1a. Objectives (from AD-416):
The long-term objective of this project is to provide wheat, barley, and sorghum producers with new pest resistant crops and technologies that will protect their crops from insect pests. Specifically, during the next five years we will focus on the following objectives. Objective 1: Identify new sources of resistance to aphids and other insects in wheat, barley, sorghum, and related species. Subobjective 1A. Evaluate available germplasm resources (national germplasm collections and accessible exotic resources) to identify new sources resistant to insect pests [Russian wheat aphid (RWA), greenbug (GB), and bird cherry-oat aphid (BCOA)] in wheat, barley, sorghum, and related species. Subobjective 1B. Define a visual rating scale for use in greenhouse screening of wheat and barley seedlings for identification of BCOA resistance. Subobjective 1C. Determine the field resistance of barley lines which exhibit a unique visual plant response to RWA feeding as seedlings in the greenhouse which is not clearly assigned to a resistance level by Webster's scale of 1 - 9. Objective 2: Characterize the mechanisms and genetics of new sources of aphid resistance in wheat, barley, and sorghum. Subobjective 2.A. Develop and evaluate genetic populations to determine the genetic control of host resistance to GB, RWA, and BCOA in barley. Subobjective 2.B. Develop and evaluate genetic populations to determine levels of genetic diversity of host resistance to GB, RWA, and BCOA in wheat, barley, and sorghum. Subobjective 2.C. Develop and identify molecular markers to facilitate identification of resistance QTLs and cloning of the resistance gene(s), and to aid selection of breeding lines through marker-assisted selection. Subobjective 2.D. Conduct functional genomics studies on host response to GB attack, leading to advanced understanding of the defense mechanisms in the hosts and discovery of genes and factors that affect host defense against insect pests (i.e., GB) in sorghum and related species. Objective 3: Develop improved germplasm of wheat, barley, and sorghum incorporating new sources of insect resistance and other desired traits into elite, adapted backgrounds for the United States. Subobjective 3.A. Develop high performance wheat, barley, and sorghum germplasm with enhanced resistance to GB, RWA, or BCOA, and release to the public. Subobjective 3.B. Develop genetically improved barley and sorghum cultivars and hybrids for use as feedstocks for bioethanol, animal feed, and forage-grazing potential under expanded growth conditions.
1b. Approach (from AD-416):
The long-term goal of this project is to provide wheat, barley, and sorghum producers with new pest resistant crops and technologies that will protect their crops from insect pests. To accomplish the research objectives, the project will search available germplasm collections to find new, effective sources of resistance to virulent aphid pests, including Russian wheat aphid (RWA), greenbug (GB), and bird cherry-oat aphid (BCOA). The genetic diversity and genetic control of resistance in these crops will be characterized using genetic and genomic approaches, leading to advanced understanding of the defense mechanisms in the hosts and discovery of genes and factors that regulate host defense against insect pests. The identified resistance genes will be transferred into elite, adapted genetic backgrounds. Plant genotyping will be conducted to map aphid resistance genes to the crop chromosomes and to develop molecular markers for marker-assisted selection to facilitate the breeding process. The research team of the project will work closely with collaborating plant breeding programs to obtain elite breeding lines to use as parents in backcrossing procedures to transfer aphid resistance and other value-added (e.g., enhanced ethanol production) traits. The genetically improved germplasm will be field-tested for agronomic and quality performance prior to release. The project will provide testing and selecting support to assure these desirable genes move through the various breeding programs on their way to the producers via cultivar and hybrid releases.
3. Progress Report:
We have screened a large set of wheat germplasm for bird cherry-oat aphid (BCOA) resistance, and identified two new resistance sources, which were subsequently used to derive mapping/breeding populations (this work relates to Objective 1A of the project). We have also completed a study aimed at characterizing a gene conferring resistance to Russian wheat aphid biotype 2 (RWA2) in PI 682675 (formerly PI 624151-1-2), a reselection line we developed from the Iranian landrace PI 624151. Our results suggested that the RWA2 resistance gene in PI 682675 is a new gene, and the curator of "Catalogue of Gene Symbols for Wheat" has designated it Dn10. Moreover, six powdery mildew resistance genes in wheat landraces and historical cultivars have been mapped using molecular markers. The curator of "Catalogue of Gene Symbols for Wheat" has designated one of these genes as Pm59. Pm59 was identified in Afghanistan landrace PI 181356, and confers high resistance to Bgt isolates collected in the Great Plains and the state of Montana. We expected that more powdery mildew resistance genes we identified will be officially designated. In addition, we have developed a recombinant inbred line (RIL) population to map QTL for BCOA resistance in a resistant germplasm we previously identified (Objective 2C). Approximately 4,000 barley accessions from the USDA-ARS National Small Grains Collection were screened for resistance to BCOA. Surviving seedlings were transplanted in the greenhouse, harvested, and seed prepared for further screening. Aphids were collected in the field in a multi-state survey to determine the adaptation of a new invasive aphid pest (Hedgehog Grain Aphid) of small grains in the U.S. If this new pest becomes established and is damaging small grains, screening for resistance will begin immediately (NP301, Sub-objective 1A). Seed of 12 genetic populations were increased for future testing to determine the level of genetic diversity for greenbug resistance in barley. For sorghum, more than 30 germplasm lines have been identified as the new sources of resistance to sugarcane aphid (SCA) following screening of a large group of diverse sorghum germplasm accessions. Most of these lines have been purified through several generations of testing and selection, thus confirmed. Many of them have been amplified, and we are now writing the manuscript for germplasm releasing. This year, we plan to release at least 10 sugarcane resistant lines (Objective 1A). Genetic characterization of the newly discovered germplasm is underway, and other agronomic traits of those lines are being investigated using the field trails through collaboration with entomologists and extension agronomists in Oklahoma and Texas. Once the genetic data and phenotypic traits are collected, we will make them available to the sorghum community, which should benefit both private and public breeding programs. Further, using the genomic approach, we have mapped a major gene that offers the genetic resistance to SCA. Meanwhile, we are developing DNA markers that are tightly linked to the resistance, and those markers can facilitate both marker-assisted breeding and cloning the SCA resistance genes in sorghum (Objective 2B). Seedlings from 3250 heads were screened for resistance to both RWA and greenbug. 5200 resistant seedlings were selected and transplanted to the greenhouse for seed increase. Plants were harvested, processed, and planted in the field for agronomic evaluation in FY18. One hundred and eighteen elite lines were evaluated in the field at three locations in Oklahoma towards the release of winter feed barley cultivars/germplasm for the southern plains. 3,000 aphid-resistant single rows were evaluated in the field and superior lines selected for evaluation in yield trials at several locations in FY18. 3,900 heads were selected from 65 populations and will be screened for resistance to both RWA and GB in FY18. Two hundred crosses were made between winter feed barley cultivars adapted to the Southern Plains and elite winter malting barley germplasm/cultivars from several northern states to begin the development of winter malting barleys for the Great Plains (Objective 3B). We continued to evaluate breeding populations in the field to select high yielding aphid-resistant lines, with an emphasis on greenbug and BCOA resistance (Objective 3A). In addition, several resistance sources have been crossed into elite sorghum lines, including BTx623, for development of both mapping populations and breeding populations. Now a couple of such populations are already at F5 or F6 generation (i.e., recombinant inbred lines). Some of the promising SCA inbred lines can be released within 1-2 years (Objective 3B).
1. Identification of a new Russian wheat aphid (RWA) resistance gene in wheat. Russian wheat aphid biotype 2 (RWA2) is virulent to most RWA resistance genes, and new resistance sources are urgently needed. ARS scientists at Stillwater, Oklahoma have identified a new RWA2 resistance gene in PI 682675 (formerly PI 624151-1-2), a reselection line they developed from Iranian landrace PI 624151. They characterized the RWA2 resistance gene using molecular markers, and mapped it to the long arm of chromosome 7D. The identified gene is new, and has been designated as Dn10. Dn10 can be used as a surrogate for the rye-derived Dn7 gene, which is currently used in wheat breeding but associated with undesirable bread-making quality.
2. Regulatory mechanism of the brown midrib (bmr) trait in sorghum. Plant biomass (i.e. lignocellulosic materials) has great potential for use as a feedstock for bioethanol production. But challenges with respect to processing steps in converting biomass to liquid transportation fuel like pretreatment, hydrolysis, microbial fermentation, and separation still exist. Lignin is a major component of plant biomass and negatively affects the saccharification and bioethanol production. This study focused on characterization of the gene that encodes a key enzyme for lignin biosynthesis in plants. As a result, ARS scientist in Stillwater, Oklahoma discovered an 8-basepair DNA deletion in its 5'-untranslated region of the gene (SbCAD2), which confers the spontaneous brown midrib (bmr) trait and lignin deficiency in sorghum germplasm line PI595743. Moreover, the results of their molecular experiments indicated that the SbCAD2 promoter was functionally conserved in terms of driving a specific expression pattern in lignifying vascular tissues. In summary, the findings from this study will be beneficial to design preferred lignocellulosic feedstocks for bio-ethanol processing to meet the industry's need for cost-effective biofuel production.
3. Identification and characterization of Pm59, a novel powdery mildew resistance gene in wheat. Powdery mildew, cause by Blumeria graminis f. sp. tritici (Bgt), is an important foliar disease of wheat worldwide, and Bgt isolates virulent to powdery mildew resistance genes widely used in the Great Plains were previously identified. ARS scientists at Stillwater, Oklahoma identified a new gene conferring resistance to Bgt isolates in the Great Plains, and located it to the terminal region of chromosome 7AL. This new gene is a valuable addition to the wheat powdery mildew resistance gene pool, and the curator of "Catalogue of Gene Symbols for Wheat" has designated it Pm59.
Li, G., Bai, G., Carver, B.F., Elliott, N.C., Bennett, R.S., Wu, Y., Hunger, R., Bonman, J.M., Xu, X. 2017. Genome-wide association study reveals genetic architecture of coleoptile length in wheat. Theoretical and Applied Genetics. 130(2):391-401.
Li, G., Xu, X., Bai, G., Carver, B.F., Hunger, R.M., Bonman, J.M. 2017. Novel sources of leaf rust resistance in winter wheat. Crop Science. 57(2):865-876.
Park, S.J., Huang, Y. 2016. Characterization and expression analysis of two cDNAs encoding Xa1 and oxysterol binding proteins in sorghum (Sorghum bicolor). Academy of Agriculture Journal. 1(4):70-75.
Mornhinweg, D.W., Hammon, R.W., Obert, D.E. 2017. Registration of 'Mesa' Russian wheat aphid-resistant winter feed barley. Journal of Plant Registrations. 11(2):85-88.
Li, G., Xu, X., Bai, G., Carver, B.F., Hunger, R., Bonman, J.M., Kolmer, J.A., Dong, H. 2016. Genome-wide association mapping reveals novel QTL for seedling leaf rust resistance in a worldwide collection of winter wheat. The Plant Genome. 9(3):1-12. doi:10.3835/plantgenome2016.06.0051.