Salt Stress and Endogenous Antioxidants Melatonin and Proline Modulate Stress Responses and Expression of Epigenetic Regulatory Genes in Rice

Moderate levels of salt, which may not significantly affect plant metabolism, can still influence a plant’s transcriptome. Our previous results showed that proline (25 mM) and melatonin (100 µM) induced the antioxidant levels equivalent to 150 mM salt treatment. Proline and melatonin-induced epigenome level changes in the promoter of their biosynthesis genes lead to their transcription regulation has been reported previously. The present study evaluates the effect of 150 mM salt on selected stress-alleviating transcripts and how the antioxidants proline and melatonin regulate their expression when supplied exogenously. The results indicate that there is a clear delineation between the functions of proline and melatonin under 150 mM salt stress. While the photosynthetic pigments Chlorophyll a (Chl a) and Chl b were more abundant when melatonin was used with salt, proline and salt+ proline improved carotenoid content in rice. Exogenous melatonin significantly reduced lipid peroxidation in salt-stressed plants. Transcript levels of antioxidants except Ascorbate peroxidase (APX) was significantly high in salt and melatonin combination. A moderate level of salt in the presence of proline did not induce the transcript of any of the proline biosynthesis enzymes. While moderate salt increased all the biosynthesis pathway genes of melatonin, exogenous melatonin suppressed the transcription of enzymes except (SNAT1). Moderate salt and melatonin upregulated melatonin biosynthesis genes significantly, except (T5H). Transcript-level analysis of methylases and demethylases indicates differential expression under the conditions tried. As the previous studies used high salt concentrations, which normally absent in the real-life situations, the 150 mM salt study may help us to identify the salinity response pathways modulated by proline and melatonin.