Innovative Synthesis of Phenolic Monoterpene Derivatives with Enhanced Antifungal Activity: Mechanistic Insights and Potential for Sustainable Plant Protection

Plant pathogenic fungi pose a threat to agricultural production, necessitating the search for safer fungicidal alternatives to conventional synthetic fungicides. Phenolic monoterpenes (PMs) and their derivatives have shown promise in fungicide development due to their potent biological activity. In this study, a ″group introduction″ strategy was employed to synthesize novel antifungal derivatives of PMs, incorporating nonclassical bioisosteres such as methylamino and acetylamino groups into carvacrol and thymol. The antifungal efficacy of these derivatives was evaluated against six plant pathogenic fungi through in vitro experiments, complemented by propidium iodide (PI) staining, ergosterol content analysis, and RNA-seq, and exogenous ergosterol complementation assays to investigate their mechanisms of action involving fungal cell membrane permeability and ergosterol biosynthesis. A greenhouse pot experiment on tomato plants was also conducted to evaluate the in vivo control efficacy of representative compounds and assess potential phytotoxicity. Results indicated that the newly synthesized PM derivatives exhibited enhanced antifungal activity compared to the original compounds, with 8a and 10a showing notable effects against Botrytis cinerea. Additionally, 7a and 7b displayed superior efficacy against various fungal strains, outperforming the positive control chlorothalonil. Greenhouse validation demonstrated that 10a effectively controlled tomato gray mold without inducing phytotoxicity at effective concentrations. These findings highlight the significant antifungal potential of the novel PM derivatives, particularly against resistant pathogenic fungi, through their mode of action involving enhanced fungal cell membrane permeability and disruption of ergosterol biosynthesis.