by Shivani Nagar, Ajay Arora, Krishna Kumar G, Dipanker Barman, Rajkumar Dhakar, Sandeep Kumar
Melatonin (N-acetyl-5-methoxytryptamine) is a tryptophan derived indolic compound. Because of its widespread presences, conserved structure and diverse action it is been considered as new master regulator in living beings. In plants melatonin is establishing itself as new signaling molecule and plant growth regulator. It controls diverse function and physiological processes such as circadian rhythm, seed germination, tropic responses, growth promotion, rooting induction, cellular and cytoskeleton development, reproduction, abiotic and biotic stress protection and senescence regulation. Here in this article we have discussed about the discovery of molecule, its biosynthesis and role in plants with special emphasis on its role under as stress protectant.
In year 1958, Lerner and coworkers first isolated an active factor from beef pineal gland. This compound was able to cause aggregation of melanin granules present in melanocytes of isolated frog skin. They named it as melatonin because of its ability to lighten skin color by inhibiting melanocyte stimulating hormone. Next year Lerner and coworkers successfully isolate melatonin from peripheral nerve of humans, monkey and cow using improved extraction methods. Later its presence was established in organisms as different as bacteria, algae, protozoans, fungi, dinoflagellates, invertebrates and vertebrates, vascular and non-vascular plants (Mayer et al., 1997; Arnao, 2014). Later when its biosynthesis pathway was exploited it was found that serotonin was precursor of melatonin. Presence of serotonin was well established in plant (West, 1958; Udenfriend et al., 1959) which increased the probability of presence of melatonin in plants. In higher plants, in 1993 van Tassel and O’Neill first published abstract in plant physiology congress on identification of endogenous melatonin in Pharbitis nil as potential dark signal in plants. In 1995 two independent scientific groups working in Japan and Germeny published full report on isolation of melatonin from plants (Hatoori et al., (1995); Dubbles et al., (1995)). It was discovered as source of melatonin for human. In animals it controls varied physiological process such as circadian clock, mood, sexual behavior, sleep regulation etc. Melatonin content in plant tissues varies from picograms to micrograms per gram of tissue weight. Consumption of food rich in melatonin can increase melatonin level in blood. Melatonin is used for treatment of depression, obesity, insomnia, reproductive disorders, aging, skin lightening, as antioxidant, antiaging, antidepressant; antioxidant, circadian rhythm regulator, immunity inducer. In recent time many studies have focused on developing plants with improved melatonin for food and nutraceutical purpose as substitute for externally taken melatonin.
Biosynthesis of melatonin
Melatonin is synthesized from tryptophan amino acid both in plants and animals. Two pathways of melatonin biosynthesis have been deciphered in living beings. In plants tryptophan is first decarboxylated by tryptophan decarboxylase forming tryptamine followed by hydroxylation forming serotonin (5-hydroxy tryptamine) (Figure 1; Blue arrow). Methylation of serotonin gives 5- Methoxytryptamine and via its acetylation reaction by serotonin N-acetyltransferase melatonin is formed. In another pathway serotonin can also be acetlylated to N- acetylserotonine and followed by methylation forming melatonin. In second pathway which is present in animal’s serotonin is formed by first hydroxylation followed by decarboxylation of tryptophan (Red arrow). Serotonin is acetlylated to N- acetylserotonine and followed by methylation forming melatonin.
Role of melatonin in plant growth and stress
Phytomelatonin is involved in multiple physiological process such as it regulates the growth of epicotyls, hypocotyls, coleoptiles, roots, shoots, tropic response, seed germination, lateral and adventitious roots induction, delaying induced leaf senescence similar to auxin activity and stress tolerance. In coleoptiles of canary grass, wheat, oat and barley 10 -55% of relative auxinic activity was observed for phototropic response of coleoptiles (Ruiz et al., 2005). Kolar et al., 1997 studied the change in melatonin level in Chenopodiumrubrum L. and first time reported the oscillating behavior of melatonin i.e. low melatonin levels during day and high levels during night which was similar to studies in animals. Later many workers confirmed it that endogenous content of melatonin shows circadian pattern. Thus this molecule may have role in regulation of physiological and metabolic process showing circadian rhythm. Role of melatoinin in reproductive development was studied by Murch andSaxena (2002), in Hypericumperforatum L on hormonal profiling they reported higher levels of IAA, serotonin and melatonin during flower development. Regeneration potential of isolated anthers containing high levels of melatonin was also more. Concentration dependent response for delaying the loss in chlorophyll content was observed on incubation of barley leaves in melatonin. Incubation in 1 mm melatonin was most effective in delaying the senescence (Arnao and Ruiz, 2009).
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