Location: Hard Winter Wheat Genetics Research2009 Annual Report
1a. Objectives (from AD-416)
Objective 1: Develop adapted hard red or white wheat germplasm lines with improved resistance to emerging or intractable problems in wheat production and marketing. Objective 2: Increase understanding of the molecular basis of parasite virulence, host resistance, and stress tolerance for these problems. Objective 3: Develop and apply phenotypic and genotypic selection technology for these traits to hard red or white winter wheat germplasm or cultivar development.
1b. Approach (from AD-416)
Production of high quality hard red or white winter wheat is limited by recurring intractable problems such as leaf rust, Fusarium head blight, Hessian fly, and heat stress during the grain filling period. In addition, new emerging problems such as stripe rust, stem rust, and Karnal bunt threaten the production or marketing of high quality grain. The first objective of this project is to develop adapted hard red or white wheat germplasm lines with improved resistance or tolerance to these problems. We will utilize existing sources and identify new sources of resistance, introgress them into desirable backgrounds, and then release them for use as parents of commercial cultivars. The second objective is to increase our understanding of the molecular basis of parasite virulence, host resistance, and stress tolerance to support strategic development and deployment of genetic resistance. Greater understanding of secreted virulence/avirulence effectors in the Hessian fly and the leaf rust pathogen may lead to better strategies for durability. Greater understanding of the mechanisms of durable rust resistance and heat tolerance may lead to discovery of new genes or alleles with complementary mechanisms and to optimized gene combinations in new cultivars. The third objective is to develop and apply phenotypic and genotypic selection technology for these traits to hard red or white winter wheat germplasm and cultivar development. This is an essential component of the technology transfer effort. Large-scale phenotypic screening data for Hessian fly and Karnal bunt resistance and genotypic marker data will be provided to cooperators.
3. Progress Report
Development of genetic populations to investigate tolerance to heat stress is continuing on schedule. BC1F1 and F1-derived populations have been produced and advanced. Wheat stem rust resistance gene Sr35, which is effective against strain Ug99, was genetically mapped and flanking DNA markers were identified. For this effort, two wheat mapping populations were characterized by stem rust screening of F3 families and molecular markers on chromosome 3A long arm. Markers flanking Sr35 have been applied to pyramiding of stem rust resistance genes (along with Sr22, Sr40) in advanced backcross-derived materials. Lines homozygous for one and two genes have been identified and are being increased for distribution to national wheat breeders. DNA markers for newly discovered Ug99-effective resistance genes SrHv6, Sr5S, SrAse3 and SrAeg5 were identified. For SrAse3, a complete set of translocation lines were developed. These genes are present on compensating translocation chromosomes and may now be used in wheat breeding, although further manipulation to reduce the likelihood of linkage drag is underway. A bacterial artificial chromosome (BAC) library of wheat cultivar Sumai 3 was screened to identify clones spanning the Fusarium head blight resistance locus, Fhb1. To date, three BAC clones covering the Fhb1 region have been sequenced from this library and further analysis is ongoing to identify genes in the Sumai 3 sequence of this region and to confirm a complete contig of this region has been developed. This BAC library has been used to construct PCR-screenable plate pools that will ease future molecular genetic screening and may be of benefit to broad wheat genome/map-based cloning studies. To our knowledge, this is the only PCR-screenable whole-genome BAC library of hexaploid wheat in the United States (or world). Progress is continuing on transfer of the FHB resistance gene Fhb1 from cultivar Sumai 3 into adapted US hard winter wheat cultivars including Wesley, Jagger, Overley, and Overland. About 100 Wesley-Fhb1 lines were selected using molecular markers and evaluated in Nebraska and Kansas disease nurseries. Leaf rust is one of the most significant wheat pathogens worldwide. The objective of this work was to compare important genomic regions of leaf rust with stem rust to determine how closely they were related and also gain preliminary data for the leaf rust genome sequencing project. Three BACs were isolated from leaf rust that contained genes with secretory peptide signals. These signals are a flag for proteins involved in avirulence. The three BACS were sequenced, assembled and characterized for gene content. Then, the sequence was compared to stem rust to determine how closely the two fungi were related. In general, gene order was maintained; however, there was significant sequence divergence between leaf and stem rust. This information will be useful for the project to sequence the entire genome of leaf rust.
1. A physical map of the Hessian fly genome. The Hessian fly is one of the most destructive insects of wheat world-wide. Resistant wheat is the best strategy to control the damage caused by this pest. The challenge for the use of resistant wheat is the ability of the insect to develop new populations that can overcome resistance once resistant wheat is deployed to the field. To find how the insect can overcome wheat resistance, the genes that are responsible for the Hessian fly to attack wheat need to be identified. This work is designed to provide a detailed map of the Hessian fly genome with specific markers. The work provides a foundation for identification of specific Hessian fly genes and for whole genome sequencing.
2. Wheat, barley, and rice differ in response to Hessian fly. The Hessian fly is one of the most destructive insects of wheat world-wide. The insect is also suspected to damage barley in the west coast region of USA. This research discovered that Hessian fly populations collected from wheat fields can survive on barley seedlings, but with high mortality and slow development. ARS scientists in Manhattan, KS also found that most barley lines responded to Hessian fly attacks with a combination of resistance and tolerance. The evidence gathered so far points to the direction that Hessian fly populations from wheat fields may not be able to cause serious damage to barley. Further research is needed on this issue. Also in this research, scientists found that rice, a nonhost of the Hessian fly, is resistant to the insect with a mechanism that differs from resistance mediated by major genes in wheat.
3. Virulence analysis of Hessian fly from Texas, Oklahoma, Kansas. The most effective means to control Hessian fly damage is to develop and deploy wheat varieties that are resistant to the insect. The challenge for the host plant resistance strategy is the dynamic change of Hessian fly biotypes in the field. Specific resistance genes can lose effectiveness due to changes in Hessian fly biotypes. In order for the host plant resistance strategy to be continuously successful, scientists need to monitor which resistance genes are still effective periodically. This research investigated the effectiveness of 20 known resistance genes to field Hessian fly populations from Texas, Oklahoma, and Kansas. Five of the 20 tested genes, H13, H21, H25, H26, and Hdic, conferred high levels of resistance (> 80% of plants scored resistant) to all tested geographic populations. However, resistance levels for other genes varied depending on which Hessian fly population they were tested against. This study should provide useful information to breeders when they select resistance genes for their breeding programs and to wheat growers when they select wheat varieties for planting.
4. New molecular markers (SFPs) developed for wheat. Poor molecular marker coverage in wheat limits high resolution mapping of many important traits. To develop new markers, wheat seedling RNA from the recombinant inbred mapping population Ning 7840/Clark and their parents were hybridized with Affymetrix microarrays to identify possible single-feature polymorphism (SFP) markers in wheat. A map with 850 SFP markers was developed using the mapping population. A selected set of SFP markers has been validated by DNA sequencing to contain single nucleotide polymorphisms (SNPs). The genetic map has been published and will be an important marker source for high-resolution mapping and marker–assisted breeding.
5. Genetic mapping of wheat stem rust resistance gene Sr40. Stem rust was historically one of the most destructive diseases of wheat worldwide. Extensive use of resistant cultivars successfully prevented rust damage over the past several decades. Recently, a new race of the stem rust pathogen called Ug99 has appeared in Africa and has defeated many existing resistance genes. Stem rust resistance gene Sr40 has been transferred from the wild wheat Triticum timopheevii ssp. armeniacum and provides resistance against Ug99 stem rust. In this study, several molecular markers for Sr40 were discovered that will be useful in marker-assisted selection for resistance.
6. Genetic mapping of wheat leaf rust resistance gene Lr42. Leaf rust is an important foliar disease of wheat worldwide. Leaf rust resistance gene Lr42 from the wild wheat relative, Aegilops tauschii, has been used as a source of rust resistance in breeding programs. To identify molecular markers closely linked to Lr42, a segregating population of near-isogenic lines (NILs) contrasting for the presence of Lr42 was developed in the hard winter wheat cultivar Century background and evaluated for rust infection type at both seedling and adult-plant stages. Two markers closely linked to Lr42 were identified on the short arm of wheat chromosome 1D. These markers will be useful for marker-assisted selection for Lr42 in new wheat varieties.
7. Maize EF-Tu protein increases heat tolerance of wheat. Heat stress is a major constraint to wheat production and negatively impacts grain quality, causing tremendous economic losses, and may become a more troublesome factor due to global warming. At the cellular level, heat stress causes aggregation of proteins and injury to membranes leading to reduction in photosynthesis. Here ARS scientists in Manhattan, KS report on the development of transgenic wheat, expressing a maize gene for a chloroplast protein called EF-Tu, that displays reduced thermal aggregation of leaf proteins, reduced heat injury to photosynthetic membranes, and enhanced rate of photosynthesis after exposure to heat stress. The results suggest that heat tolerance of wheat, and possibly other crop plants, can be improved by modulating expression of chloroplast EF-Tu and/or by selection of plants with increased natural levels of this protein.
8. A new resistance gene, Fhb3, for Fusarium head blight (FHB). FHB can cause substantial yield and grain quality losses in wheat crops of the Great Plains. Intensive resistance breeding efforts have resulted in a narrow genetic basis of FHB resistance, with most new wheat varieties sharing the same source of resistance. In order to broaden the basis of resistance, a new source of FHB resistance, named Fhb3, was identified from a wild relative of wheat, Leymus racemosus. This source is different from previously reported FHB resistance genes in wheat, providing a new source for wheat breeding programs. Fhb3 had a large effect in reducing disease severity in experimental wheat lines and may be rapidly deployed in wheat breeding programs.
9. Complete sequence of Triticum mosaic virus. Plant viruses are an important constraint on wheat production in the Great Plains. A new virus was recently found with symptoms similar to wheat streak mosaic virus. A coat protein covers most plant viruses and the amino acid sequence of the new virus coat protein was different than that of any other virus characterized. Thus, the virus was given the new name Triticum mosaic virus (TriMV). There is strong evidence that the virus belongs to the Potyvirus family.
10. New hard red winter wheat cultivar with resistance to Karnal Bunt. The experimental hard winter wheat line TX03M1096 was tested in the Karnal bunt screening nurseries in Mexico and India for the past 3 years, and has consistently been shown to have resistance to Karnal bunt. This line was released by Texas AgriLife Research in Fall, 2008 as TAM 401 and is marketed by AgriPro Wheat. TAM 401 is a high yielding disease resistant awnless hard red winter wheat adapted to Central and North Central Texas, including the 5 counties where Karnal bunt was found in 2001. The majority of wheat grown in this area is grazed and producers prefer awnless wheat. The release of TAM 401 is a very significant accomplishment, since TAM 401 has better disease resistance and higher grain yield than currently grown awnless cultivars. It will likely be adopted and grown on significant acreage thus reducing the likelihood of a Karnal bunt epidemic in the future.Prasad, V., Pisipati, S., Ristic, Z., Bukovnik, U., Fritz, A.K. 2008. Impact of Nighttime Temperature on Physiology and Growth of Spring Wheat. Crop Science. 48:2372-2380.