view of the complexity of host : pathogen interactions, genetic diversity must be seen as a key ingredient in large scale sustained control of plant diseases. It has been argued that even where specific or major resistance genes are used, genetic diversity can be used as insurance against lack of durability and hence as a means of reducing genetic vulnerability. Above all, responsible use of resistance genes depends upon an understanding of the resistance genes present in cultivars and breeding populations, and monitoring pathogen populations with respect to deployed resistances, are crucial in ensuring that the genetic bases of resistances are not narrowed.
Detection of significant new races of the wheat stem rust pathogen in Africa and Middle East
BGRI 2015 Plenary Abstract M. Patpour
Aarhus University, Denmark
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.
Phenotyping adult plant resistance to leaf rust in wheat under accelerated growth conditions
BGRI 2015 Plenary Abstract Adnan Riaz
The University of Queensland, Queensland Alliance for Agriculture and Food Innovation, Australia
Watch presentation (YouTube)
Leaf rust (LR), caused by Puccinia triticina, is among the most important diseases of wheat (Triticum aestivum L.) crops globally. The most sustainable method for controlling rust pathogens is deployment of cultivars incorporating durable forms of resistance, such as adult plant resistance (APR). However, phenotyping breeding populations or germplasm collections for LR resistance in the field is dependent on weather conditions and limited to only once a year. In this study, we report a protocol for phenotyping APR to LR incorporating ‘speed breeding’ technology, which utilizes controlled temperature regimes and 24-hour light to provide accelerated growth conditions (AGC) – enabling up to 6 plant generations of wheat per year. A panel of 22 genotypes, including disease standards carrying known APR genes along with a diversity panel comprising 300 accessions (including winter types and landraces) were characterized for resistance to LR under AGC and in the field. Analysis of genotypes displaying APR revealed that disease response expressed on flag–2 leaves under AGC was highly correlated with field-based measures (R2 = 0.76). Analysis of the diversity panel indicated that APR was expressed by plants that had obtained the stem elongation stage (i.e. GS≥30) prior to inoculation. Despite the high degree of genetic diversity in the panel, strong correlations between LR response under AGC and the field were observed, and were further improved when field response was adjusted based on growth stage (R2 = 0.81). The diversity panel was also screened with DNA markers for known APR genes (Lr34, Lr46 and Lr67), which identified 22 accessions carrying potentially novel sources. This method integrates assessment at both seedling and adult growth stages and requires only seven weeks to complete, enabling up to seven consecutive assays annually. When coupled with ‘speed breeding’, this approach could also accelerate introgression of resistance genes into adapted wheat cultivars.
Metabolomics and plant physiology during the wheat-stripe rust interaction
BGRI 2015 Plenary Abstract Veronica Roman-Reyna
Australian National University
Stripe rust is one of the major diseases of wheat worldwide. The causative fungus, Puccinia striiformis f.sp. tritici (Pst), keeps the infected tissue alive even after sporulation phase, a strategy that is referred to as biotrophy. The compatible interaction is divided into three phases; colonization, growth, and sporulation, the last occurring ~14 days after germination of spores. During the growth phase plant apoplast is completely occupied by hyphae, and the fungus develops special invasive structures called haustoria within plant cell. Both hyphae and haustoria are thought to take up nutrients from the host, but haustoria are specialized for this role. However, it is still unknown how the fungus obtains nutrients; perhaps by direct manipulation of host metabolic pathways related to photosynthesis or by changes in whole plant metabolite fluxes by acting as a sink. Also, it is unclear why wheat plants do not detect either the fungus itself, or the consequent loss of nutrients. The aim of this study is to understand the changes during the three phases of infection, comparing metabolites and plant photosynthetic efficiency in healthy and infected tissue, and correlating this with fungal growth. The results show that CO2 assimilation rates decreased only at the sporulation phase, which correlates with a reduction in transitory starch accumulation. However, glucose and fructose levels were lower only during colonization phase. Interestingly, although the infection alters the nutrient balance, this did not seem to affect the development of young leaves. In addition to these results, we found that stripe rust grows faster in younger leaves, which might be related to their morphology and the nutrient availability and fluxes within the leaf. This research suggests that the fungus is undetected until sporulation, and will aid future studies to understand the mechanisms of adult plant resistance conferred by transporter proteins. The research will aid future studies to understand the dynamic of adult plant resistance conferred by transporter proteins. The knowledge in wheat physiology and metabolism during rust infection could help to explain the role of transporter proteins during wheat-stripe interaction in different plant growth stages.
Training agricultural scientists for a more globalized world: Monsanto’s Beachell-Borlaug International Scholars Program after 7 years
BGRI 2015 Plenary Abstract Edward Runge
Texas A&M University
Monsanto’s Beachell-Borlaug International Scholars (MBBIScholars) Program was established on March 25, 2009, on Dr. Norman Borlaug’s 95th birthday. Monsanto initially funded the MBBIScholars program for $10 MM ($2 MM per year for 5 years) and extended the program with a second grant for $3 MM ($1 MM per year for 3 years). As of February 2015 (6 Years of funding) the program has supported 70 students. The 70 MBBIScholars were selected from 359 applications. MBBIScholars are from 25 countries with India having 20 scholars. MBBIScholars from other countries are – Argentina 3, Bangladesh 2, Brazil 2, China 4, Columbia 4, Ecuador 1, Egypt 1, England 1, Ethiopia 4, Kenya 2, Korea 2, Iran 3, Italy 1, Mali 1, Nepal 2, Pakistan 1, Philippines 1, Syria 2, Tajikistan 1, Thailand 1, Tunisia 1, USA 4, and Uruguay 2. Forty scholars studied wheat breeding and 30 studied rice breeding. Twenty seven scholars were young ladies. Applications for the 7th round were due on or before February 1, 2015. A unique feature of the MBBIScholars Program is the requirement that scholars must complete part of their PhD program in both developed and developing/transition countries. Scholars have worked with developed country scientists as follows – Australia 4, Canada 3, USA 43, and Western Europe 20. The program pays for the MBBIScholars to participate in a 3 day Leadership course prior to attending the World Food Prize during their first 2 years. It has been a good experience to see MBBIScholars gain self-confidence after attending the Leadership Course and World Food Prize, and as they study and conduct research in developed and developing/transition countries. They also gain many lifelong contacts in the plant breeding community. Based on the current funding agreement with Monsanto, the final round of MBBIScholars will be selected from applications due February 1, 2016. In view of the great success of this model of training international plant breeders, it would be highly desirable for donors to support and extend this PhD training program to include additional crops of interest in developed and developing countries.
Wheat stripe (Puccinia striiformis f. sp. tritici,=Pst) and stem (Puccinia graminis f. sp. tritici =Pgt) rusts are the most important wheat disease in Egypt as well as present in all wheat growing areas. This study to evaluate a set of tester lines of wheat carrying stripe Yr's, stem Sr's rust genes and selected Egyptian varieties have been studied for their response to Pst and Pgt at adult plant stage under field conditions in Sakha Agriculture Research Station, during the 2011 to 2014 growing seasons. The results revealed that stripe rust, it has been observed that the new race Yr27-virulence to Pst. In addition pathotypes were virulent for Yr2, Yr6, Yr7, Yr8, Yr9, Yr27, while Yr18 showed moderate susceptibility. On the other hand, Yr1, Yr5, Yr10, Yr15, Yr17, Yr32 and YrSP exhibited high levels of resistance. Regarding, evaluation of resistance genes sources of stem rust on ICARDA, CIMMYT wheat germplasm, and Egyptian wheat varieties released i.e. Misr1 and Misr2 which having Ug99_resistance genes Sr2 and Sr25 were found susceptible to Pgt, also Sr31 recorded infection moderately susceptible to susceptible at adult stage. Genes Sr2 complex, Sr24, Sr26, Sr27, and Sr32 were resistant at adult plant stages. The combination of Sr26 with Sr2 and Sr25 provided stem rust resistance in some CIMMYT wheat germplasm. The objectives of this work are: race analysis of wheat stem and stripe rust disease, evaluation the level and distribution of wheat stripe and stem rust in Egypt, and identification the resistance genes in commercial varieties or new promising lines using standard and molecular genetic markers. Egyptian germplasm such as Misr1, and Misr2 and others tester lines of wheat carrying stem rust Sr's were evaluative under field condition at adult stage in Egypt during 2014 growing season, Egyptian cultivars Misr1 and Misr2 were susceptible rated 10S-20S and Sr31 rated MSS. that results clearly presence a new Sr31-virulence. On other hand, genes Sr2 complex, Sr24, Sr26, Sr27 and Sr32 were resistant and combination of Sr26 with (Sr2 and Sr25) produced stem rust resistance in some CIMMYT wheat germplasm. Shahin et al., 2015, in APS Annual Meeting, Aug. 1-5, Pasadena, CA, US, (In Press).
A key objective of BGRI is to breed high yielding, stem rust resistant spring wheat germplasm suitable for releases as successful varieties in wheat growing countries of Africa, Middle East, Asia and Latin America. High emphasis was given to select adult plant resistance (APR) to stem rust in achieving this goal that is especially important in East African highlands where various variants belonging to the Ug99 race group and other lineages of stem rust fungus are now known, disease is endemic and present throughout the year on wheat crops. Recent molecular mapping studies show that combinations of partially effective APR gene Sr2 with 3 to 4 additional APR genes such as Sr55, Sr56, Sr57, Sr58 and other undesignated quantitative trait loci confer adequate to high levels of resistance to stem rust. A ‘Mexico-Kenya shuttle breeding scheme’ was initiated in 2008 to select APR to stem rust under high disease pressures at Njoro, Kenya while selecting for resistance to other rusts, yield, agronomic and quality traits in Mexico. This selection scheme, combined with phenotyping of advanced lines for multiple seasons in Kenya has resulted in identifying a small frequency of high yielding lines that possess a high level of resistance with a stable and low stem rust severity performance over seasons/locations under high disease pressures. These near-immune wheat lines are the best candidates for release in East Africa to achieve durable disease control and simultaneously curtail, or reduce, further selection of new virulences. A significantly higher proportion of wheat lines were also developed with moderate levels of resistance that is considered suitable for deployment in wheat growing areas where rust builds up later in the season. The worldwide distribution of the wheat lines derived from Mexico-Kenya shuttle breeding initiated in 2012 through the international yield trials and nurseries from CIMMYT. Potential releases and cultivation of these lines in different countries together with a reduction in area sown to susceptible varieties are expected to reduce the threat from stem rust.
Recent events in worldwide populations of the fungal pathogen Puccinnia striiformis, which causes the yellow rust disease on wheat and other cereals, have suggested that other factors than shifts in virulence can lead to epidemic events. For instance, the spread of two strains across four continents that has occurred within the last 10-15 years seems to be a result of high temperature adaptation combined with a relatively short latent period (Hovmøller et al. 2008; Milus et al. 2009). Variation for quantitative traits like latent period has often been hypothesized to play a significant role in population shift but only very few experimental data have been generated. Here we report difference for components of aggressiveness which included latent period and lesion growth for 17 isolates derived from a selfing of an aggressive isolate using Berberis vulgaris. A group of offspring isolates had a significantly longer latent period and higher lesion growth than the parental isolate. Interestingly, the two traits were found to be positively correlated where a long latent period was correlated with a higher lesion growth rate. This may suggest a trade-off between latent period and lesion growth. All isolates were assessed on seedlings of two highly susceptible host varieties and the two hosts gave similar results. In a previous study the progeny isolates showed segregation for virulence/avirulence and SSR markers (Rodriguez-Algaba et al. 2014). In conclusion, this study demonstrates genetically inheritable variability for latent period and lesion growth in P. striiformis, even within a single parental isolate. The results contribute to a better understanding of the ability of P. striiformis to adapt to new host varieties and changing environments at the quantitative level.
We will present an update on the BBSRC-funded SCPRID project “Maximizing the potential for sustainable and durable resistance to the wheat yellow rust pathogen”. This aims to understand the molecular basis of Puccinia striiformis f. sp. tritici (PST) pathogenicity and exploit this information to design effective breeding strategies that maximize the potential for durable disease resistance in the field. We have established a PST genomics platform through sequencing of PST genomes (UK, European, African, and Indian races) and analysis of expression time courses during infection (Cantu et al 2013). Using this platform we have characterised the PST effector complement, identified putative candidates and have begun their validation. The latest results of this will be presented. We have also evaluated a collection of hexaploid wheat landraces for resistance to PST across continents and have initiated single seed descent mapping populations and initial characterisation in F2:3 populations. We will exemplify the use of new genomic technologies to develop closely linked markers to enable deployment of resistance loci in breeding programmes (Ramirez-Gonzalez et al 2014). We will also provide an update of a new technique, called Field Pathogenomics (Hubbard et al 2015). This method uses transcriptome sequencing of PST-infected wheat leaves to describe pathogen diversity and also identify the host variety. This analysis uncovered a dramatic shift in the PST population in the UK and suggests a recent introduction of a diverse set of exotic PST lineages that may have displaced previous PST populations.
Rapid detection of micro-RNAs associated with APR to rust pathogens in wheat
BGRI 2015 Poster Abstract Singh
The University of Queensland, Queensland Alliance for Agriculture and Food Innovation, Australia
The identification of R-genes using traditional map-based approaches is a long, laborious process, not to mention the time required for subsequent development of cultivars incorporating the new resistances. Breeders seek to reduce the length of breeding cycles, and researchers require new tools to accelerate discovery and understanding of mechanisms associated with durable resistance, especially adult plant resistance (APR). A new method for rapid generation advancement, known as ‘speed breeding’, significantly reduces the length of breeding cycles, provide increased recombination during line development and enable selection in early generations. The speed breeding protocol uses controlled temperature regimes and 24h light to accelerate plant growth and development. Phenotyping methods adapted for use in the speed breeding system permit year-round evaluation of APR to rust pathogens within 5 weeks from time of sowing. RNA sequencing (RNA-Seq) technology has revolutionized gene expression profiling in plants. We previously used RNAseq to identify novel transcripts and miRNAs associated with seedling resistance (Lr28) leading to identification of transcription factors and miRNA families (e.g. miR36, miR37 and miR39) involved in signalling and defense response (Kumar et al. J. Nuc. Acids 2014:570176). In this study we report the application of speed breeding and RNAseq technologies for the purpose of rapidly identifying transcripts and miRNA associated with APR. Wheat landraces harbouring novel sources of resistance were grown under speed breeding conditions and sampled for RNA at key growth stages, before and after inoculation, which enabled discovery of differentially expressed miRNAs. Our next steps are aimed at validating these genetic factors associated with APR in order to better understand the signalling pathways and deliver tools to assist the assembly of robust wheat cultivars for the future.