Discovery of New Botanical Insecticides: Identification and Insecticidal Activity of Saponins from Clematis obscura Maxim and Insights into the Stress Response of Acyrthosiphon pisum Harris
Hao, Nan, Yinyin Qi, Long Zhao, Shuangshuang Liang, Wenjing Sun, Sunao Zhang and Xiangrong Tian
Journal of Agricultural and Food Chemistry
https://doi.org/10.1021/acs.jafc.3c06557
ABSTRACT
To discover new botanical products-based insecticide candidates, 14 triterpenoid saponins (<bold>1</bold>-<bold>14</bold>) including four new ones, obscurosides A-D (<bold>1</bold>-<bold>4</bold>), were isolated from Clematis obscura Maxim as potential agrochemicals against Acyrthosiphon pisum Harris and Plutella xylostella (L.). Compounds <bold>1</bold>-<bold>3</bold> were characterized by a rare ribose substitution at C-3, and <bold>4</bold> was a bidesmoside glycosylated at the rare C-23 and C-28 positions of the oleanane aglycone. Compounds <bold>10</bold> (median antifeeding concentration, AFC(50) = 1.10 mg/mL; half-lethal concentration, LC50 = 1.21 mg/mL) and <bold>13</bold> (AFC(50) = 1.09 mg/mL, LC50 = 1.37 mg/mL) showed significant insecticidal activities against third larvae of P. xylostella at 72 h. All saponins displayed antifeedant activities against A. pisum with the deterrence index of 0.20-1.00 at 400 mu g/mL. Compound <bold>8</bold> showed optimal oral toxicity (LC50 = 50.09 mu g/mL) against A. pisum, followed by compounds <bold>1</bold>, <bold>5</bold>-<bold>7</bold>, <bold>9</bold>, and <bold>14</bold> (LC50 = 90.21-179.25 mu g/mL) at 72 h. The shrinkage of the cuticle and the destruction of intestinal structures of microvilli, nucleus, endoplasmic reticulum, and mitochondria were toxic symptoms of <bold>8</bold>-treated A. pisum. The significantly declined Chitinase activity in <bold>8</bold>-treated A. pisum with an inhibition rate of 79.1% at LC70 (70% lethal concentration) could be the main reason for its significant oral toxicities. Molecular docking revealed favorable affinities of compounds <bold>1</bold> and <bold>8</bold> with group I Chitinase OfChtI (Group I Chitinase from Ostrinia furnacalis) through conventional hydrogen bonds and alkey/pi-alkey interactions by different patterns. These results will provide valuable information for the development of novel botanical pesticides for the management of insect pests, especially against A. pisum.