l‑Isoleucine-Derived Amide-hydrazide\nCompounds Evaluated as a Novel Potential Agricultural Fungicide

Building\nupon previous structure–activity relationships\nabout the fungicidal amide and hydrazide lead structures, 24 novel\namide-hydrazide compounds were designed and synthesized with <i>L</i>-isoleucine as the initial skeleton to explore the impact\nof substituents in the hydrazide bridge on the fungicidal activity.\nAmong these compounds, <b>A5</b> exhibited excellent and broad\nspectrum inhibitory activity, along with satisfactory <i>in vivo</i> protective efficiency against R. solani at concentrations of 200 and 50 μg·mL<sup>–1</sup>. Scanning electron microscopy (SEM) and transmission electron microscopy\n(TEM) observations revealed that compound <b>A5</b> induced\nsignificant morphological changes in the R. solani mycelium coupled with vacuole rupture and cytoplasmic inhomogeneity\nin cellular structures. Transcriptomic and metabolomic analyses indicated\nthat, following <b>A5</b> treatment, the differentially expressed\ngenes and metabolites were significantly enriched in carbohydrate\nmetabolism-related pathways as well as in lipid metabolism-associated\npathways, including glycerophospholipid metabolism, steroid biosynthesis,\narachidonic acid metabolism, and sphingolipid metabolism. Additionally,\ncompound <b>A5</b> demonstrated low toxicity to zebrafish, with\nsurvival rates of 100% and 60% at concentrations of 1 and 10 μg·mL<sup>–1</sup>, respectively, over a period of 7 days. The above\nresults provide theoretical guidance for the development of novel\ngreen hydrazide fungicidal candidates.