Location: National Peanut Research Laboratory
2023 Annual Report
Objectives
Objective 1
Identify and integrate beneficial genes from disease-resistant peanut and wild peanut sources into genetically stable peanut germplasm.
Sub-objective 1A.
Screening for aflatoxin accumulation under laboratory conditions.
Sub-objective 1B.
Identification of disease resistance-associated genes and plant defense mechanisms.
Sub-objective 1C.
Integration of beneficial alleles from wild diploid Arachis species into genetically stable peanut germplasm.
Objective 2
Define specific defensive roles of peanut phytoalexins against Aspergillus spp. and other fungal pathogens and identify their genetic sources as potential resistance to fungal diseases and aflatoxin contamination.
Sub-objective 2A.
Determination of phytoalexin profiles in experimental and field seeds and search for new phytoalexins.
Sub-objective 2B.
Study of potential involvement of pegs in contamination of peanut seeds with aflatoxins.
Approach
Peanut (Arachis hypogaea) is one of the major food crops in the world. Most of the pathogens that attack peanuts are of fungal origin and are evident etiological factors of over 40 economically important peanut diseases. Aspergillus flavus and A. parasiticus are opportunistic fungal parasites that often invade peanut seeds and produce carcinogenic aflatoxins. Contamination of peanuts with aflatoxins is an important food safety issue and threatens the competitiveness of the United States agriculture in the world market. Aflatoxin monitoring and reprocessing of contaminated peanuts is a passive and costly practice to prevent aflatoxins from entering the food chain. Current peanut cultivars often demonstrate limited resistance to fungal pathogens. Therefore, wild peanut species have received substantial consideration as sources of disease resistance because the narrow genetic base of cultivated peanuts cannot provide the necessary levels of resistance to defend the peanut plant. A prospective approach to reduce disease pressure and aflatoxin contamination is to develop resistant peanut cultivars through introgression of beneficial genes and alleles from wild peanut species into elite cultivars. To achieve this goal, the first objective to identify and integrate beneficial genes from disease-resistant peanut and wild peanut sources into genetically stable peanut germplasm will use this approach. In conjunction with this objective, another promising strategy/second objective is to define specific defensive roles of peanut phytoalexins against Aspergillus spp. and other fungal pathogens and identify their genetic sources as potential resistance to fungal diseases and aflatoxin contamination. Both approaches will generate new knowledge on the mechanisms of peanut resistance to fungal invasion and a faster release of enhanced germplasm and cultivars. The ultimate goal of this project is to reduce peanut disease load and to develop improved germplasm. The beneficiaries of the successful accomplishment of the project goal are breeders and all segments of the peanut industry.
Progress Report
Significant progress has been made in achieving Objective 1. A total of 330 samples from 12 accessions of six wild diploid Arachis species were evaluated for resistance to aflatoxin accumulation. Seven to ten-day-old plants grown from half seed were transplanted into pots and grown to full maturity in screenhouses. Genetic characterization of wild peanut sources and breeding materials was completed using high throughput SNP genotyping. For generation advancement of two segregating mapping population, 480 individual plants and 196 plots were planted in screenhouses at the ARS location in Dawson, Georgia and at Bolton farm, Georgia, respectively. A second year of interspecific crosses involving two A-genome and three non-A genome species was completed. The F1 hybrids from these crosses were validated with molecular markers. Synthetic allotetraploids, S0 and S1 generations from one of these hybrids were obtained by chromosome doubling and subsequently validated by flow cytometry and single nucleotide polymorphism markers. The project also contributed to publication of the first draft genome of Thecaphora frezii, causal agent of peanut smut disease, and Cercosporidium personatum, causal agent of late leaf spot disease of peanut.
Substantial progress has been made to achieve Objective 2. Determination of phytoalexin profiles in experimental and field seeds was performed simultaneously with the quantitative analyses of aflatoxins. Over 500 samples were analyzed for peanut phytoalexins, representing 8-12 major, structurally related, but exhibiting different biological activity compounds. Correlation of production of specific phytoalexins with aflatoxin contamination was determined and used as an additional tool for the evaluation of resistance of wild Arachis species to aflatoxin production by Aspergillus species.
Study of potential involvement of pegs in contamination of peanut seeds with aflatoxins was started with a thorough design of the greenhouse experiments. Environmentally safe screenhouse experimental setup for the study of peg-fungus interaction in planta was developed and tested in a series of limited scale experiments that used a color mutant fungus (A. parasiticus NRRL 6111, norsolorinic acid producer) at this stage. Peanuts were grown in round shallow pods. Pegs growing outside the main pot were pointed into smaller pots with common field soil. Escape of fungal spores from small pods protected by silica gel was confirmed.
First year research on real-time aflatoxin accumulation in association with production of defensive stilbenoid phytoalexins was conducted on green peanut pods using the developed analytical procedures described above. The results are being processed and will be published in a peer-reviewed journal.
Accomplishments
1. Generation of peanut genetic resources and molecular tools to facilitate the transfer of desirable traits from wild relatives into cultivated peanut. Interspecific hybrids are necessary to overcome the cross-incompatibility between wild and cultivated peanuts. The development of new plant material through hybridization between peanut species, followed by chromosome doubling is key to the transfer of desirable traits from wild resistant plants. For the first time, ARS scientists at Dawson, Georgia, generated, validated, and characterized interspecific hybrids between two wild diploid species followed by whole-genome duplication. This plant material, also called synthetic allotetraploids or amphidiploids, carries favorable genes to increase disease resistance and to reduce aflatoxin accumulation. Along with the development of molecular markers, this new genetic resource is ready to be used as a potential source for next generation breeding with existing peanut cultivars to introduce favorable genes with desirable traits. It is anticipated that peanut breeders will incorporate this new germplasm into their breeding programs. A medium throughput method for the simultaneous quantification of aflatoxins and peanut phytoalexins in ¼ cuts of single seeds has been developed, validated, and used in the analyses of hundreds of samples. The scientific manuscript was invited by the Editorial Board for publication in the peer reviewed Journal of Visualized Experiments in 2023.
Review Publications
Rosso, M.H., De Blas, F.J., Massa, A.N., Oddino, C., Giordano, D.F., Arias De Ares, R.S., Soave, J.H., Soave, S.J., Butteler, M.I., Bressano, M. 2023. Two QTLs govern the resistance to Sclerotinia minor in an interspecific peanut RIL population. Crop Science. 63:613-621. https://doi.org/10.1002/csc2.20875.
Arias De Ares, R.S., Dobbs, J., Stewart, J., Cantonwine, E., Orner, V.A., Sobolev, V., Lamb, M.C., Massa, A.N. 2023. First draft genome and transcriptome of Cercosporidium personatum, causal agent of late leaf spot disease of peanut. BMC Research Notes. 16:58. https://doi.org/10.1186/s13104-023-06331-0.
Arias De Ares, R.S., Conforto, C., Orner, V.A., Soave, J., Massa, A.N., Lamb, M.C., Bernardi-Lima, N., Rago, A. 2023. First draft genome of Thecaphora frezii, causal agent of peanut smut disease. BMC Genomics. 24:9. https://doi.org/10.1186/s12863-023-01113-w.
