Overexpression of Caffeic Acid O-Methyltransferase 1 (COMT1) Increases Melatonin Level and Salt Stress Tolerance in Tomato Plant

Melatonin is a natural phytohormone that occurs in most plants. It can regulate not only plant growth and development but also alleviate biotic and abiotic stress in plants. However, no studies have reported on endogenous melatonin increasing tomato tolerance to salt stress. In the present study, we observed that the overexpression of caffeic acid O-methyltransferase 1 (SlCOMT1) increased melatonin levels in tomato plants. We also observed that transgenic plants exhibited higher salt stress tolerance. SlCOMT1 overexpression could maintain the balance of Na+/K+ and decrease ion damage by activating salt overly sensitive (SOS) pathway under salt treatment. In addition, SlCOMT1 overexpression significantly enhanced the antioxidant capability, with higher antioxidant enzyme activity observed, including superoxide dismutase, peroxide and catalase activity, and higher ascorbic acid (AsA) and glutamate (GSH) accumulation levels. SlCOMT1 overexpression also maintained good nutrient homeostasis in the tomato plants. In addition, SlCOMT1 overexpression upregulated some stress-related genes (AREB1, AIM1, MAPK1, WRKY33 and CDPK1), which resulted in the activation of downstream signaling pathways and could be partly responsible for the improvement in salt stress tolerance.