Melatonin Mediated Photoprotection in Arabidopsis: Insights from RNA-Seq Analysis

Melatonin, an antioxidant and regulator of cellular processes, plays a variety of roles in plant growth and development under changing environmental demands. Under conditions of excess light, Arabidopsis thaliana plants suffer from stress, and melatonin alleviates this stress. To better understand the mechanisms of melatonin (MT) effects, a comparative RNA-Seq analysis was performed. As a result, a set of genes specifically regulated by melatonin under both the control and prolonged light stress conditions was identified, indicating that, in some cases, melatonin may play opposing roles in regulating the expression of the same genes. Under mild light, a substantial effect of melatonin was associated with the genes involved in stress responses despite favourable growth conditions, as well as with the genes related to cell communication and the regulation of transcription. The genes that were specifically targeted by melatonin under high light (HL) included melatonin-affected receptors, kinases, calcium signalling genes, transcription factors and hormone-related genes. A number of differentially expressed genes (DEGs) have been implicated in cell wall modification, carbon metabolism, lipid and fatty acid metabolism and protein quality control, in addition to a special set of genes associated with sulfur metabolism affecting acclimation responses to oxidative stress. A detailed in silico analysis of the promoter regions of representative MT-responsive genes for putative regulatory cis-elements revealed that the most abundant cis-regulatory elements were associated with light and hormone responses, as well as with transcription factors involved in growth and environmental signalling. To query the physiological processes underlying these transcriptional changes, the physiological parameters affected by MT were analysed immediately after stress and after 24 h of recovery. MT contributed to the maintenance of higher levels of antioxidant system elements and parameters related to the functional activity of photosystem II, which is consistent with the formation of photoprotective physiological responses.