Delayed germination of Brassica parachinensis seeds by coumarin involves decreased GA₄ production and a consequent reduction of ROS accumulation

The plant allelochemical coumarin effectively inhibits the germination of Brassica parachinensis (B. parachinensis) seeds. Quantification of endogenous phytohormones showed that contents of abscisic acid (ABA), ABA glucose ester, gibberellin A20 (GA₂₀), GA₃, GA₁₅, GA₂₄, GA₉ and GA₄ were higher in germinating seeds than in seedlings. Moreover, the presence of coumarin significantly reduced the content of bioactive GA₄ which is thought to positively regulate seed germination. Histochemical staining and spectrophotometry of reactive oxygen species (ROS) revealed that exogenous GA₃ and GA₄₊₇ could effectively promote the production of endogenous ROS during germination and that the GA synthesis inhibitor paclobutrazol could effectively inhibit production of ROS. Coumarin significantly inhibited the accumulation of ROS, especially superoxide anion radical ( ${\rm O}_2^{{\cdot}{-}} $ ). This inhibitory effect could be restored by the addition of exogenous GA₃ and GA₄₊₇. Coumarin also inhibited the activity of the ROS-degrading enzymes such as superoxide dismutase, catalase and peroxidase as well as β-amylase in seeds and seedlings. Taken together, we propose a model for the regulation of seed germination in B. parachinensis by coumarin, Gas and ROS, in which coumarin may delay seed germination by reducing endogenous GA₄, thus decreasing the accumulation of ROS.