A Phloroglucinol from Dryopteris crassirhizoma Acts as a 20E Antagonist with Insecticidal Activity by Targeting the Ecdysone Receptor

The 20-hydroxyecdysone (20E) signaling pathway is a key target for developing insect growth regulators, yet few effective 20E antagonists have been identified. Through bioassay-guided fractionation using a 20E-responsive reporter assay in Drosophila Kc cells, a symmetric dimeric phloroglucinol, methylene-bis-phlorobutyrophenone (compound A3), was isolated from the rhizomes of Dryopteris crassirhizomaNakai. Compound A3 potently inhibited 20E-induced transcriptional activation without intrinsic agonistic activity. Two-hybrid and microscale thermophoresis (MST) assays suggested that A3 might compete with 20E for binding to the ecdysone receptor (EcR) ligand-binding domain, thereby blocking 20E-induced heterodimerization with USP. Molecular docking revealed that A3 occupies the same hydrophobic pocket as 20E and forms two conserved hydrogen bonds. In vivo, A3 delayed pupariation and caused lethal metamorphic defects in both Drosophila melanogaster and the lepidopteran pest Ostrinia furnacalis, which phenocopied 20E signaling disruption. These results identify methylene-bis-phlorobutyrophenone as a novel 20E antagonist and a promising lead for developing eco-friendly insecticides targeting the ecdysone receptor.