Fig. 6
Summary of protective mechanisms of melatonin against selenium phytotoxicity. A schematic diagram showed the mitigating effects of exogenous MT on Brassica napus L. seedlings under Se (IV) stress. Se heightened its toxicity by (I) Over-accumulating ROS that leads to chlorophyll degradation and ultimately growth reduction. (II) Induction of electrolyte leakage and lipid peroxidation reflects the damages in cellular membrane. (III) Disturbances in the synchronization of the defense system by increasing SOD and APX activities, proline, and free amino acids but declined the key enzymes (CAT and GR) and soluble sugar. (IV) Osmotic stress by lowering relative water and sugar contents. (V) An increase in the levels of thiol compounds (GSH, GSSG, NPTs, cysteine, and PCs) depicted the greater potential of Brassica napus plants to confer Se tolerance. Exogenous MT ameliorated the Se toxicity by enhancing photochemical efficiency and osmo-protection, which is linked with the enhanced plant growth and biomass production. In addition, exogenous MT induced the endogenous MT content which assist in the protective role of MT against Se-prompted ROS generation by inducing enzymes involved in AsA-GSH cycle (APX and GR), ROS-detoxifying enzymes (mainly SOD and CAT), biosynthesis of thiol components (especially GSH and phytochelatins), and the enzymes involved in thiol metabolism (γ-ECS, GST and PCS). The greater accumulation of MT and thiol components in roots suggested roots as greater site for the detoxification of Se as compared with leaves. Diagram indicates O2•– (superoxide), H2O2 (hydrogen peroxide), SOD (superoxide dismutase), CAT (catalase), APX (ascorbate peroxidase), GR (glutathione reductase), GSH (reduced glutathione), GSSG (oxidized glutathione), RWC (relative water content), Pro (proline), WSG (water soluble sugar), FAA (free amino acids), REL (relative electrolyte leakage), MDA (melondialdehyde), NPTs (non-protein thiols), PCs (phytochelatins), cyst (cysteine), γ-ECS (gamma-glutamylcysteine synthase), GST (glutathione-S-transferase) and PCS (phytochelatins synthase)