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Abstract
Fusarium verticillioides is one of the most prevalent seed- and wind-borne fungal pathogens on maize, causing severe ear rot symptoms. Alternative to its pathogenic effect, F. verticillioides often exists as a symptomless endophyte. Living inside maize tissues, F. verticillioides has to cope with antimicrobial phytochemicals such as 2-benzoxazolinone (BOA), which the fungus tolerates better than other fungi due to its ability to hydrolyze the -lactam ring via a lactamase encoding gene, MBL1. Inspired by the hydrolytic function of MBL1, I examined genomes from across the fungal kingdom for lactamase encoding genes. A strong positive correlation was clearly evident between environmental niche complexity and the number of lactamase encoding genes in a fungal species. A resulting central hypothesis is that many fungal hydrolytic lactamases are responsible for the degradation of, and thus resistance to, plant or microbial xenobiotic lactam compounds encountered in their respective niches (Chapter 2). Pyrrocidine A and B are two lactam-containing antimicrobial compounds produced by a protective maize endophyte, Sarocladium zeae, which co-inhabits maize kernels with F. verticillioides but maintains tissue specificity (i.e., S. zeae in the embryonic area, and F. verticillioides in the pedicel). RNA-seq experiments were carried out to elucidate F. verticillioides responses to pyrrocidine exposure and its tolerance mechanism(s). Among 10 pyrrocidine up-regulated genes selected for functional characterization, an ABC transporter gene (FVEG_11089, FvABC3) was a primary determinant of pyrrocidine tolerance. Further, a pyrrocidine induced zinc-binding dehydrogenase encoding gene (FVEG_00314, FvZBD1) appears to be a heretofore unidentified repressor of fumonisin biosynthesis and lies immediately adjacent to the fumonisin biosynthetic gene cluster. This may be a mechanism by which S. zeae pyrrocidines silence F. verticillioides secondary metabolism during sympatric interactions in the plant (Chapter 3). Lastly, BOA, 2-oxindole, 2-coumaranone, and chlorzoxazone are four lactam and/or lactone-containing compounds with similar chemical structures, and RNA-seq analysis of F. verticillioides exposed to these compounds identified gene clusters responsive to each of the four compounds, indicating substrate and enzymatic specificity (Chapter 4). Overall, this work represents the first in-depth exploration of fungal lactamases and antifungal lactams, underlying their relevance to fitness and resistance to antimicrobials in the environment.