Exogenous 3,3′-Diindolylmethane Improves Vanadium Stress Tolerance in <i>Brassica napus</i> Seedling Shoots by Modulating Antioxidant Enzyme Activities

3,3′-diindolylmethane (DIM) belongs to a family of indole glucosinolate compounds that have been shown to improve <i>Brassica napus</i> growth through the modulation of reactive oxygen species when applied exogenously. The <i>B. napus</i> cultivar AV Garnet was previously identified as a vanadium-sensitive cultivar. Therefore, in this study we investigated whether exogenous DIM could improve the vanadium tolerance of AV Garnet. We performed the following experiments: seed germination assessment, dry weight assessment, cell viability assay, chlorophyll content assay, malondialdehyde (MDA) assay, conjugated diene (CD) content assay, hydrogen peroxide (H₂O₂) content assay, superoxide (O₂⁻) content determination, methylglyoxal (MG) content determination, hydroxyl radical (<b>·</b>OH) concentration determination, ascorbate peroxidase (APX) activity assay, superoxide dismutase (SOD) activity assay, glyoxalase I (Gly I) activity assay, glutathione S-transferase (GST) activity assay and inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis for vanadium content determination. Under vanadium stress, exogenous DIM increased the seed germination percentage, shoot dry weight, cell viability and chlorophyll content. Exogenous DIM also led to a decrease in MDA, CD, H₂O₂, O₂⁻, MG and <b>·</b>OH, under vanadium stress in the shoots. Furthermore, DIM application led to an increase in the enzymatic activities of APX, SOD, Gly I and GST under vanadium stress. Interestingly, under vanadium stress, DIM treatment did not alter vanadium content in <i>B. napus</i> shoots. Our results indicate that exogenous application of DIM can improve <i>B. napus</i> seedling shoot growth and biomass under vanadium stress by priming the antioxidant enzymes via reactive oxygen species (ROS) signaling.