GEORGE MEYER
Major Professor: Dr. Joshua Widhalm
PhD DEFENSE SEMINAR
July 23rd @ 1:00 PM
Room: HORT 222
“Biochemical Investigations into the Phytotoxicity, Metabolism, and Agricultural Applications of Juglon”
Abstract: Plant specialized 1,4-naphthoquinones are a class of metabolites that mediate diverse plant-biotic interactions. Their convergent evolution across multiple plant lineages highlights the significance of their ecological roles. The black walnut tree (Juglans nigra), which is native to the central and eastern parts of the United States, synthesizes the phytotoxic 1,4-naphthoquinone juglone. The focus of my dissertation research was on fundamental questions about juglone’s biochemical mechanism of action, biosynthesis, and its potential as an agrochemical. Although juglone’s phytotoxicity has long been attributed to inhibiting H+-ATPase and generating reactive oxygen species, we propose that its mechanism of action is likely broader. Juglone has dual chemistry as a redox cycler and its electrophilic nature gives it the ability to alkylate nucleophilic thiol groups. My dissertation research expands juglone’s mechanism of action to include the depletion of small antioxidants, such as the tripeptide glutathione, and the modification of exposed and unreduced cysteine groups on peptides and proteins.
The new insights gained on juglone’s biochemical mechanism of action led us to reassess how black walnut trees prevent autotoxicity and how juglone is biosynthesized. In black walnut, juglone exists primarily in its glucosylated form, hydrojuglone glucoside. We hypothesize that the black walnut protects itself from juglone by storing juglone in its glucoside form and spatially separating hydrojuglone β-glucosidase in the apoplast so that juglone is released outside of the cell. This mechanism draws similarity to how glucosinolate-producing plants protect themselves from the electrophilic isothiocyanates released from spatially separated myrosinase. However, the genes involved in glucosylating juglone and hydrolyzing hydrojuglone glucoside remain elusive.
Juglone is often suggested to be a potential natural product-based herbicide. However, its lack of molecular specificity, its broad reactivity, and the need for sustained exposure raise questions about its suitability as an herbicide. These properties, however, make it useful for other agricultural applications, such as a soil urease inhibitor. Chemical urease inhibitors are commonly coated on urea granules, which localizes the inhibitor to the granule and allows the urea to penetrate the soil surface before it is converted to volatile ammonia by urease. My dissertation research included studying juglone and juglone-containing tissue as an inexpensive and sustainable urease inhibitor alternative to improve fertilizer efficiency. Taken together, insights from my work lay the groundwork for future studies characterizing the metabolism of other specialized 1,4-naphthoquinones and their potential use in agriculture.