Coumarin Promotes Hypocotyl Elongation by Increasing the Synthesis of Brassinosteroids in Plants

Coumarins are natural products commonly found in plants and are typical allelopathic substances that strongly affect the growth of plants after being exudated from the root and help plants absorb Fe in cases of iron deficiency. Although coumarins have been found to have multiple effects, this understanding is still relatively limited. Here, we show that coumarin significantly promotes the elongation of the hypocotyl by enhancing cell elongation. Further research has found that coumarin increases the content of BR in plants by enhancing the expression of brassinosteroid (BR) synthesis genes. The effect of coumarin on promoting hypocotyl elongation is completely blocked by the mutation of the BR synthesis gene <i>DEETIOLATED 2</i> (<i>DET2</i>) or the co-addition of the BR synthesis inhibitor brassinazole (BRZ). Genetic analysis using Arabidopsis mutants showed that coumarin promoting hypocotyl elongation depends on the signaling pathway of the BRs. Overall, coumarin promotes elongation of the hypocotyl by increasing the synthesis of BRs in plants. These results provide us with new insights into the role of coumarins and offer strong theoretical support for the mechanisms of interactions between plants.