Exogenous Inhibitors Enhance Plant Resistance to Rice Planthoppers Through Boosting Phenylpropanoid Pathway Under Elevated CO ₂

The damage caused by the Nilaparvata lugens to rice plants will become aggravated under elevated CO₂ (eCO₂), emphasizing the urgent necessity to enhance rice defences against N. lugens under eCO₂ conditions. The phenylpropanoid pathway is composed of several branches and is known to enhance plant resistance. Inhibiting key enzymes in certain branches can compensatorily activate other branches, thereby strengthening plant defence. In this study, rice plants were treated with the cinnamic acid-4-hydroxylase (C4H) inhibitor piperonylic acid (PA) and the 4-coumarate:CoA ligase (4CL) inhibitor 3,4-(methylenedioxy) cinnamic acid (MDCA), and their effects on N. lugens and rice plants were investigated under ambient CO₂ (aCO₂) and eCO₂. The results demonstrated that PA and MDCA treatments reduced N. lugens feeding and survival, especially under eCO₂. Additionally, PA and MDCA treatments activated the ROS system and phenylpropanoid pathway in rice plants, and increased hydroxybenzoic acid derivatives contents under eCO₂, with only minor changes observed under aCO₂. Further analysis illustrated that these compounds suppressed N. lugens feeding and survival, and could bind to and induce the activities of detoxification enzymes in N. lugens. These findings demonstrate that PA and MDCA enhance rice resistance under eCO₂, providing a potential strategy to mitigate pest damage exacerbated by climate change.