Location: National Peanut Research Laboratory
2021 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. New genetic and genomic resources will be developed to identify mechanisms of pre-harvest aflatoxin resistance in selected wild peanut germplasm. Understanding these mechanisms is fundamental to efficiently introgress favorable alleles from wild Arachis species to reduce aflatoxin in elite peanut cultivars. 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
Significant progress has been made by ARS scientists at Dawson, Georgia toward Objective 1; a better understanding of the fungus-host interactions and the influence of stilbenoid phytoalexins on the aflatoxin production by Aspergillus. ARS scientists at Dawson, Georgia demonstrated that several peanut phytoalexins are capable of completely blocking, or significantly reducing aflatoxin production by toxigenic Aspergillus species in feeding experiments in vitro. ARS scientists at Dawson, Georgia directly confirmed this finding in a series of experiments with selected phytoalexins, all of which downregulated genes in the fungal Aflatoxin-Biosynthesis Gene Cluster. 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. Also, substantial progress has been made in the simultaneous analytical determination of major presumably defensive compounds of interest. Data were obtained through the use of a multi-platform approach, including ultra-performance liquid chromatography, mass spectrometry, and nuclear magnetic resonance analyses, and lead to the discovery of new peanut bioactive constituents whose presence in fungal-challenged peanut seeds has been evidenced by past research. This approach allowed ARS scientists at Dawson, Georgia to discover unique properties of a prenylated polyphenol and to elucidate its structure. It is capable of forming a water-insoluble jell-like polymer within a fraction of a second. ARS scientists at Dawson, Georgia suggest that such a property may be a part of the seed defense mechanism. The mechanism of this phenomenon needs further study since this compound may act as a seed wound-sealing gel preventing fungal invasion. The presence of substantial concentrations of this compound could be considered a useful trait for breeding tolerant peanut lines. Significant progress has been made by ARS scientists at Dawson, Georgia toward Objective 2, prospective peanut accessions with resistance to early leaf spot (caused by Passalora arachidicola), late leaf spot (caused by Nothopassalora personata), and aflatoxin accumulation were incorporated into the pre-breeding pipeline developed at ARS Dawson, Georgia. This includes the development of inter-specific F1 hybrids and amphidiploids, evaluation of pre-harvest aflatoxin resistance subjected to late-season heat and drought stress, high-throughput genotyping, transcriptome analysis, and marker development. Transcriptome sequence and analysis of Aspergillus-challenged seeds was completed by for another set of 24 aflatoxin-resistant and susceptible peanut genotypes. Combined with genome-wide SNP genotyping data, levels of aflatoxin accumulation, and phytoalexin response, this information is being used to advance ongoing trait introgression, genetic studies, marker development, and gene discovery. Research progress were presented at the 53rd Virtual Annual Meeting of the American Peanut Research and Education Society on July 12-16, 2021.
Accomplishments
1. Inhibition of aflatoxin formation in Aspergillus species by peanut stilbenoids in the course of peanut-fungus interaction. ARS scientists at Dawson, Georgia, found that Aspergillus flavus, common fungal species, are widely distributed in nature and, under certain conditions, invade preharvest peanut seeds and produce highly carcinogenic aflatoxins that pose a threat to human and animal health and are responsible for the multimillion dollar loses in all segments of the peanut industry. Under favorable conditions, the fungus-challenged peanut seeds produce defensive phytoalexins, structurally related stilbenoids, capable of retarding fungal vitality. Understanding and potential manipulation of this mechanism may help to resolve the preharvest aflatoxin contamination problem. ARS scientists at Dawson, Georgia, revealed for the first time that in the course of peanut-fungus interaction, aflatoxin formation was completely suppressed by peanut phytoalexins in Aspergillus strains tested. The results were published in a peer-reviewed journal. This discovery will lead to new genetic approaches at ARS Dawson, Georgia, to control aflatoxin production in toxigenic fungi based on the phytoalexin profiles with particular attention to the known aflatoxin-blocking phytoalexins.
2. Identification of candidate genes, genomic regions, and single nucleotide polymorphism (SNP) markers associated with disease resistance in peanuts. ARS scientists at Dawson, Georgia, found a high-resolution RNA sequencing technology used to analyze the interaction between peanut seeds and the aflatoxigenic fungus Aspergillus captured the transcriptional changes that occur in both seed and pathogen simultaneously. A gene co-expression network analysis was further conducted to identify modules of genes associated with the observed transcriptional responses. Phytoalexin biosynthesis genes, including those encoding isoflavonoids and stilbenes, were among the peanut genes that differentially responded to Aspergillus infection. These genes are potential target for aflatoxin resistance. An efficient and cost-effective single nucleotide polymorphisms genotyping approach provided statistically significant evidence that the United States peanut germplasm collection contains at least one putative smut-resistant germplasm line. The identification of smut-resistant germplasm is critical given the potential risk of a global spread. ARS scientists in collaboration with other peanut researchers, genotyped and characterized a recombinant inbred line (RIL) population derived from a cross between an elite line of Arachis hypogaea, susceptible to peanut smut (Thecaphora frezzi), and a resistant synthetic amphidiploid obtained from crosses between A. batizocoi x (A. correntina x A. cardenasii). A genetic map was constructed with 1819 SNP markers spanning a genetic distance of 2531.81 centiMorgans. The quantitative trait loci analysis (QTL) identified two QTLs located on chromosomes A08 and A02/B02. These two studies were published in peer reviewed journals.
Review Publications
Massa, A.N., Bressano, M., Soave, J.H., Buteler, M.I., Seijo, G., Sobolev, V., Orner, V.A., Oddino, C., Soave, S.J., Faustinelli, P.C., De Blas, F.J., Lamb, M.C., Arias De Ares, R.S. 2021. Genotyping tools and resources to assess peanut germplasm: smut-resistant landraces as a case study. PeerJ. https://doi.org/10.7717/peerj.10581.
Gell, R.M., Horn, B.W., Carbone, I. 2020. Genetic map and heritability of Aspergillus flavus. Fungal Genetics and Biology. (144):103478. https://doi.org/10.1016/j.fgb.2020.103478.
Power, I.L., Faustinelli, P.C., Orner, V.A., Sobolev, V., Arias De Ares, R.S. 2020. Analysis of small RNA populations generated in peanut leaves after exogenous application of dsRNA and dsDNA targeting aflatoxin synthesis genes. PLoS One. 10:13820. https://doi.org/10.1038/s41598-020-70618-6.
De Blas, F.J., Bruno, C.I., Arias De Ares, R.S., Ballen-Taborda, C., Mamani, E., Oddino, C., Rosso, M., Costero, B., Bressano, M., Soave, J.H., Soave, S.J., Buteler, M.I., Seijo, G.J., Massa, A.N. 2021. Genetic mapping and QTL analysis for peanut smut resistance. Biomed Central (BMC) Plant Biology. 21:312. https://doi.org/10.1186/s12870-021-03023-4.
