Dihydrocoumarin Transcriptionally Activates EcIAA9 to Repress Root Growth of Echinochloa crus−galli

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.