Date on Senior Honors Thesis

5-2020

Document Type

Senior Honors Thesis

Degree Name

B.S.

Department

Biology

Degree Program

College of Arts and Sciences

Author's Keywords

Genetics; Fungus; Mitochondria; Pathogen; Infection

Abstract

Ustilago maydis and Sporisorium reilianum are both dimorphic fungi that cause infection in the host plant, corn (Zea mays). In order for infection to be successful, compatible haploid mating types must form a dikaryon that later leads to the development and dispersal of teliospores. In order to sustain growth, energy is produced via the electron transport chain within the mitochondrion. Host plants often produce harsh reactive oxygen species, resulting in the need to use an alternative respiration pathway such as employing an alternative oxidase. Therefore, this study attempted to investigate the role of the aox gene in pathogenicity for these fungi. This was achieved by disrupting the gene of interest and assessing mating, response to stress, and virulence. Virulence was found to be lower in seedlings infected with U. maydis deletion strains, suggesting aox could play a role in pathogenicity. Mating assays and stress tests did not show any differences in growth. Unfortunately, data could not be collected from S. reilianum infections, whose assessment requires full development of infected plants, but further study will investigate virulence and teliospore germination rates. There was a slight difference in mating, but this could not be quantified and should be compared to mating during infection as a future point of investigation. Additionally, there were no differences in morphology or growth ability of deletion strains when compared to wild type during stress tests.

Lay Summary

Ustilago maydis and Sporisorium reilianum are two types of fungus that cause infection of corn (Zea mays). Although they do not produce exactly the same symptoms, spores of both fungi are produced in structures found on infected plants. For U. maydis, tumors or galls result on above-ground tissue including stems, leaves, tassels, and ears. In contrast, spores of S. reilianum are found in white sori produced in the tassels and/or the ear. The infected ear of the corn appears very small and tear-drop shaped. The cob is replaced by the white sori, which are the structures that make and hold the spores of the fungus. For infection to occur successfully in both types of fungus, two separate compatible mating type partners must come together, activating genes that lead to infection symptoms. In order to proliferate, the fungus must produce energy which is achieved via an organelle called the mitochondrion. The mitochondrion uses a reaction pathway to produce energy, but the corn plant can produce compounds that decrease energy output or can cause damage to the fungal DNA. To combat this, both U. maydis and S. reilianum have an alternative oxidase (aox) gene to allow the fungi to effectively deal with the stress. The main focus of this paper was to explore the role of the aox gene in virulence, or severity of infection. To better understand its role, the gene was deleted and the mutants that resulted were examined for effects on mating, response to stress, and infection. Mating of S. reilianum showed a slight difference qualitatively; however, this effect could not be further assessed quantitatively. U. maydis did not show any differences in mating. There were also no differences in growth when exposed to various stressors such as salt and hydrogen peroxide. U, maydis did show a significant difference in virulence as measured by the disease index when comparing the mutants and wild type strains. S. reilianum infections could not be assessed because symptoms had not fully developed at the time of this report.

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