Melatonin alleviates chromium toxicity by altering Cr6+ subcellular distribution and enhancing antioxidant metabolism in wheat seedlings

It was reported that Melatonin (MT) is essential for plants and can mitigate multiple abiotic stresses. However, the mechanism used by endogenous MT to alleviate chromium phytotoxicity is still unclear. In this study, the effects of MT pretreatment on growth, root architecture, subcellular Cr distribution, phytochelatin synthesis and antioxidant metabolism of wheat seedlings under chromium (50 µM Cr6+) stress were researched. Our results showed that Cr stress markedly inhibited seedlings growth and increased ROS accumulation in both roots and leaves. Importantly, growth inhibition and oxidative stress were significantly improved by MT under Cr stress. On the one hand, Cr accumulation and transport from roots to leaves were reduce by MT, which was owed to enhanced vacuolar sequestration via up-regulated phytochelatin (PC) accumulation, which was derived from that MT up-regulated the expression of key genes for PC synthesis (TaPCS and Taγ-ECS). On the other hand, MT pretreatment obviously alleviated Cr-induced oxidative damage, which proved by the diminished lipid peroxidation and cell apoptosis, profiting from the enhanced scavenging ability of reactive oxygen species (ROS) as a result of the MT-induced increment of the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR) and the related encoding genes expression levels of TaSOD2, TaCAT, TaAPX and TaGR. Taken together, all these results revealed that MT alleviated Cr toxicity to wheat seedlings probably through altering Cr6+ subcellular distribution and enhancing antioxidant metabolism. And these findings may be helpful to increase wheat production under Cr stress.