Discovery of Novel Pyrazole Amides as Potent Fungicide Candidates and Evaluation of Their Mode of Action

Abstract

A rational molecule design strategy based on scaffold hopping was applied to discover novel leads, and then a series of novel pyrazole amide derivatives were designed, synthesized, characterized, and evaluated for their antifungal activities. Bioassay results indicated that some target compounds such as <b>S3</b>, <b>S12</b>, and <b>S26</b> showed good in vivo antifungal activities; among them, <b>S26</b> exhibited commendable in vivo protective activity with an 89% inhibition rate against <i>Botrytis cinerea</i> on cucumber at 100 μg/mL that is comparable to positive controls boscalid, isopyrazam, and fluxapyroxad. Microscopy observations suggested that <b>S26</b> affects the normal fungal growth. Fluorescence quenching analysis and SDH (succinate dehydrogenase) enzymatic inhibition studies validated that <b>S26</b> may not be an SDH inhibitor. Based on induction of plant defense responses testing, <b>S26</b> enhanced the accumulation of <i>RBOH, WRKY6</i>, <i>WRKY30</i>, <i>PR1</i>, and <i>PAL</i> defense-related genes expression and the defense-associated enzyme phenylalanine ammonia lyase (PAL) expression on cucumber. These findings support that <b>S26</b> not only displayed direct fungicidal activity but also exhibited plant innate immunity stimulation activity, and it could be used as a promising plant defense-related fungicide candidate.