Melatonin Affects <i>Peucedanum praeruptorum</i> Vegetative Growth and Coumarin Synthesis by Modulating the Antioxidant System, Photosynthesis, and Endogenous Hormones

The dried root of Peucedanum praeruptorum is often used medicinally and has high pyran- and furanocoumarin content. Although exogenous melatonin (MT) impacts the regulation of plant growth, stress responses, secondary metabolism, etc., it remains unclear whether MT regulates the vegetative growth and development of P. praeruptorum. Thus, the aim of the current study is to characterize the effects of different exogenous MT concentrations on the physiological functions, photosynthesis, antioxidant systems, hormone induction, and coumarin synthesis of P. praeruptorum. Different MT concentrations exert distinct regulatory effects on P. praeruptorum growth and the expression of genes related to coumarin synthesis. Treatment of P. praeruptorum with low concentrations of MT increases photosynthesis and leaf growth compared to the control, while high concentrations reduce root vitality and elongation and decrease the expression of photosynthetic system genes. Low concentrations of MT also significantly increase antioxidant enzyme activity and photosynthetic pigment content and modulate the levels of IAA, gibberellic acid, salicylic acid, jasmonic acid, abscisic acid, and endogenous MT. Moreover, MT increases the activity of the MT synthesis enzymes tryptophan decarboxylase, tryptophan hydroxylase, tryptamine-5-hydroxylase, serotonin N-acetyltransferase, acetylserotonin O-methyltransferase, and caffeic acid O-methyltransferase, and promotes the accumulation of isoscopoletin, scopoletin, peucedanocoumarin II, praeruptorin A, praeruptorin B, and praeruptorin E. MT also upregulates most genes associated with coumarin synthesis, including PAL1, C4H, 4CL-3, C3H-1, F6H-1, CCoAMT, OMT-1, CYP71AJ1, CYP84A1-1, S8H-1, PT-1, and COSY-1. These findings demonstrate that MT may improve P. praeruptorum growth and development while promoting the synthesis of coumarin components.