Invasive alien plants seriously threaten native plant biodiversity and agricultural production. The development of environmentally friendly agriculture requires sustainable weed control techniques to manage these invasive alien weeds. This study evaluated the allelopathic effects of ethanol extract from Artemisia frigida against five invasive alien plants (Ageratum conyzoides, Bidens pilosa, Ipomoea purpurea, Eclipta prostrata, and Amaranthus retroflexus). The main components in the extract were identified using high-performance liquid chromatography–tandem mass spectrometry (LC-MS/MS), and we assessed their allelopathic effects on seed germination of the five species. The results showed that the ethanol extract of A. frigida completely inhibited seed germination of all five invasive plants at 5 g·L⁻¹. Thirteen components were identified, among which 4-ethyloctanoic acid, cis-jasmone, and p-anisic acid exhibited significant inhibitory effects. Notably, 4-ethyloctanoic acid demonstrated broad-spectrum herbicidal activity. At 50 mg·L⁻¹, it completely inhibited B. pilosa growth and had the strongest inhibitory effects on A. conyzoides and E. prostrata. This compound disrupted redox homeostasis and induced oxidative stress by modulating antioxidant enzyme activities, including superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT). These findings indicate that 4-ethyloctanoic acid is the main allelochemical with herbicidal potential in A. frigida, providing a theoretical basis for developing novel herbicides and environmentally friendly control techniques for invasive alien plants.

Echinochloa crus−galli is one of the world’s ten most pernicious weeds and is mainly controlled by chemical herbicides. However, the application of herbicides over long periods has escalated weed resistance, highlighting the need for eco−friendly alternatives. As a degradable natural allelochemical, dihydrocoumarin displays auxin−like activity and inhibits root growth in E. crus−galli in a manner dependent on concentration, phenotypically mimicking exogenous IAA. Previous studies have shown that dihydrocoumarin suppresses weed growth largely via auxin signaling. Here, we identified EcIAA9, a gene that responds to dihydrocoumarin and encodes a nuclear protein interacting with EcARF7. Heterologous rice assays showed that at 25 mg/L dihydrocoumarin, EcIAA9 overexpression increased primary−root inhibition from 30.81% (WT) to 54.44% and 52.00% (EcIAA9−OE). Conversely, OsIAA9 knockout lines consistently exhibited a milder root inhibition trend compared to the overexpression lines. These differential phenotypes collectively establish IAA9 as a key positive regulator in this pathway. Notably, EcIAA9 overexpression rice showed different responses to dihydrocoumarin and IAA, and combined treatments produced additive inhibitory effects. Knockout of OsARF7 failed to fully abolish dihydrocoumarin−triggered inhibition, indicating that ARF7 participates in this regulatory module, although functional compensation among ARF homologs may underlie this observation. In summary, the EcIAA9–EcARF7 regulatory module contributes to the repression of root growth triggered by dihydrocoumarin. These findings show that dihydrocoumarin phenotypically mimicked the inhibitory effect of IAA on barnyardgrass but acted through a somewhat different mechanism. This study provides new insights into the crosstalk between allelochemicals and auxin signaling.