Melatonin Alleviates Copper Toxicity via Improving Copper Sequestration and ROS Scavenging in Cucumber.

Melatonin plays an important role in stress tolerance in plants. In this study, exogenous melatonin significantly alleviated the dwarf phenotype and inhibited the decrease of plant fresh weight induced by excess copper (Cu²⁺). Our results indicated that melatonin alleviated Cu²⁺ toxicity by improving Cu²⁺ sequestration, carbon metabolism and ROS (reactive oxygen species) scavenging, rather than by influencing the Cu²⁺ uptake under excess Cu²⁺ conditions. Transcriptome analysis showed that melatonin broadly altered gene expression under Cu²⁺ stress. Melatonin increased the levels of glutathione and phytochelatin to chelate excess Cu²⁺ and promoted cell wall trapping, thus keeping more Cu²⁺ in the cell wall and vacuole. Melatonin inhibited ROS production and enhanced antioxidant systems at the transcriptional level and enzyme activities, thus building a line of defense in response to excess Cu²⁺. The distribution of nutrient elements was recovered by melatonin which was disturbed by Cu²⁺. In addition, melatonin activated carbon metabolism, especially glycolysis and the pentose phosphate pathway, to generate more ATP, an intermediate for biosynthesis. Taken together, melatonin alleviated Cu²⁺ toxicity in cucumber via multiple mechanisms. These results will help to resolve the toxic effects of Cu²⁺ stress on plant growth and development. These results can be used for new strategies to solve problems associated with Cu²⁺ stress. [ABSTRACT FROM AUTHOR]