Location: National Peanut Research Laboratory
2018 Annual Report
Objectives
1. Evaluate pathogen-host interactions, including enzyme production by host and pathogen during aflatoxin accumulation, and identify potential resistance genes for aflatoxin control.
2. Mine the diploid Arachis germplasm collections in peanut to identify resistance to various pathogens, characterize novel sources of resistance to important fungal pathogens, and introgress genes into cultivated peanuts.
2a. Screen wild peanut germplasm collection to identify useful germplsm for resistance to important fungal pathogens (e.g. Aspergillus, Cercospora, Cercosporidium, and Sclerotinia spp.).
2b. Transfer economically important genetic traits from wild Arachis species to cultivated peanuts.
3. Develop integrated strategies for management of fungus-associated peanut diseases.
Approach
Mycotoxins are toxic secondary fungal metabolites. Contamination of crops with mycotoxins, particularly aflatoxins, is an important food safety issue and threatens the competitiveness of United States agriculture in the world market. Aflatoxins are strong carcinogens produced in crops by the fungus Aspergillus (A.) flavus. Contamination of crops with aflatoxins is an important food safety issue. The purpose of this project is to develop effective integrated strategies for controlling mycotoxin accumulation and fungal diseases that cause yield losses in peanut. One strategy for reducing aflatoxins is to prevent Aspergillus from invading crops. To achieve this goal, the first objective will evaluate fungus-plant interactions, gene expression and chemical profiling of host and pathogen during aflatoxin accumulation. Another strategy for aflatoxin reduction is to prevent its formation by the fungus. This strategy is based on data from our recent research showing that selected peanut stilbenoids significantly reduce or completely block aflatoxin biosynthesis in A. flavus. The second objective will explore wild Arachis germplasm collections to identify resistance to A. flavus, determine and characterize novel sources of resistance to important fungal pathogens, including species causing early and late leaf spot and white mold diseases, and introgress genes into cultivated peanuts. The third objective, which is related to the first and second objectives, is to combine knowledge and methodology obtained from these objectives on the reduction of aflatoxin in peanut. The ultimate goal of this objective is to establish new peanut germplasm with increased resistance to toxigenic A. flavus.
Progress Report
In 2018, a new simple, sensitive, and accurate method of peanut phytoalexin analysis was developed. The method uses a charged aerosol detector, which makes the quantification of phytoalexins possible without the use of deficient/unavailable phytoalexin standards. Phytoalexin profiles now can be directly compared; it saves time and effort to search for new potent phytoalexins in wild Arachis species. In vitro experiments, two presumably new phytoalexins were identified in various wild species. Structural elucidation of these phytoalexins will be performed after substantial multiplication of the seeds in the field. The search for new phytoalexins was completed. Wild Arachis species were demonstrated to biosynthesize the same major stilbenoid-type phytoalexins as the popular peanut cultivars. Earlier, within this project, ARS scientists at Dawson, Georgia demonstrated in vivo experiments that major peanut stilbenoid phytoalexins are capable of completely blocking, or significantly reducing aflatoxin production by important toxigenic Aspergillus fungal species. This discovery led to new genetic approaches at the Laboratory in Dawson, Georgia to control aflatoxin production in toxigenic fungi. Based on the phytoalexin profiles, with particular attention to the known aflatoxin-blocking phytoalexins, 50 fungal-resistant and susceptible wild peanut accessions were identified and used by the geneticist to select potentially useful peanut accessions.
During 2017 a total of 250 accessions were grown at ARS research farms (800 plots) in Dawson, Georgia, and evaluated for resistance to peanut diseases, including early leaf spot, late leaf spot, and Tomato Spotted Wilt Virus. In collaboration with university peanut researchers, 500 additional plots were evaluated in North Carolina. Resistant genotypes were identified and used for interspecific crosses. A second year of field evaluations are being conducted and will be completed in 2018. Genetic characterization of wild Arachis species was performed with a high throughput single nucleotide polymorphism (SNP) array. The genotyping of twenty-seven species of section Arachis (168 accessions) was completed with more than seven thousand high-confidence single nucleotide polymorphism markers. This information provides the molecular basis for genetic introgression of wild Arachis germplasm. Research progresses was presented at the International Plant and Animal Genome Meeting, San Diego, CA, Jan 13-17, 2018, and the 50th Annual Meeting of the American Peanut Research and Education Society, Williamsburg, VA, July 10-12, 2018.
Accomplishments
1. Systematic study of wild peanut defensive phytoalexin chemical profiles and their use for the identification of resistant peanut genotypes. Peanuts accumulate over 20 defensive phytoalexins in response to the presence of soil fungal pathogens. ARS researchers at Dawson, Georgia, demonstrated that several peanut phytoalexins are capable of completely blocking, or significantly reducing aflatoxin production by toxigenic Aspergillus species. This discovery allowed ARS scientists to develop new approaches and strategies for aflatoxin control in toxigenic fungi by identifying and introgressing genes responsible for the production of phytoalexins from resistant wild species into cultivars. The first important stage of the research has been completed: based on the phytoalexin profiles, with particular attention to the known aflatoxin-blocking phytoalexins, 50 fungal-resistant and susceptible wild peanut accessions were identified. The data was conveyed to the project geneticist, who has combined the data with the genetic information obtained throughout the years 2017-2018 to select potentially useful peanut accessions. These new approaches are promising and are expected to have significant impact on conventional peanut breeding programs nationwide. Gene introgression, offered by the Dawson scientists, opposed to genetic modification, is a preferable choice for the peanut industry since non-genetically-modified peanuts are in great demand by the peanut consumers.
2. Identification of new genetic sources of resistance to peanut pathogens. Contamination of peanuts with mycotoxins, particularly aflatoxins, secondary metabolites of common soil fungi, is an important economic and food safety issue that threatens the competitiveness of United States agriculture in the world market. To date, no aflatoxin resistant cultivars have been developed by conventional breeding methods, and given the narrow genetic base of cultivated peanut, wild species represent the primary source of genetic diversity for peanut improvement. ARS researchers at Dawson, Georgia, used an integrated approach that combines chemical profiles and genetic evaluations to identify potentially useful wild peanut germplasm. This approach has led the scientists to identify novel sources of resistance to aflatoxin accumulation, which represents the foundation for cultivar development; a paper has been accepted for publication. Another outcome of this research is the genetic characterization of wild germplasm of section Arachis, which covers over 90 percent of the accessions of section Arachis currently available at the United States peanut germplasm collection in Griffin, Georgia. Section Arachis is of great interest because only species within this section can hybridize with cultivated peanut. Consequently the information generated by this work is expected to have significant impact on pre-breeding and breeding programs worldwide.
