Exogenous Melatonin Boosts Heat Tolerance in Rosa hybrida via RhCOMT1 Modulation

<i>Rosa hybrida</i> is one the most commonly cultivated ornamental plant of economic importance and faces major challenges under heat stress. Melatonin has been widely shown to regulate plant stress response; however, the exact mechanism involved in heat stress in <i>R. hybrida</i> has yet to be determined. Here, we observed that <i>R. hybrida</i> in vitro plantlets supplemented with melatonin in the culture medium exhibited higher chlorophyll content, relative ion leakage, and fresh weight after 12 d of high-temperature treatment; the optimal concentration was established at 5 mg/L. Using molecular and biochemical techniques, we explored the roles of a melatonin synthase gene <i>RhCOMT1</i>, which expression was influenced by heat stress and melatonin. RhCOMT1 was located in the nuclear-cytoplasmic under ambient conditions, while heat stress translocated the distribution of RhCOMT1 to chloroplasts. Overexpression of <i>RhCOMT1</i> in rose petal enhanced thermotolerance, and silencing of <i>RhCOMT1</i> reduced thermotolerance via affect H₂O₂ content and relative ion leakage. These findings collectively emphasize the pivotal role of melatonin in enhancing thermotolerance to <i>R. hybrida</i> by alleviation of oxidative stress, through modulation of <i>RhCOMT1</i> expression and location.