The role of Fusarium mycotoxins in seedling infection of soybeans, wheat and maize

Bruns, Tracy
Major Professor
Gary P. Munkvold
Committee Member
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Plant Pathology and Microbiology

Fusarium graminearum and Fusarium verticillioides are fungal plant pathogens that can cause yield losses, reductions in grain quality, and produce mycotoxins that can cause serious diseases in animals and humans when contaminated grain is consumed. Both pathogens can infect and cause disease in seedlings. F. graminearum infects cereals and other crops and produces mycotoxins including deoxynivalenol (DON), which can act as a virulence factor for Gibberella ear rot in maize and head blight of wheat. F. verticillioides infects maize and produces fumonisins, a group of mycotoxins with phytotoxic properties. Previous research into the role of these mycotoxins in seedling diseases has been conflicting.

To assess the roles of deoxynivalenol and fumonisins in seedling disease, wild-type and mycotoxin non-producing mutants of both fungi were used to inoculate seeds in rolled-towel experiments. F. verticillioides isolates were used to infect dent and sweet varieties of maize while F. graminearum isolates were used to infect dent maize, soybeans and wheat. Plant weights, shoot lengths and disease severity (soybean only) were measured at 7 days. Additionally, infection levels were compared by quantifying fungal biomass present in plant tissue between fungal isolates using qPCR. Finally, an experiment to determine changes in gene expression in maize inbred B73 when infected with wild-type or deoxynivalenol non-producing isolates of F. graminearum was performed to better understand the infection process and response of the plant to DON.

Results of experiments with F. graminearum differed among the varieties for all crop species. In soybean, only the susceptible cultivar demonstrated significant differences between plants inoculated with the wild-type and the DON non-producing mutant for plant weight and length. In wheat, the wild type, but not the mutant, caused reductions in weight and length in the susceptible cultivar. The partially resistant variety was reduced in plant weight by both isolates, but neither isolate reduced shoot or root length. In maize, the wild type and the mutant had an impact on shoot length and plant weight for all three hybrids tested. In one susceptible hybrid the wild-type caused greater effects than the DON non-producing mutant. Results indicate that DON production is not required for pathogenicity in seedlings, but the wild-type isolate generally produced greater symptoms and there are interactions between host genotype and DON effects. Gene expression changes in maize in response to infection with wild-type and a DON- mutant revealed that different genes were expressed during early infection and plant germination in comparison to genes expressed after 7 days. There were minimal differences between the wild-type and the mutant isolates for any of the time points sampled. There were greater differences between wild-type infected plants and mock-inoculated plants.

Experiments with Fusarium verticillioides in sweet and dent maize did not provide conclusive results about the effects of fumonisins on seedling disease. For hybrid dent maize, shoot length and plant weight were reduced by only two out of ten isolates tested, regardless of fumonisin production. Fungal biomass was higher in the root tissues than mesocotyl tissues but did not show consistent differences between fumonisin producing and non-producing isolates. In sweet maize there were no significant differences in plant weight or shoot length between plants inoculated with fumonisin-producing and non-producing isolates. All isolates were significantly different from the control for both plant weight and shoot length. Fungal biomass quantification for F. verticillioides in sweet maize was highly variable but fumonisin-producing strains did not consistently colonize the plant tissue to a greater extent than non-producing strains.