Z. A. Pretorius

Leaf rust and stripe rust resistance genes transferred to common wheat from Triticum dicoccoides

Linked leaf rust and stripe rust resistance genes introduced from Triticum dicoccoides protected common wheat seedlings against a range of pathotypes of the respective pathogens. The genes were chromosomally mapped using monosomic and telosomic analyses, C-banding and RFLPs. The data indicated that an introgressed region is located on wheat chromosome arm 6BS. The introgressed region did not pair with the ‘Chinese Spring’ 6BS arm during meiosis possibly as a result of reduced homology, but appeared to pair with 6BS of W84-17 (57% of pollen mother cells) and ‘Avocet S’. The introgressed region had a very strong preferential pollen transmission (0.96–0.98) whereas its transmission through egg cells (0.41–0.66) varied with the genetic background of the heterozygote. Homozygous resistant plants had a normal phenotype, were fertile and produced plump seeds. Symbols Lr53 and Yr35 are proposed to designate the respective genes.

Identification of adult plant resistance to stripe rust in the wheat cultivar Cappelle-Desprez

Following the appearance of stripe rust in South Africa in 1996, efforts have been made to identify new sources of durable resistance. The French cultivar Cappelle-Desprez has long been considered a source of durable, adult plant resistance (APR) to stripe rust. As Cappelle-Desprez contains the seedling resistance genes Yr3a and Yr4a, wheat lines were developed from which Yr3a and Yr4a had been removed, while selecting for Cappelle-Desprez derived APR effective against South African pathotypes of the stripe rust fungus, Puccinia striiformis f. sp. tritici. Line Yr16DH70, adapted to South African wheat growing conditions, was selected and crossed to the stripe rust susceptible cultivar Palmiet to develop a segregating recombinant inbred line mapping population. A major effect QTL, QYr.ufs-2A was identified on the short arm of chromosome 2A derived from Cappelle-Desprez, along with three QTL of smaller effect, QYr.ufs-2D, QYr.ufs-5B and QYr.ufs-6D. QYr.ufs-2D was located within a region on the short arm of chromosome 2D believed to be the location of the stripe rust resistance gene Yr16. An additional minor effect QTL, QYr.ufs-4B, was identified in the cv. Palmiet. An examination of individual RILs carrying single or combinations of each QTL indicated significant resistance effects when QYr.ufs-2A was combined with the three minor QTL from Cappelle-Desprez, and between QYr.ufs-2D and QYr.ufs-5B.

Challenges for sustainable cereal rust control in South Africa

The cultivation of small grain cereals was introduced to South Africa by Dutch settlers in the 17th Century. According to historical records the first documented epidemic of wheat stem rust occurred in the south-western parts of the current Western Cape in 1726. Recurring stem and leaf rust epidemics were associated with expanding wheat production and became particularly severe in the winter-rainfall regions of the Western and Eastern Cape, as well as in the summer-rainfall regions of the Free State. The wheat stripe rust pathogen was first detected in South Africa in 1996. Due to susceptibility of cultivars at the time of this exotic introduction, stripe rust has caused significant losses in commercial wheat production over the past 10 years. Pathotype surveys of Puccinia graminis and P. triticina were initiated in the 1920s, but were discontinued until research on wheat stem rust was resumed in the 1960s. Recent evidence has shown that P. graminis f. sp. tritici continues to evolve. In addition, the annual number of wheat stem rust collections is increasing, emphasising the sustained threat of this damaging pathogen. A stem rust pathotype first detected in 2000, with newly acquired virulence for Sr8b and Sr38, currently constitutes more than 80% of all collections. Leaf and stem rust diseases also occur on barley, oat, triticale, and rye and are important production constraints in several regions. Some studies have described variability in these pathogens but long-term records of pathogenicity changes in barley and oat rust are not available. Cereal rust diseases have clearly played an important role in South African agriculture and many production regions remain favourable for rust development. Current expertise in cereal rusts covers most technologies necessary to study the respective host–pathogen systems. However, a general lack of capacity and fragmentation of research groups prevent a unified approach and remain a challenge for sustainable cereal rust control in South Africa. A national strategy for cereal rust control, with particular emphasis on pathogen and host resources, and breeding for resistance, is urgently needed.

A study of modified forms of the Lr19 translocation of common wheat

Abstract Following the induction of allosyndetic pairing between the Thinopyrum-derived Lr19 translocation in ‘Indis’ wheat and homoeologous wheat chromatin, eight suspected recombinants for the Lr19 region were recovered. These selections were characterised for marker loci that were previously used to construct a physical map of the Lr19 segment. At the same time near-isogenic lines were developed for some of the selected segments and tested for seedling leaf-rust resistance in order to confirm the presence of Lr19. It appeared that three of the four white-endosperm selections do not possess Lr19 and only one, 88M22-149, is a true Lr19 recombinant. The resistance gene in the three non-Lr19 selections resides on chromosome 6B, appears to derive from ‘Indis’, and was selected unintentionally during backcrossing. The pedigree of ‘Indis’ is suspect and it is believed that the Lr19 translocation in ‘Indis’ is in reality the Th. ponticum-derived (T4) segment rather than being of Th. distichum origin as was believed earlier. The white-endosperm recombinant, 88M22-149, retained the complete Lr19 resistance and was apparently re-located to chromosome arm 7BL in a double-crossover event. 88M22-149 has lost the Sd1 gene and often shows strong self-elimination in translocation heterozygotes. This effect may result from additional gametocidal loci or from an altered chromosome structure following re-location of the segment. 88M22-149 in fact contains a duplicated region involving the Wsp-B1 locus. Three selections had partially white endosperms and expressed Lr19 and other Thinopyrum marker alleles. Polymorphisms for the available markers confirmed that the translocated segment in at least one of them had been shortened through recombination with chromosome arm 7DL. Further markers need to be studied in order to determine whether the translocation in the remaining two partially white recombinants had also undergone recombination with wheat. The eighth selection has yellow endosperm and appears to self-eliminate in certain translocation heterozygotes. No evidence of recombination could be found with the markers used. If the latter selections are in fact recombinants they may prove useful in attempts to unravel the complex segregation distortion mechanism.

Borlaug Global Rust Initiative provides momentum for wheat rust research

The discovery in 1999 in Uganda (Pretorius et al. 2000) of stem rust on wheat varieties with the 1BL.1RS translocation carrying the gene known as Sr31 came as re-awakening, even when it should not have been unexpected. The worldwide deployment of wheat varieties with the translocation probably had an effect of reducing inoculum, resulting in low incidence of the disease over an extended period. Stem rust resistance was only one of a number of advantageous features of the translocation. With acquisition of virulence for Sr31, race Ug99 as it became known, was subsequently found in epidemic proportions in Kenya and Ethiopia. Stem rust had not been a significant problem in Kenya for many years prior to 2000 although the region was a historic hotspot for the disease. In Ethiopia, with about 40% of the area growing durum the situation was different; stem rust continued to be a recurrent problem on durum whereas bread wheat was generally resistant. Subsequent events of rapid mutational changes in Ug99 to attack varieties with Sr24 and Sr36, and its further migration to Yemen and Iran, caused major concern and a prediction of the likelihood of further movement to the important grain production areas of the Indian subcontinent and beyond. Previously races of the stripe rust pathogen virulent for Yr9, also in the same 1BL.1RS translocation, were observed sequentially from equatorial Africa to India suggesting the entire wheat area in Asia may comprise a single epidemiologic zone (Singh et al. 2004, 2008). Preliminary laboratory and field tests with Ug99 confirmed that it not only had virulence for Sr31, but it also possessed virulence to other genes deployed in contemporary wheat varieties along with obvious fitness attributes. Most importantly it could cause disease on the widely used variety (mega-variety) Attila and sibs that were being widely grown from Kenya to India and Afghanistan under several different names. International concern regarding the threat to world wheat production caused by Ug99 led to the durable rust resistance in wheat project at Cornell University funded by the Bill and Melinda Gates Foundation and establishment of a Global Rust Initiative (later Borlaug Global Rust Initiative, BGRI). The BGRI led by Cornell University, CIMMYT, ICARDA and FAO is supported by many national governments and research centres. Clearly, stem rust is only one of the three rusts that threaten wheat, and currently there is a group of rapidly spreading races of the stripe rust pathogen that is a major threat to production at least as great as Ug99. One of the objectives of BGRI is to conduct an annual meeting. The first meeting was held at Ciudad R. A. McIntosh (&) The University of Sydney Plant Breeding Institute Cobbitty, Private Bag 4011, Narellan, NSW 2567, Australia e-mail: robert.mcintosh@sydney.edu.au

Reduction of Aegilops sharonensis chromatin associated with resistance genes Lr56 and Yr38 in wheat.

The Lr56/Yr38 translocation consists primarily of alien-derived chromatin with only the 6AL telomeric region being of wheat origin. To improve its utility in wheat breeding, an attempt was made to exchange excess Ae. sharonensis chromatin for wheat chromatin through homoeologous crossover in the absence of Ph1. Translocation heterozygotes that lacked Ph1 were test-crossed with Chinese Spring nullisomic 6A tetrasomic 6B and nullisomic 6A-tetrasomic 6D plants and the resistant (hemizygous 6A) progeny were analyzed with four microsatellite markers. Genetic mapping suggested general homoeology between wheat chromosome 6A and the translocation chromosomes, and showed that Lr56 was located near the long arm telomere. Thirty of the 53 recombinants had breakpoints between Lr56 and the most distal marker Xgwm427. These were characterized with additional markers. The data suggested that recombinants #39, 157 and 175 were wheat chromosomes 6A with small intercalary inserts of foreign chromatin containing Lr56 and Yr38, located distally on the long arms. These three recombinants are being incorporated into adapted germplasm. Attempts to identify the single shortest translocation and to develop appropriate markers are being continued.

Chromosome locations of leaf rust resistance genes in selected tetraploid wheats through substitution lines

Langdon durum D-genome disomic substitution lines were used to study the chromosome locations of adult-plant leaf rust resistance genes identified from tetraploid wheat accessions. The accessions are 104 (Triticum turgidum subsp. dicoccum var. arras) and 127 (T. turgidum subsp. durum var. aestivum). The complete sets of the substitution lines were crossed as female parents with the accessions and F1 double monosomic individuals selected at metaphase I. Segregating F2 individuals were inoculated during the flag leaf stage with pathotype UVPrt2 of Puccinia triticina. The substitution analysis involving accession 104 showed that the gene for leaf rust resistance is located on chromosome 6B. The analysis with accession 127 indicated that chromosome 4A carries a gene for leaf rust resistance. The two novel genes are temporarily designated as Lrac104 and Lrac127, respectively from accessions 104 and 127.

Stem Rust Resistance in a Geographically Diverse Collection of Spring Wheat Lines Collected from Across Africa

Following the emergence of the Ug99 lineage of Puccinia graminis f. sp. tritici (Pgt) a collective international effort has been undertaken to identify new sources of wheat stem rust resistance effective against these races. Analyses were undertaken in a collection of wheat genotypes gathered from across Africa to identify stem rust resistance effective against the Pgt races found in Eastern and Southern Africa. The African wheat collection consisted of historic genotypes collected in Kenya, South Africa, Ethiopia, Sudan, Zambia, Morocco, and Tunisia, and current South African breeding lines. Both Bayesian cluster and principal coordinate analyses placed the wheat lines from Sudan in a distinct group, but indicated a degree of genetic relatedness among the other wheat lines despite originating from countries across Africa. Seedling screens with Pgt race PTKST, pedigree information and marker haplotype analysis confirmed the presence of Sr2, Sr36, Sr24, Sr31, and Lr34/Yr18/Sr57 in a number of the lines. A genome-wide association study (GWAS) undertaken with Diversiry Arrays Technology (DArT) and stem rust (Sr) gene associated markers and Stem Area Infected (SAI) and Reaction Type (RT) field phenotypes, collected from trials carried out across two seasons in Kenya in 2009 and in South Africa in 2011, identified 29 marker-trait associations (MTA). Three MTA were in common between SAI and RT, with the biggest effect MTA being found on chromosome 6AS. Two wheat lines, W1406 and W6979 that exhibited high levels of adult plant stem rust resistance were selected to generate bi-parental mapping populations. Only the MTA on chromosomes 6AS and 3BS, and the locus Lr34/Yr18/Sr57 were confirmed following QTL mapping. Additional stem rust resistance QTL, not detected by the GWAS, were found on chromosomes 2BS, 2DL, 3DL, and 4D.