Melatonin promotes plant growth by maintaining integration and coordination between carbon and nitrogen metabolisms.

KEY MESSAGE: Melatonin-induced growth promotion of maize seedlings is associated with maintaining coordination between gene expressions and activities of key enzymes involved in carbon and nitrogen metabolisms. Melatonin is a pleiotropic molecule that influences many diverse actions to enhance plant growth. The effect of melatonin on maintaining a necessary balance between carbon and nitrogen metabolisms that underpins the growth process in higher plants remains unclear. In this study, the influence of melatonin on nitrogen assimilation, mitochondrial respiration, and photosynthesis, which are major pathways related with carbon and nitrogen metabolism, was investigated on the basis of the seedling growth of maize. Melatonin applications (10, 100, and 1000 μmol L-1) significantly increased the growth parameters assessed by root elongation, plant height, leaf surface area, and the contents of protein, carbohydrate, and chlorophyll in comparison to the control seedlings. They also had a strong encouraging effect on the activities and gene expressions of enzymes (nitrate reductase, nitrite reductase, glutamine synthase, glutamate 2-oxoglutarate transferase, and NADH-glutamate dehydrogenase) involved in the nitrogen assimilation process. While melatonin applications elevated nitrate and nitrite concentrations, they markedly lowered ammonium content compared to control. Similarly, the activity of citrate synthase, the first enzyme of citric acid cycle providing carbon skeleton for nitrogen assimilation, was significantly augmented by melatonin applications. Moreover, melatonin considerably upregulated the gene expressions of citrate synthase and cytochrome oxidase, an enzyme responsible for ATP production. Remarkable increments were recorded at Rubisco activity and gene expressions of Rubisco and Rubisco activase in melatonin-treated seedlings. In conclusion, all these data put together reveal that melatonin-induced growth promotion of maize seedlings resulted from its coordinating effect on carbon and nitrogen metabolisms.