Abstract
Mulberry (Morus alba L.) leaves and fruit are traditional Chinese medicinal materials with anti-inflammatory, immune regulatory, antiviral and anti-diabetic properties. Melatonin performs important roles in the regulation of circadian rhythms and immune activities. We detected, identified and quantitatively analyzed the melatonin contents in leaves and mature fruit from different mulberry varieties. Melatonin and three novel isoforms were found in the Morus plants. Therefore, we conducted an expression analysis of melatonin and its isomer biosynthetic genes and in vitro enzymatic synthesis of melatonin and its isomer to clarify their biosynthetic pathway in mulberry leaves. MaASMT4 and MaASMT20, belonging to class II of the ASMT gene family, were expressed selectively in mulberry leaves, and two recombinant proteins that they expressed catalyzed the conversion of N-acetylserotonin to melatonin and one of three isomers in vitro. Unlike the ASMTs of Arabidopsis and rice, members of the three ASMT gene families in mulberry can catalyze the conversion of N-acetylserotonin to melatonin. This study provides new insights into the molecular mechanisms underlying melatonin and its isomers biosynthesis and expands our knowledge of melatonin isomer biosynthesis.
Highlights
Melatonin, a kind of biogenic indolamine, is a pineal secretory product and was first discovered in the bovine pineal gland in 1958 (Lerner et al, 1958)
UPLC-mass/mass spectrometry (MS/MS) was used to determine whether the retention time and the collision-induced ion fragments of mulberry samples in positive mode were consistent with those of the melatonin standard (Figure 1)
A similar result was confirmed in the other two mulberry varieties (“Jialing NO. 30” and “Zhongsang 5801”) (Supplementary Figure 3). These results suggested that MaASMT4 and MaASMT20 may be involved in the synthesis of melatonin isomer (MI-3), which was only detected in mulberry leaves
Summary
A kind of biogenic indolamine, is a pineal secretory product and was first discovered in the bovine pineal gland in 1958 (Lerner et al, 1958). Melatonin has been detected in other species, such as bacteria, algae, fungi, animals, and plants (Pöggeler et al, 1991; Hattori et al, 1995; Manchester et al, 1995; Sprenger et al, 1999). Melatonin was first reported in plants in 1995, and is contained in almost all tested plants, at concentrations ranging from pico- to nano-grams per gram of tissue (Garcia-Parrilla et al, 2009; Gomez et al, 2013).
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