Stereoselective\nBioactivity and Action Mechanism of\nthe Fungicide Isopyrazam

Understanding the stereoselective bioactivity of chiral\npesticides\nis crucial for accurately evaluating their effectiveness and optimizing\ntheir use. Isopyrazam, a widely used chiral SDHI fungicide, has been\nstudied for its antifungal activity only at the racemic level. Therefore,\nto clarify the highly bioactive isomers, the stereoselective bioactivity\nof isopyrazam isomers against four typical phytopathogens was studied\nfor the first time. The bioactivity ranking of the isomers was <i>trans-</i>1<i>S</i>,4<i>R</i>,9<i>R</i><i>-</i>(+)-isopyrazam > <i>cis</i>-1<i>R</i>,4<i>S</i>,9<i>R</i>-(+)-isopyrazam\n> <i>trans</i>-1<i>R</i>,4<i>S</i>,9<i>S</i>-(−)-isopyrazam > <i>cis-</i>1<i>S</i>,4<i>R</i>,9<i>S</i>-(−)-isopyrazam.\nSDH activity was assessed by molecular docking simulation and actual\ndetection to confirm the reasons for stereoselective bioactivity.\nThe results suggest that the stereoselective bioactivity of isopyrazam\nis largely dependent on the differential binding ability of each isomer\nto the SDH ubiquitin-binding site, located within a cavity formed\nby the iron–sulfur subunit, the cytochrome b560 subunit, and\nthe cytochrome b small subunit. Moreover, to reveal the molecular\nmechanism of isopyrazam stereoselectively affecting mycelial growth,\nthe contents of succinic acid, fumaric acid, and ATP were measured.\nFurthermore, by measuring exospore polysaccharides and oxalic acid\ncontent, it was determined that 1<i>S</i>,4<i>R</i>,9<i>R</i>-(+)- and 1<i>R</i>,4<i>S</i>,9<i>R</i>-(+)-isopyrazam more strongly inhibited the ability\nof Sclerotinia sclerotiorum to infect\nplants. The findings provided essential data for the development of\nhigh-efficiency isopyrazam fungicides and offered a methodological\nreference for analyzing the enantioselective activity mechanism of\nSDHI fungicides.