Coumarin Inhibits Primary Root Growth by Modulating Auxin Signaling via Neddylation

The allelopathic compound coumarin inhibits root growth across numerous species, but its mechanism is unknown. Through a genetic screen in Arabidopsis mutants, we identified the mutants for AXR1 (AUXIN RESISTANT 1) and ECR1 (E1 C-TERMINAL RELATED 1), two subunits of the NEDD8-activating enzyme (NAE), that are resistant to coumarin. Conversely, overexpression of the NEDD8-encoding gene RUB1 (RELATED TO UBIQUITIN 1) caused hypersensitivity, while the NAE inhibitor MLN4924 blocked coumarin’s effect. Since neddylation regulates auxin signaling, we analyzed downstream AUX/IAA proteins and found that the loss-of-function mutant of AUXIN RESISTANT 2 (AXR2) (also known as IAA7) was resistant to coumarin. We further showed that coumarin treatment leads to the accumulation of the AXR2 protein. Taken together, these results demonstrate that coumarin inhibits primary root growth by modulating auxin signaling via neddylation.