Objective 1: Identify new germplasm sources of resistance to cereal aphids, including greenbugs, Russian wheat aphid, bird cherry-oat aphid, and other important insect pests, in wheat, barley, and sorghum. Subobjective 1A. Evaluate available germplasm resources (U.S. germplasm collections and accessible exotic resources) to identify new sources resistant to insect pests [Russian wheat aphid (RWA), greenbug (GB), bird cherry-oat aphid (BCOA)], and other important insect pests in wheat, barley, sorghum, and related species. Objective 2: Characterize the genes controlling pest resistance and other related traits that are important for sustainable cereal crop production. 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 and genes controlling the resistance to GB, RWA, SCA, and BCOA in wheat, barley, and sorghum. Subobjective 2.C. Map genes conferring resistance to cereal aphids, and develop genomic tools for cloning and marker-assisted selection of aphid resistance genes. Subobjective 2.D. Conduct functional genomics studies on host response to attack by GB and sugarcane aphids (SCA), 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 and SCA) in grain sorghum, forage sorghum, and related species. Subobjective 2.E. Reveal the genetic architecture of BCOA resistance using genomewide association study (GWAS), and develop genomic tools to facilitate rapid introgression of aphid resistance genes into adapted germplasm. Objective 3: Develop enhanced germplasm and new varieties of sorghum, wheat, and barley with resistance to insect pests. Subobjective 3.A. Develop high performance wheat, barley, and sorghum germplasm with enhanced resistance to GB, RWA, SCA, or BCOA, and release to the public. Subobjective 3.B. Develop winter malting barley germplasm and cultivars for the Great Plains with enhanced insect resistance.
Wheat, barley and sorghum are the major cereal grains in the United States but their production is often threatened by destructive aphid pests, specifically the Russian wheat aphid, greenbug, bird cherry-oat aphid, and sugarcane aphid. Identification of natural resistance and use of genetically pest-resistant cultivars and hybrids in an integrated pest management program are the most economical and environmentally sound methods to reduce the negative economic impact of these damaging aphids. The overall goal of this project is to develop high performance wheat, barley, and sorghum with resistance to aphid pests. To accomplish this goal, the project will search available germplasm collections to find new, effective sources of resistance to aphid pests that are notorious for overcoming resistance through biotype evolution. The genetic diversity and resistance mechanisms will be analyzed, and resistance genes will be characterized and transferred into adapted genetic backgrounds. Plant genotyping will be conducted to map aphid resistance genes to the crop chromosomes and to develop molecular markers to facilitate marker-assisted selection and map-based gene cloning. 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 traits. The genetically improved germplasm and varieties will be field-tested for agronomic and quality performance prior to release. The project will provide testing and selecting support to assure that these desirable genes move through the various breeding programs on their way to producers via improved cultivar and hybrid releases.
This report documents progress for Project Number 3072-21000-009-00D, which started in March 2018 and continues research from Project Number 3072-21000-008-00D, entitled "Identification, Characterization, and Development of Insect-Resistant Wheat, Barley, and Sorghum Germplasm". We began to screen a mini-core set of germplasm for resistance to different biotypes of Russian wheat aphid, intending to find wheat accessions offering high resistance to multiple biotypes. The identified resistance sources were used to derive breeding/mapping populations. In addition, we have screened about 2,000 germplasm for bird cherry-oat aphid (BCOA) resistance, and identified a couple of lines exhibiting resistance to BCOA. The newly identified resistant lines will be retested in 2019, and used in germplasm enhancement. (Objective 1A) Aphids were collected in a multi-state field survey to determine the adaptation of a new invasive aphid pest of small grains, the hedgehog grain aphid (HGA). Also 2000 winter barley and 1000 spring barley accessions from the NSGC were screened in the greenhouse for BCOA resistance and 76 crosses were made to transfer BCOA resistance into malting barley. (Objective 1A) Twelve genetic populations were increased in the greenhouse for future evaluation to determine genetic diversity in 10 sources of greenbug (GB) resistance. (Objective 2B) We have screened over 10,000 accessions and identified a few resistant plants from PI 595379. Bioassays indicated that the reselection line PI 595379-1 was highly resistant to greenbug biotypes B, C, E, and I. Using a set of F2:3 lines derived from a cross between PI 595379-1 and PI 243735, we mapped the greenbug resistance gene in PI 595379-1, designated Gb595379, to the long arm of chromosome 7D, where Gb3 and Gb7 were harbored. Given that greenbug biotype B was virulent to Gb3 and Gb7, Gb595379 is a new gene/allele. We further made two crosses to derive segregating populations for allelism tests aimed at determining the relationships of Gb595379 with Gb3 and Gb7. We expect to complete allelism tests in 2019. (Objective 2C) To develop sugarcane resistance in sorghum, 46 crosses have been made this year using the newly identified sources of resistance to sugarcane aphids. The resulted hybrid seeds were planted for developing into subsequent breeding populations for the further evaluation of transferring the resistance trait to adapted sorghum materials. (Objective 3A) 4,150 barley heads selected from 84 winter feed barley F3 populations in the field were screened in the greenhouse to both Russian wheat aphid (RWA) and GB and selected seedlings increased in the greenhouse for future field evaluation (Obj. 3B). A total of 150 heads from 2 elite hulless winter barley lines were screened to both RWA and GB to determine purity prior to germplasm release. Field evaluation was conducted of 42 hulless barley germplasm lines prior to germplasm release. (Objective 3A) 260 crosses were made and 95 F1 populations were increased to the F2 in the greenhouse towards the development of winter malting barley adapted to the Great Plains. In addition, 106 elite lines with potential as parents of winter malting barley were evaluated in the field for adaptation to the Great Plains. (Objective 3B)
1. Identification of a novel leaf rust resistance gene in wheat. Leaf rust, caused by Puccinia triticina (Pt), is the most common and widespread rust disease in wheat. Pt races evolve rapidly in the Southern Great Plains and leaf rust resistance genes often lose their effectiveness shortly after their deployment in wheat production. ARS scientists at Stillwater, Oklahoma discovered a novel leaf rust resistance gene, designated Lr470121, in Croatian breeding line PI 470121, and mapped Lr470121 to the terminal region of chromosome 2AS. Lr470121 confers a broad spectrum of resistance to Pt races in the U.S. and can be widely used in U.S. wheat breeding programs to enhance leaf rust resistance.
2. Molecular characterization of a new powdery mildew resistance gene in wheat. Powdery mildew poses a persistent threat to wheat production, and identification of powdery mildew resistance genes is essential for sustainable improvement of wheat disease resistance. ARS scientists at Stillwater, Oklahoma discovered a novel powdery mildew resistance, designated Pm628024, in Iranian landrace PI 628024. Pm628024 was mapped to the long arm of chromosome 2B, and molecular markers closely linked to Pm628024 were developed. Pm628024 confers high resistance to a considerable number of representative Bgt isolates collected from different regions of the USA, and molecular markers developed in this study can facilitate its introgression into adapted germplasm.
3. Development of molecular markers for selecting sugarcane aphid resistance trait in sorghum. Molecular (i.e. DNA) markers have enormous potential to improve the efficiency and precision of crop breeding via marker-assisted selection. ARS scientists in Stillwater, Oklahoma, developed precise DNA markers that can be used for identifying the genetic components for sugarcane aphid resistance in sorghum, and for selecting promise breeding materials or lines that carrying such a resistance trait. The DNA markers developed according to the cloned genes are right on the target gene, thus are reliable for precise selection of the trait at early stage of cultivar and hybrid development. The genes and DNA markers discovered at ARS will help public research programs and sorghum seed companies in effective breeding sorghum for sugarcane aphid resistance.
4. A better understanding of molecular responses of host plants to aphid pests. Aphid pests have been a severe threat to cereal crops in the U.S. Host plant resistance has proved to be the most economical and environmentally sound methods to control such pests to reduce the negative impact of those damaging aphids. To better understand the genetic mechanisms of host plant defense, transcriptional profiles of the sorghum plants in response to greenbug attack were developed by ARS scientist in Stillwater, Oklahoma using the next-generation sequencing technology, which led to the identification of hundreds of differentially expressed genes. Analysis of those differentially expressed genes suggests that plant resistance is not only regulated by jasmonic acid signaling pathway but also through a network of the signaling cascades including ethylene. It also appeared that certain defense pathways in host plants switched on as a defensive response to greenbug infestation. This research has led to new insights to the molecular basis and genetic mechanisms of the host plant defense against aphid pests and offers the new tool for insect resistance breeding in sorghum.
Mornhinweg, D.W., Armstrong, J.S., Carver, B.F., Springer, T.L. 2018. Registration of STARS 1501B and STARS 1502B barley germplasm with Rsg2 resistance to greenbug. Journal of Plant Registrations. 12(2):237-240. https://doi.org/10.3198/jpr2017.09.0061crg.