Date of Graduation

5-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Plant Science (PhD)

Degree Level

Graduate

Department

Entomology and Plant Pathology

Advisor/Mentor

Bluhm, Burton H.

Committee Member

Rupe, John C.

Second Committee Member

Egan, Martin J.

Third Committee Member

Shi, Ainong

Fourth Committee Member

Alverson, Andrew J.

Keywords

Appressorium; Cercospora; Cercosporin; Pathogenesis; Secondary metabolism; Stomatal infection

Abstract

Gray leaf spot is a globally important foliar disease of maize caused by the fungus Cercospora zeae-maydis. In C. zeae-maydis, light is a critical environmental signal linked to pathogenesis and secondary metabolism. However, the mechanisms by which the fungus senses and responds to light are not fully understood. Thus, the overarching goal of this project was to unravel the connection between light and pathogenesis in C. zeae-maydis. The conserved fungal protein White Collar-1 (WC-1), designated Crp1 in C. zeae-maydis, senses light and initiates physiological responses. In fungi, WC-1 functions in the White Collar Complex via interaction with White Collar-2 (WC-2). In this study, the WC-2 ortholog of C. zeae-maydis, Crp2, was found to be involved in many of the same processes as Crp1, including pathogenesis. The light-mediated response initiated by Crp1 included transcriptional induction of the conserved circadian clock component frequency (FRQ1), which also displayed circadian rhythmicity. Deletion of FRQ1 impaired pathogenesis. Although overexpression of frequency in other fungi impairs clock function, overexpression of FRQ1 did not impact virulence in wild-type C. zeae-maydis but partially restored virulence in a Crp1-deficient strain. Characterization of the Defective in Binding DNA (DBD) region of Crp1 revealed a conserved role in the fungal circadian clock, but the domain was dispensable for pathogenesis. Additionally, Crp1 and Frq1 regulated the biosynthesis of, and self-protection against, cercosporin, a light-regulated secondary metabolite produced by C. zeae-maydis. Specifically, the stress response MAPKKK gene CZK3 and the cercosporin transporter gene CFP1 showed circadian rhythmicity, and the physiological response to cercosporin was dependent on the circadian clock. Lastly, Set3, a conserved component of histone deacetylation, was found to be important for pathogenesis, secondary metabolism, and transcriptional regulation of FRQ1. These findings suggest that (1) C. zeae-maydis has a circadian clock, (2) a Crp1/Crp2-mediated light response drives pathogenesis through regulation of FRQ1, (3) Set3 modulates FRQ1 following Crp1/Crp2 activation possibly through changes in histone acetylation, and (4) Frq1 has clock-independent role in pathogenesis and a clock-dependent role in cercosporin resistance. These phenomena have not been previously described and substantially advance the fundamental understanding of how C. zeae-maydis utilizes light to regulate pathogenesis.

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