Location: Corn Host Plant Resistance Research
2023 Annual Report
Objectives
1. Identify new sources of maize germplasm with resistance to Aspergillus (A.) flavus infection and aflatoxin accumulation.
1.A: Screen maize germplasm obtained from the GEM project, CIMMYT, and other sources for resistance to aflatoxin accumulation to identify new and potentially useful sources of resistance.
1.B: Determine optimal combinations of A. flavus and A. parasiticus isolates for screening maize germplasm for resistance to aflatoxin accumulation under different environmental conditions.
2. Identify new sources of maize germplasm with resistance to fall armyworm, southwestern corn borer, and corn earworm.
2.A: Screen maize germplasm from CIMMYT, the GEM project, and other sources for resistance to leaf feeding by fall armyworm and southwestern corn borer.
2.B: Screen maize germplasm for resistance to ear feeding by fall armyworm, southwestern corn borer, and corn earworm.
3. Develop and characterize genetic mapping populations, identify and elucidate functions of genes associated with resistance, and develop molecular markers for enhancing maize germplasm resistance to A. flavus/aflatoxin and insects.
3.A: Identify genetic loci associated with resistance to A. flavus infection and aflatoxin accumulation via analysis of linkage and association mapping and sequencing data.
3.B: Identify genetic loci associated with resistance to lepidopteran insect feeding via analysis of linkage and association mapping, pathway analysis, or sequencing data.
3.C: Identify genes and elucidate functions of genes associated with resistance to A. flavus/aflatoxin and insects.
4. Develop and release maize germplasm with resistance to Aspergillus flavus infection, aflatoxin accumulation, and insect damage.
4.A: Develop and release maize germplasm lines with resistance to A. flavus infection and aflatoxin accumulation using conventional breeding methods.
4.B: Develop and release lines with resistance to feeding by southwestern corn borer and fall armyworm using conventional breeding methods.
4.C: Develop lines with resistance to A. flavus infection, aflatoxin accumulation, and insects using molecular markers and release together with marker information.
Approach
Objective 1: Identify new sources of maize germplasm with resistance to Aspergillus (A.) flavus infection and aflatoxin accumulation. Screen maize germplasm obtained from the Germplasm Enhancement of Maize project (GEM) project, International Center for Maize and Wheat Improvement (CIMMYT), and other sources for resistance to aflatoxin accumulation to identify new and potentially useful sources of resistance. Determine optimal combinations of A. flavus and A. parasiticus isolates for screening maize germplasm for resistance to aflatoxin accumulation under different environmental conditions.
Objective 2: Identify new sources of maize germplasm with resistance to fall armyworm, southwestern corn borer, and corn earworm. Screen maize germplasm from CIMMYT, the GEM project, and other sources for resistance to leaf feeding by fall armyworm and southwestern corn borer. Screen maize germplasm for resistance to ear feeding by fall armyworm, southwestern corn borer, and corn earworm.
Objective 3: Develop and characterize genetic mapping populations, identify and elucidate functions of genes associated with resistance, and develop molecular markers for enhancing maize germplasm with resistance to flavus/aflatoxin and insects. Identify genetic loci associated with resistance to A. flavus infection and aflatoxin accumulation via analysis of linkage and association mapping and sequencing data. Identify genetic loci associated with resistance to lepidopteran insect feeding via analysis of linkage and association mapping, pathway analysis, or sequencing data. Identify genes and elucidate functions of genes associated with resistance to A. flavus/aflatoxin and insects.
Objective 4: Develop and release maize germplasm with resistance to A. flavus infection, aflatoxin accumulation, and insect damage. Develop and release maize germplasm lines with resistance to A. flavus infection and aflatoxin accumulation using conventional breeding methods, and develop and release lines with resistance to feeding by southwestern corn borer and fall armyworm using conventional breeding methods. Develop lines with resistance to A. flavus infection, aflatoxin accumulation, and insects using molecular markers and release together with marker information.
Progress Report
Obj. 1: In 2022 and 2023, we are evaluating GEM lines for the USDA GEM project. These lines are evaluated for response to inoculation by A. flavus and F. verticillioides. Results are published at the GEM cooperators’ meeting at ASTA. In 2023, we are evaluating a set of 120 segregating lines donated to the GEM project by commercial collaborators (Novasem, Bayer Crop Science). The Bayer lines were derived from crosses between elite Corn Belt inbreds and Latin American accessions derived from 17 races of maize. The Novasem lines were derived from crosses between GEM lines and commercial inbreds from Mexico. CHPRRU acquired 500 of these lines and will evaluate the material annually. Lines that perform well will be incorporated into our breeding program. A set of 95 inbreds recently developed by CIMMYT was planted in 2022 and evaluated for response to A. flavus and F. verticillioides. In 2023, we are conducting the second year of a multi-location trial evaluating 80 inbred lines. This trial includes every publically available inbred reported as resistant to aflatoxin accumulation, additional lines that showed resistance to either A. flavus or F. verticillioides in previous trials, and ex-PVPs with agronomic potential as recurrent parents in breeding crosses. In all of these trials we evaluate germplasm for ear-rot ratings after inoculation with A. flavus and with F. verticillioides, mycotoxin accumulation (aflatoxin and fumonisin), general agronomic performance, insect ear-feeding, and response to heat and drought stress when conditions allow. 2022 was an especially hot and dry year, allowing for valuable evaluation of reaction to heat and drought stress. 2023 has been especially hot, but there has been above average rain during crop development.
Over the 5-year life of this project, germplasm has been screened annually from all of the sources just described. In the first 2-3 years the evaluations focused almost exclusively on aflatoxin accumulation. In the final 3 years of the project, the phenotyping was expanded to include F. verticillioides inoculations in addition to A. flavus. During this same period the phenotyping was also expanded to evaluate agronomic traits plus insect ear-feeding and heat and drought stress when conditions allow. The rationale for evaluating these other biotic and abiotic stresses is that susceptibility to ear-rots and mycotoxin accumulation is a complex trait and heavily influenced by these environmental factors. The rationale for the increased emphasis on agronomic traits is that all known sources of resistance to aflatoxin accumulation possess undesirable traits (late maturity, low combining ability for yield, tall plants, high rates of lodging, lack of fit in standard heterotic groups). These traits make the lines difficult for industry stakeholders to utilize, and they make it difficult for basic researchers to study the interaction between maize and A. flavus. The goal is to fully characterize germplasm during our screening trials, so that breeding crosses can address agronomic weaknesses of the germplasm and combine the traits underlying resistance to mycotoxin accumulation in maize (disease resistance plus resistance to insect ear-feeding and tolerance to heat and drought stress).
Obj. 2: More than 100 germplasm accessions from CIMMYT were screened in replicated field trials for resistance to fall armyworm (FAW) leaf feeding. A few accessions exhibited moderately high levels of resistance but not as high as the resistant checks developed by CHPRRU. These newly identified lines should be useful in efforts to produce corn hybrids with resistance to FAW damage. We worked with our collaborations with CIMMYT in Kenya to identify germplasm with FAW resistance that could be used in developing hybrids to fight FAW invasion in Africa. Among those CIMMYT lines, CML67, CML71, CML122, CML139, CML370, CML371, and CML484, exhibited resistance in multi-year evaluations comparable to some of the lines that we developed and released. Lines from the GEM project were also evaluated for resistance to FAW damage. Twenty GEM lines were evaluated for resistance to FAW leaf feeding damage in 2021 and 2022; GEMN-0135, GEMN-0292, GEMN-0270, and GEMS-0294 sustained the least damage in 2021. GEMS-0294, GEMN-0292, GEMS-0270, and GEMN-0135 sustained the least damage in 2022. These lines exhibited only moderate resistance to FAW damage in our trials, but they may be useful in breeding crosses with some of our highly resistant lines that lack other desirable agronomic qualities.
In addition to evaluations for FAW leaf feeding damage to whorl stage corn, we initiated research to identify resistance to ear damage. Ear damage was evaluated following infestation of developing ears with neonate or third instar FAW or SWCB larvae at anthesis. Damage to husks, ears, ear shanks, and stalks was and is continuously being assessed. We conducted a study to compare damage sustained by single cross hybrids infested with FAW or SWCB larvae at either the mid-whorl stage of growth or at anthesis. A study was conducted in 2022 and is continued in 2023 to evaluate a multiple-infestation strategy for identifying resistance to FAW at various growth stages throughout the growing season. In 2023 we initiated a study to compare infestation rates of FAW neonates on inbred lines to distinguish susceptible vs resistant genotypes for leaf feeding damage.
We evaluated 20 hybrids derived from crosses between maize and teosinte or maize and Trypsacum for resistance to FAW leaf feeding damage. Although none of these hybrids exhibited levels of resistance as high as those of the hybrids produced from lines developed and released by CHPRRU, some of them exhibited intermediate resistance and may provide new sources of resistance.
Obj 3: During the 5-years of the project, CHPRRU published 4 articles reporting the results of 5 bi-parental QTL mapping studies focused on QTL for resistance to aflatoxin accumulation. These studies used Mp715, Mp717, Mp719, and CML69 as the resistant parents. A marker-assisted selection (MAS) project was initiated to introgress the most promising QTL identified in the Mp715 study. The MAS has progressed through 4 generations of backcrossing and two generations of selfing to derive F3BC4 near-isogenic lines. These lines will be evaluated during the next project plan to assess the utility of the QTL for practical breeding through MAS.
Mp705 served as the resistant parent in a bi-parental QTL mapping population evaluated for leaf feeding damage. Significant QTL were identified on Chromosomes 4 and 9 that accounted for 35% of the phenotypic variance associated with FAW leaf feeding damage. Results of this study are now being used in MAS to develop maize germplasm lines with resistance to FAW with desirable agronomic qualities.
A diverse set of 300 germplasm lines was evaluated for resistance to FAW damage as part of a 3-year GWAS for resistance to FAW in replicated trials in 2019, 2020, and 2021. This study provides additional information on molecular markers that can be used in breeding for resistance. Germplasm lines that showed promise as sources of resistance to FAW were further evaluated in 2022 and 2023.
A gene expression study for FAW was initiated. In 2021, 2022, and 2023, leaf tissues from FAW infested and non-infested corn plants at different growth stages from resistant and susceptible corn germplasm lines were collected following infestation of plants at the mid-whorl stage of growth. This tissue will be processed to conduct gene expression studies and to identify metabolites associated with resistance to FAW leaf damage.
Obj. 4: Across 2022 and 2023, 162 segregating populations were advanced through selfing and backcrossing. These populations were derived from crosses between three CHPRRU aflatoxin accumulation resistant lines used as donors (Mp715, Mp717, and Mp719) and ex-PVPs used as recurrent parents. While these populations were advanced, the F1 of each cross was evaluated for agronomics, yield, and resistance to ear-rot and aflatoxin accumulation in multi-location trials in 2022 and 2023. These F1 evaluations will be used to determine the heterotic pattern of each donor line. At the same time, all of the donor and recurrent parents were evaluated per se in multi-location trials in 2022 and 2023. These F1 and inbred evaluations will allow for selections to be made between breeding populations. In 2023, the segregating breeding lines are being inoculated with A. flavus allowing for selections to be made within populations. The selected lines will be planted ear-to-row in 2024 and advanced through selfing. Line development was also initiated in 2023 in the segregating material obtained from Bayer and Novasem described earlier. Their 2023 evaluations will allow for selections between lines, and further selections will take place within selected lines. Additional breeding crosses were made in 2023 summer nursery. Many of these crosses focused on lines that showed decreased ear-feeding in 2022.
Over the course of the 5-year project, existing germplasm lines were fully characterized as were potential recurrent parents. Second-cycle lines are being derived that combine disease resistance with improved agronomic characteristics. New germplasm sources were also introduced into the breeding program, primarily from the GEM project and GEM collaborators, and line development was initiated in this new germplasm while it is being evaluated.
We incorporated germplasm identified as potential source of resistance to FAW into our breeding program. In 2022 and 2023 a study was conducted to evaluate the potential benefits of resistance to FAW and SWCB feeding in reducing aflatoxin accumulation in corn hybrids with varying levels of resistance to these insects. We initiated near-isogenic lines (NILs) for a single QTL associated with resistance to FAW on Chromosome 9 that was identified in Mp704 and Mp705.
Accomplishments
Review Publications
Warburton, M.L., Woolfolk, S.W., Smith, J.S., Hawkins, L.K., Castano-Duque, L.M., Lebar, M.D., Williams, W.P. 2023. Genes and genetic mechanisms contributing to fall armyworm resistance in maize. The Plant Genome. 16(2):e20311. https://doi.org/10.1002/tpg2.20311.
Warburton, M.L., Jeffers, D., Smith, J.S., Scapim, C., Uhdre, R., Thrash, A., Williams, W.P. 2022. Comparative analysis of multiple GWAS results identifies metabolic pathways associated with resistance to A. flavus infection and aflatoxin accumulation in maize. Toxins. 14(11). Article 738. https://doi.org/10.3390/toxins14110738.
Tiwari, A., Williams, W.P., Shan, X. 2022. MatGel: A MATLAB program for quantitative analysis of 2D polyacrylamide electrophoresis (2D-PAGE) protein gel images. MethodsX. 9(101930):1-5. https://doi.org/10.1016/j.mex.2022.101930.
Abel, C.A., Frei, U.K., Woolfolk, S.W. 2023. Evaluating founding landraces of maize population PI 674097 for resistance to leaf-feeding Spodoptera frugiperda. Southwestern Entomologist. 48(1):83-88. https://doi.org/10.3958/059.048.0108.
Woolfolk, S.W., Matthews Jr, G.A., Williams, W.P. 2023. Evaluation of germplasm lines of maize for resistance to fall armyworm and southwestern corn borer leaf-feeding damage. Southwestern Entomologist. 48(2):347-352. https://doi.org/10.3958/059.048.0208.
