Puccinia graminis f. sp. tritici (Pgt) is the major wheat production constraint in Ethiopia causing recurrent epidemics that resulted in the withdrawal of widely grown wheat cultivars from production. Among the current Pgt races detected in Ethiopia, TKTTF is the most frequent and has caused a severe epidemic in the south wheat growing regions (Bale and Arsi) after its first detection in 2012. Therefore, to avert the current situation, identifying sources of resistance to race TKTTF in breeding germplasm is a top priority to the National Wheat Breeding Program. Hence, 82 promising bread wheat lines including five check cultivars were evaluated in Debre Zeit in a TKTTF single race nursery for three consecutive seasons, 2014-2016. Ethiopian bread wheat cultivar Digalu was used as a spreader row and was inoculated using a single isolate of race TKTTF at different growth stages. The nursery was bounded by oat to reduce interference with any other stem rust race. The 82 lines were tested in the greenhouse at Cereal Disease Laboratory and were also tested with known diagnostic molecular markers. Twenty-nine lines displayed low levels of terminal stem rust severity in the field and low coefficient of infections. Fourty-one lines were resistant to race TKTTF at the seedling stage. Bread wheat lines resistant to TKTTF are valuable sources of resistance that can be deployed in wheat growing regions of Ethiopia prone to stem rust.
Primary Author: Bulbula, Ethiopian Institute of Agricultural Research
Argentina and Uruguay are neighboring countries located in the same rust epidemiological area. The last significant stem rust epidemic occurred in 1950. Since then, stem rust was frequently observed in experimental fields and off-season nurseries, but was mostly absent in commercial fields. During 2014, 4.6 million ha of wheat were grown, and there was a widespread incidence of stem rust, reaching levels of 80S on susceptible cultivars in both countries. Yield losses of 13 to 21% were estimated in experimental trials in Argentina. The epidemic was probably caused by the increasingly widespread cultivation of highly susceptible, but high yielding French cultivars during the last decade. In Argentina 42.3% of the commercial cultivars were susceptible to stem rust, and in Uruguay 23.0% were susceptible, 6.8% moderately susceptible and 20.3% were intermediate in reaction. However, the actual area sown to susceptible cultivars in Uruguay has continued to increase, from 22% of the wheat area in 2009 to 53.3% in 2014. Conductive weather conditions of high rainfall and warmer than average temperatures during the winter and spring, favored early infection. Cultivars with resistance genes Sr31 and Sr24 continue to be resistant in the region and are believed to be the most important genes currently providing resistance. Some Argentinean and Uruguayan cultivars that do not carry Sr31 and/or Sr24 were susceptible in 2011, but resistant in 2014, indicating a narrower range of virulence in 2014 compared to 2011. Both countries are working to improve resistance to local races and to the Ug99 race group. Disease modeling would be useful for understanding and predicting the occurrence and severity of this disease.
Primary Author: Campos, National Institute of Agricultural Technology (INTA), Argentina
Leaf rust is the most important wheat disease in Argentina; 4.2 M ha of wheat were grown in 2014. The objective of the study was to identify avirulence/virulence phenotypes of the Pt population in leaf rusted samples collected in wheat-growing areas during 2013. Single uredinial isolates were taken from samples and tested on Thatcher near-isogenic lines and some local varieties. Resistance genes in sets of four included: Lr1, Lr2a, Lr2c, Lr3; Lr9, Lr16, Lr24, Lr3ka; Lr11, Lr17, Lr30, Lr10; Lr14a, Lr19, Lr20, Lr21; Lr23, Lr25, Lr26, Lr27+31; Lr29, Lr36, Lr39/41, Lr42, Lr43; Lr44 and Lr47. Race designations were based on the first three sets proposed by Long and Kolmer (1989, Phytopathology, 79: 525-529) and the gene designations Lr10 and/or Lr20 were appended to indicate virulence on lines with those genes. Among 141 single uredinial isolates, 18 races were identified. Race MFP, was the most frequent, accounting for 27% of isolates; race MDP was second at 22.7%. Both races were isolated for the first time in 2005. The first three races were present in similar frequencies to 2012. Two new races were found, MKJ 10 and MGJ 10. Virulence was not found for genes Lr19, Lr21, Lr25, Lr29, Lr36, “Lr43”, Lr44 and Lr47. Race DBB 10,20 was the most frequent race on durum wheat. Virulence to Lr16 appears to be increasing.
Primary Author: Campos, National Institute of Agricultural Technology (INTA), Argentina
Stem rust is an important disease of wheat and barley. Barley is genetically vulnerable to stem rust with few identified resistance genes and sources. Stem rust resistance breeding is largely based on the gene Rpg1, first incorporated into North American barley cultivars in the 1940s. To identify potentially new resistance sources, the USDA-ARS National Small Grains Barley iCore Collection (1,860 accessions) was evaluated for reaction to stem rust at the seedling and adult stages. The adult stage evaluations were conducted at Njoro, Kenya (race TTKSK/TTKST), and St. Paul, Minnesota (race QCCJB), for two seasons, and the seedling tests (race TTKSK) in the BSL-3 greenhouse at St. Paul, Minnesota. At St. Paul, between 7 and 10% (132-203) of the accessions exhibited resistance, whereas in Kenya, 11-14% (218-261) were resistant. Correlation between years was higher in Kenya (0.60) than it was at St. Paul (0.48). Approximately 15% (277) of the collection gave moderately low to low reactions to TTKSK at the seedling stage. From these initial tests, 290 accessions were chosen based on diversity of reaction, origin of plant material, or stability across environments. These accessions were then further evaluated with a suite of races at the seedling stage to postulate resistance genes. Of these selections, 244 gave reactions suggesting they carry adult plant resistance. The remaining 46 accessions gave low to very low reactions to one or more races. Based on country of origin and resistance spectrum 15 accessions were predicted to have the rpg4/Rpg5 complex, including a subset from Switzerland. The remaining 31 have a reaction spectrum, country of origin or pedigree that does not suggest the presence of the rpg4/Rpg5 complex. Molecular tests will be used to confirm the presence of this complex in these materials.
Primary Author: Case, Department of Plant Pathology, University of Minnesota, USA
Adult plant resistance (APR) to stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is often conferred by multiple minor genes and has the potential to be durable. A preceding project identified two Kenyan wheat lines (W1406 and W6979) from the Genome Resource Unit (Norwich, UK) that exhibit APR to Pgt. The aim of this study was to investigate the APR response to Pgt race PTKST in W1406 and W6979 compared to 37-07, a susceptible control line. Histological investigation of inoculated flag leaf sheaths indicated a significant and quantifiable decrease in Pgt colony size in the APR lines at 120 hours post inoculation (hpi). Molecular analysis supported the observed fungal biomass decrease in the APR lines at 120 hpi. RNAseq analysis identified 169 transcripts differentially expressed in W1406 and 166 transcripts in W6979 when comparing 24 and 72 hpi to 0 hpi. In W1406 transcripts encoding putative pectinesterases, lipid-transfer proteins and leucine-rich repeat-like proteins were induced at 72 hpi. In W6979 only a corresponding putative pectinesterase encoding transcript was identified. Although the induced defence response in the two APR lines exhibited some dissimilarity, it potentially involves cell wall modification in both lines. Two independent sets of peroxidases were induced at 24 and 72 hpi in both lines, suggesting independent signalling events. Expression analysis suggests the occurrence of two phases of gene expression, one at 24 hpi and another at 72 hpi; the latter seeming to correspond to the inhibition of Pgt growth, manifesting as the observed APR phenotype.
Primary Author: Castelyn, University of the Free State, South Africa
The domestication of wild emmer wheat ~10,000 years ago by early agrarian societies have led to the selection of domesticated emmer and subsequently of durum wheat through a process of selection for non-brittle rachis and free-threshing forms. Durum wheat and became established as a prominent crop only ~1,500-2,000 years ago. We have completed the 10.45 Gb assembly of the 14 chromosomes of the modern DW cultivar 'Svevo' and provides, via comparison with the wild emmer assembly, an account of the genome-wide modifications imposed by 10,000 years of selection and breeding on the genome architecture of tetraploid wheat. A number of regions that were under selection during the domestication of wild emmer or the subsequent selection of durum wheat have been identified. Furthermore, we have projected on the durum wheat genome about 1,500 QTLs for morphological phenological and quality traits, grain yield components and disease resistance reported from published biparental mapping or GWAS. NBS-LRR genes are prominently involved in signaling and plant disease resistance. The durum wheat genome contains more than 66,000 genes and among them we annotated about 1,500 complete NBS-LRR genes. A similar number was found in the wild emmer genomes, nevertheless the comparison of the two genomes has identified some NBS-LRR genes specific for durum wheat. The availability of the complete genome of durum wheat will speed up the identification and the isolation of new resistance genes as well as the breeding for high-yielding and more resilient cultivars.
Primary Author: Cattivelli, CREA Research Centre for Genomics and Bioinformatics
Stripe rust, is a major constraint to wheat production in the more than 12.8 m ha region of the Northern Hills and North Western Plains zones in India. The previously deployed resistance genes Yr9 and Yr27 are no longer effective. New sources of resistance (Yr5, Yr10, Yr15, Yr24) became available under the umbrella of an Indo-Australian collaborative project. A set of advanced backcross derivative lines out yielded the checks in preliminary evaluations and were promoted to station-level (16 lines) and national (5 lines) trials. A new cohort of resistance genes (Yr47, Yr51, Yr57) are now available and are being used in the breeding program. Resistance genes Yr17, Yr18, Yr31, Yr36, Yr40, Yr53, YrC591, and Yr70 are also being used. The recent progress in development of high yielding, stripe rust resistant lines will help to address future threats from stripe rust.
Primary Author: Chatrath, ICAR-Indian Institute of Wheat and Barley Research, Karnal
The flag leaf and spike are the prime organs in wheat (Triticum aestivum L.) which contribute majorly for spike photosynthesis and eventually aid in grain filling. In this study we have tried to elucidate the effect of abiotic stress on the grain filling and spike photosynthesis. In order to unravel the role of flag leaf, awn, and spike in wheat grain filling and spike photosynthesis, 1000-kernel weight were calculated after removing flag leaves, awns, and by shading the spike in four wheat genotypes (PBW343, C306, K7903, HD2329) for two seasons (2014-2015, 2015-2016). A significant decrease in the grain filling was observed for all the genotypes. These results indicate the role of these organs in spike photosynthesis. The role of the awn tissue was investigated in PBW343 for its role in spike photosynthesis during heat stress. Deep transcriptome sequencing of the awn tissue (PBW343) was performed and it revealed 147573 unigenes. Out of these, 394 genes were differentially expressed genes (DEGs). These DEGs constitutes 201 upregulated and 193 downregulated genes. Genes involved in photosynthesis (Ribulose bisphosphate carboxylase/oxygenase activase B, NADH dehydrogenase, Fe-S protein2), membrane integrity (ATP-dependent zinc metalloprotease FTSH6), and ion channel transporters (two-pore potassium channel3) were prominently expressed. Gene Ontology (GO) enrichment analysis represents PSII associated light-harvesting complex II catabolism, chloroplast organization, photosynthesis light harvesting in photosystemI, ethylene biosynthesis, regulation of oxidoreductase activity, stomatal closure, chlorophyll biosynthesis categories, which are highly overrepresented under heat stress conditions. Therefore, utilizing the awn transcriptome information, Rubisco activase (RCA) gene was chosen for overexpression studies in wheat and rice with the aim to enhance the photosynthetic efficiency of the spike tissue leading to higher grain filling.
Primary Author: Chaudhary, Department of Plant Molecular Biology, University of Delhi South Campus
Stem rust resistance genes Sr39 (RL6082) and Sr36 (Cook) were transferred from Aegilops speltoides and Triticum timopheevi to chromosome 2B of wheat. Both genes are located on large translocated segments. Genotypes carrying Sr36 and Sr39 produce infection types (ITs) 0; and 2, respectively, against avirulent pathotypes. This investigation was planned to study the genetic relationship between these genes with the aim of combining them in a single genotype. Seedling tests on RL6082/Cook F3 lines showed complete repulsion linkage [25 Sr39Sr39sr36sr36 (IT2-) : 53 Sr39sr39Sr36sr36 (IT2-, IT0;) : 13 sr39sr39Sr36Sr36 (IT 0;)], and preferential transmission of the Ae. speltoides segment over the T. timopheevi segment was evident from the segregation ratio. The Sr39-carrying translocation was shortened by Niu et al. (2011; Genetics 187: 1011-1021) and the genetic stock carrying the shortest segment was named RWG1. Based on the reported location of Sr39 in the smaller alien segment in RWG1, we predicted that it should recombine with Sr36. F3 lines derived RWG1/Cook were phenotyped for stem rust response at the two-leaf stage and again complete repulsion linkage between Sr39 and Sr36 was observed [23 Sr39Sr39sr36sr36 (IT2-) : 78 Sr39sr39Sr36sr36 (IT0;, IT2-) : 68 sr39sr39Sr36Sr36 (IT 0;)]. In contrast to the cross involving the large Sr39 translocation, preferential transmission of the T. timopheevi segment was observed. These results indicated that a genetic determinant of meiotic drive had been deleted in the shortened Ae. speltoides segment. Genotyping with the co-dominant STS marker rwgs28 matched the phenotypic classification of F3 families. Marker rwgs28 was diagnostic for the Ae. speltoides segment, but the rwgs28 allele amplified in Cook was not T. timopheevi-specific.
Primary Author: Chemayek, The University of Sydney, Plant Breeding Institute, Australia
Most South African winter wheat varieties display all stage resistance (ASR) to stem rust caused by Puccinia graminis f. sp. tritici (Pgt). To study inheritance, four resistant varieties were crossed to a susceptible parent (Line 37) and F2 populations were phenotyped at the seedling stage with stem rust race PTKST (Ug99 lineage). Populations derived from varieties Koonap, Komati, Limpopo and SST 387 segregated in a 3:1 ratio, indicating that a single, dominant gene confers resistance in each population. Assessment of F2 seedlings of four intercrosses between these varieties failed to deliver susceptible segregants therefore suggesting that they carry the same resistance gene. Genotyping of F2 plants with microsatellite markers produced consistent linkage of resistance with markers on chromosome 6DS. Experiments are underway to determine the relationship between resistance in the four winter wheat varieties and resistance genes Sr42, SrCad and SrTmp, all located on 6DS. Current evidence shows that ASR in the South African winter wheat varieties Koonap, Komati, Limpopo and SST 387 is based on a single gene and thus vulnerable to pathogenic adaptation in Pgt.
Primary Author: Chemonges, University of the Free State