The Physiological Function of Melatonin in Plants

Melatonin (N-acetyl-5-methoxytryptamine), a well-known animal hormone, was discovered in plants in 1995 but very little research into it has been carried out since. It is present in different parts of all the plant species studied, including leaves, stems, roots, fruits and seeds. This brief review will attempt to provide an overview of melatonin (its discovery, presence and functions in different organisms, biosynthetic route, etc.) and to compile a practically complete bibliography on this compound in plants. The common biosynthetic pathways shared by the auxin, indole-3-acetic, and melatonin suggest a possible coordinated regulation in plants. More specifically, our knowledge to date of the role of melatonin in the vegetative and reproductive physiology of plants is presented in detail. The most interesting aspects for future physiological studies are presented.Key Words: antioxidant, auxin, flowering, growth, IAA, melatonin, plant hormone, reproductive development, rooting, vegetative developmentMelatonin (N-acetyl-5-methoxytryptamine), an “old friend” and well known as an animal hormone but “new” to plant biology is arousing great interest due to its broad distribution in the biological kingdom and the recent data on its possible physiological role in plants. Many studies on melatonin, as a phytochemical compound with potentially interesting health-related properties, have recently appeared, but no more than 15–20 papers with a plant physiological focus have been published since 1995. Besides mentioning the most interesting data on melatonin related with plants, this review will hopefully trigger more studies into this molecule to deepen our understanding of the different physiological roles that it might play in plants. We shall briefly look at the well-known function of melatonin in vertebrates, its discovery in plants and other organisms, and its presence in plants as a possible medicinal phytochemical. The joint biosynthetic pathways of melatonin and the auxin indole-3-acetic acid (IAA) will be described. Thus, we reveal the new and emerging field of melatonin studies in plants, the limited physiological data available and its possible role in plants.     

Melatonin in Plants: More Studies are Necessary

Melatonin (N-acetyl-5-methoxytryptamine) is a biogenic indoleamine structurally related with other important substances such as tryptophan, serotonin, indole-3-acetic acid (IAA). In mammals, birds, reptiles and fish melatonin is a biological modulator of several timing (circadian) processes such as mood, sleep, sexual behavior, immunological status, etc. Since its discovery in plants in 1995 several physiological roles, including a possible role in flowering, circadian rhythms and photoperiodicity and as growth-regulator have been postulated. Recently, a possible role in rhizogenesis in lupin has also been proposed. Here, these actions of melatonin in plant development are commented on and some other interesting recent data concerning melatonin in plants are also discussed. The need for more investigation into melatonin and plants is presented as an obvious conclusion.Key Words: antioxidant, auxin, ethylene, flowering, growth, IAA, melatonin, rhizogenesisMelatonin (N-acetyl-5-methoxytryptamine) is well known in human and animal physiology, but is an unknown player in the physiology of plants. Many studies have clearly demonstrated its presence in different parts of plants such as the root, stem, leaf, flower, fruit and seed.^1–3 In addition to its phytochemical interest (natural melatonin is absorbed by the human digestive tract), this compound has aroused attention as a possible signal molecule in plant physiology.^4,5 From it discovery in plants in 1995, some authors have postulated many physiological roles for melatonin, although, in general, research into melatonin in plants is clearly insufficient. Only the possible role of melatonin in flowering and as growth promotor have been studied with some detail. As regards the former, the studies of Kolar''s group on the role of melatonin as plant rhythm regulator provided interesting data, pointing to melatonin''s action in the later stages of the flowering process.^6,7 Melatonin seems to have a more obvious effect in the growth process of some species, as has been demonstrated by our group. Our data showed that melatonin has a growth-promoting effect on aerial organs (epi- and hypocotyls, coleoptiles) and a growth-inhibitory effect on roots, in a similar way to auxins.^8,9 Other authors, too, have provided evidence on the possible growth-promoting activity of melatonin in Glycyrrhiza uralensis, which doubled its melatonin content in roots in the 3–6 month development period.¹⁰ A more recent paper, presented data concerning the effect that melatonin has on the rhizogenesis process. Melatonin produces and/or activates the generation of root primordia and their subsequent growth into lateral roots and adventitious roots in Lupinus albus.¹¹ Studies on melatonin in vegetative plant development pointed to a relationship between IAA and melatonin but more data are necessary to identify the particular interconnection. The most recent data in this respect, established the effect of melatonin on the enzymatic activity of ACC oxidase in hypocotyls and roots of Lupinus albus, pointing to the possible regulation of ethylene production in these vegetative organs.¹²One aspect that has slowed down research into melatonin in plants is the difficulty involves in its detection, identification and measurement of melatonin in plants. Because the high degree of interference caused by melatonin-immunodetection kits using plant samples, the habitual use of the liquid chromatography-tandem mass spectrometry (LC-MS/MS) has been crucial.¹³ The use of this sophisticated technique for melatonin identification combined with measuring levels by means of liquid chromatography with electrochemical or fluorescence detection seem to be an efficient methodological option. In this respect, studies such as that recently published by Cao et al. (2006),¹⁴ where a robust method for determining melatonin, serotonin and auxin in plant samples using LC-MS/MS was presented, clearly contribute to improving accurate research into melatonin in plants.Future studies on melatonin in plant physiology should take metabolic and molecular aspects into consideration. Thus, the participation of different enzymatic activities in melatonin biosynthesis and catabolism in plants appears to be an interesting challenge.⁵ Also, the presence of melatonin receptor(s) in plant samples would strongly suggest a role for melatonin. Other interesting aspects to be investigated are: the possible tissular transport of melatonin, its action as plant cell protector due to its excellent antioxidative properties, and its involvement in particular physiological processes such as germination, cell growth, senescence, flowering, etc. Lastly, we must not forget the involvement of melatonin in stress processes in animal cells, which may be mirrored to some extent in plant cells. As can be seen, much remains to be done.     

植物褪黑素及其抗逆性研究

褪黑素(N-乙酰-5-甲氧基色胺)是脊椎动物的松果体产生的吲哚类激素,主要参与动物昼夜节律调节.现已证实褪黑素在高等植物中也普遍存在,但对其功能的研究还不甚深入.目前,植物中褪黑素的可能功能包括清除自由基、调节光周期、参与生长调节等.本文简述了植物中褪黑素的研究概况、含量及其合成途径,重点综述了其在提高植物抗逆性方面的功能,并对其研究前景进行展望.     

Exogenous Melatonin Protects Canola Plants from Toxicity of Excessive Copper

Physiological mechanisms of canola (Brassica napus L., cv. Westar) plant protection afforded by melatonin (at 0.1–100 μM) from copper salts (at 10–100 μM) were studied. Plants were cultivated on Hoagland–Snyder medium. At the age of 5 weeks, they were subjected to melatonin, copper sulfate, or their combination for 7 days. It was found that excessive copper in a nutrient medium inhibited the dry biomass accumulation against the control by 25–85%. Copper sulfate diminished the content of chlorophylls and carotenoids and functional activity of the thylakoid membranes in the chloroplasts. It increased 2.0–2.5 times the lipid peroxidation (LPO) intensity and the proline level up to 20 times. Melatonin reduced the changes caused by copper, and the degree of the protection depended on melatonin and CuSO₄ concentrations. It was found that melatonin decreased the oxidative stress and proline accumulation, both induced by CuSO₄. At first, we established the positive correlation (with the coefficient 0.8240) between the level of oxidative stress and proline content in the presence of CuSO₄. Possible mechanisms of protection by melatonin and its biological role under conditions of technogenic stress are discussed.     

Appearance of New Transport Capacity in Wounded Plants

Gersani, M. 1985. Appearance of new transport capacity in woundedplants.—J. exp. Bot. 36: 1809–1816. Bean hypocotyls were cut so that only a narrow transverse bridgeconnected the shoot and the root. The purpose of the work wasto follow the processes of new, regenerative, vascular differentiationas expressed by the transport capacity of the wounded regions.Rapid transport of sucrose, auxin and gibberellin appeared asan exponential function of time. This transport probably expressesthe functioning of newly differentiated sieve tubes and it comesclose to the level of intact plants about 3 d after the plantsare wounded. There was also a less rapid transport that appearedearlier, within 1 d of wounding, that was specific to auxin.The results supply critical parameters concerning new vasculardifferentiation and support the hypothesis that it is inducedand oriented by the flow of auxin. Key words: Regeneration, transport, auxin     

Exogenic Melatonin Reduces the Toxic Effect of Polymetallic Stress on Barley Plants

Doklady Biochemistry and Biophysics - The effect of melatonin on growth parameters, the photochemical activity of photosystem II (PS II), the content of the main photosynthetic pigments, and lipid...     

褪黑素与植物抗逆性研究进展

褪黑素广泛存在于植物体内,对植物生长和发育方面有着重要的作用。其中,最为人们关注的是褪黑素在植物抵御干旱、高盐、极端温度和氧化胁迫等不良影响中所发挥的重要功能。随着人们对褪黑素研究的深入,褪黑素在植物体中发挥的作用和功能也更加明确,国内外在褪黑素与植物抗逆性关系的研究也取得了丰硕的成果。主要从植物体中褪黑素的合成途径、褪黑素在植物抗性反应中的作用以及内源褪黑素含量与逆境等方面进行了综述,并提出今后的研究方向。可以归纳为:植物体内褪黑素的合成机制与动物体内相似,但是确切的生物合成途径和具体的合成位点尚未明确;外源褪黑素处理能够增强植物抵御逆境的能力;逆境胁迫能够促进植物自身合成褪黑素,过表达褪黑素合成相关基因能够增加植物体内褪黑素的含量。     

Melatonin Promotes Rice Seed Germination under Drought Stress by Regulating Antioxidant Capacity

Drought stress is a serious threat to the germination of plant seeds and the growth of seedlings. Melatonin has been proven to play an important role in alleviating plant stress. However, its effect on seed germination under drought conditions is still poorly understood. Therefore, we studied the effects of melatonin on rice seed germination and physiological characteristics under drought stress. Rice seeds were treated with different concentrations of melatonin (i.e., 0, 20, 100, and 500 μM) and drought stress was simulated with 5% polyethylene glycol 6000 (PEG6000). The results showed that 100 μM melatonin can effectively improve the germination potential, rate and index; the vigor index of rice seeds; and the length of the shoot and root. In addition, that treatment also increased the activity of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), and reduced the content of malondialdehyde (MDA). The grey relational grade between the shoot MDA content and the melatonin seed-soaking treatment was the highest, which could be useful for evaluating the effect of melatonin on drought tolerance. Two-way analysis of variance showed that the effect of single melatonin treatment on rice seeds was more significant than that of single drought stress and interaction treatment of drought and melatonin (p < 0.05). The subordinate function results showed that 100 μM melatonin significantly improved the germination and physiological indexes of rice seeds and effectively alleviated the adverse effects of drought stress on rice seedlings. The results helped to improve the understanding of the morphological and physiological involvement of melatonin in promoting seed germination and seedling development under drought stress.     

褪黑素预处理对高温下猕猴桃幼苗抗氧化能力的影响

为了探明外源褪黑素对高温胁迫下猕猴桃幼苗耐热性的影响,以盆栽‘秦美’猕猴桃幼苗为材料,进行根灌褪黑素溶液和45℃高温胁迫处理,并测定相关抗逆生理指标。结果表明：(1)与高温对照相比,褪黑素预处理显著降低了猕猴桃幼苗叶片相对电导率和丙二醛含量,提高了脯氨酸含量以及过氧化氢酶和过氧化物酶活性,且褪黑素溶液浓度在100~200 μmol·L^-1时效果较好。(2)褪黑素预处理显著增加了叶片中总酚、总黄酮和总黄烷醇含量,且幼苗的抗氧化能力(ABTS法、FRAP法和DPPH法)在胁迫过程中显著高于高温对照。研究认为,外源褪黑素可以通过提高植株体内抗氧化酶活性,非酶抗氧化物质如酚类物质和类黄酮物质含量以及渗透调节物质含量来增强植物的抗氧化能力,从而有效缓解高温对于猕猴桃幼苗的伤害,增强猕猴桃幼苗的耐热性。     

Violaxanthin Cycle Pigment Contents in Potato and Tobacco Plants with Genetically Reduced Photosynthetic Capacity

The influence of photosynthetic activity on the light-dependent adaptation of the pool size of the violaxanthin cycle pigments (violaxanthin + antheraxanthin + zeaxanthin) was studied in leaves of wild-type and transgenic potato (Solanum tuberosum L.) and tobacco (Nicotiana tabacum L.) plants. The genetically manipulated plants expressed an antisense mRNA coding for the chloroplastic fructose-bisphosphatase. Chl fluorescence quenching analysis revealed that the transformed plants exhibited a greatly impaired electron transport capacity. Light-limited and light-saturated non-photochemical quenching was strongly enhanced in the mRNA antisense potato plants. After 7 d of adaptation at various high photosynthetic photon flux densities (PPFDs), the violaxanthin cycle pool size increased, with a progressive elevation in PPFD. The pool size was higher for transgenic potatoes than for wild-type plants at all PPFDs. This difference vanished when pool size was correlated with the PPFD in excess of photosynthesis, as indicated by the epoxidation state of the violaxanthin cycle. Contrasting results were obtained for tobacco; in this species, photosynthetic activity did not affect the pool size. We conclude that regulatory mechanisms exist in potato, by which photosynthetic activity can influence the violaxanthin cycle pool size. Furthermore, evidence is provided that this adaptation of the pool size may contribute to an improved photoprotection of the photosynthetic apparatus under high-light conditions. However, tobacco plants seem to regulate their pool size independently of photosynthetic activity.     

褪黑素调节烟草丁香假单胞菌抗性的功能研究

为分析褪黑素(N-乙酰-5-甲氧基色胺)在植物先天免疫中的功能及调控机理,研究以病原菌丁香假单胞杆菌(Pseudomonas syringae pv.tomato DC3000,Pst DC3000)—烟草互作系统为模型,检测了病原菌侵染对烟草褪黑素相关基因表达的影响,并探讨了褪黑素对植物叶片病原菌生长以及气孔开度和活性氧自由基(reactive oxygen species,ROS)含量的影响以及调控机理。结果表明:(1)Pst DC3000处理提高了烟草褪黑素合成(NtSNAT1)和受体(NtPMTR1)基因表达,且外源褪黑素处理降低了叶片中的病原菌含量。(2)与野生型植物相比,过表达大豆GmSNAT1基因显著提高了转基因烟草中内源褪黑素含量和NtPMTR1的表达,且转基因烟草叶片中的Pst DC3000菌落数显著下降。(3)外源褪黑素和细菌鞭毛蛋白多肽flg22处理诱导了野生型和转基因烟草保卫细胞中ROS产生和气孔关闭,且转基因植物对褪黑素和flg22诱导的气孔关闭和ROS产生比野生型烟草更加敏感。综上所述,研究表明褪黑素可能通过受体NtPMTR1介导的信号途径促进保卫细胞ROS产生,诱导气孔关闭,从而降低病原菌Pst DC3000的入侵。     

外源褪黑素对盐胁迫下紫花苜蓿幼苗抗氧化能力以及光合作用效率的影响

紫花苜蓿为多年生豆科优良牧草,土壤盐碱化是其产量的重要限制因素.该研究以'中苜1号'紫花苜蓿为材料,在盐胁迫浓度和褪黑素浓度筛选试验基础上,设置盐(150 mmol/L NaCl)和褪黑素(30、50、80 μmol/L)单独及复合处理,采用水培根灌褪黑素的方法,探究外源褪黑素对盐胁迫下紫花苜蓿幼苗生长特性、膜透性、渗...     

Dilution of Liquid Rhizobium Cultures To Increase Production Capacity of Inoculant Plants

Experiments were undertaken to test whether peat-based legume seed inoculants, which are prepared with liquid cultures that have been deliberately diluted, can attain and sustain acceptable numbers of viable rhizobia. Liquid cultures of Rhizobium japonicum and Rhizobium phaseoli were diluted to give 10⁸, 10⁷, or 10⁶ cells per ml, using either deionized water, quarter-strength yeast-mannitol broth, yeast-sucrose broth, or yeast-water. The variously diluted cultures were incorporated into gamma-irradiated peat, and the numbers of viable rhizobia were determined at intervals. In all of the inoculant formulations, the numbers of rhizobia reached similarly high ceiling values by 1 week after incorporation, irrespective not only of the number of cells added initially but also of the nature of the diluent. During week 1 of growth, similar multiplication patterns of the diluted liquid cultures were observed in two different peats. Numbers of rhizobia surviving in the various inoculant formulations were not markedly different after 6 months of storage at 28°C. The method of inoculant preparation did not affect the nitrogen fixation effectiveness of the Rhizobium strains.     

用清除有机自由基DPPH法评价植物抗氧化能力

几种抗氧化剂的浓度与其清除1,1-二苯基苦基苯肼(DPPH)能力呈显著的线性相关.不同抗氧化剂清除DPPH能力差异明显.抗坏血酸与DPPH反应的灵敏性高于其抑制肾上腺素氧化的能力.用DPPH法和亚油酸氧化法同时测定了生长在不同光强下植物叶片抗氧化能力的变化,两种方法所得结论相一致.结果表明清除有机自由基法是一种快速、简便、灵敏的评估植物抗氧化能力的可行方法.     

Melatonin

Melatonin, originally discovered as a hormone of the pineal gland, is produced by bacteria, protozoa, plants, fungi, invertebrates, and various extrapineal sites of vertebrates, including gut, skin, Harderian gland, and leukocytes. Biosynthetic pathways seem to be identical. Actions are pleiotropic, mediated by membrane and nuclear receptors, other binding sites or chemical interactions. Melatonin regulates the sleep/wake cycle, other circadian and seasonal rhythms, and acts as an immunostimulator and cytoprotective agent. Circulating melatonin is mostly 6-hydroxylated by hepatic P450 monooxygenases and excreted as 6-sulfatoxymelatonin. Pyrrole-ring cleavage is of higher importance in other tissues, especially the brain. The product, N1-acetyl-N2-formyl-5-methoxykynuramine, is formed by enzymatic, pseudoenzymatic, photocatalytic, and numerous free-radical reactions. Additional metabolites result from hydroxylation and nitrosation. The secondary metabolite, N1-acetyl-5-methoxykynuramine, supports mitochondrial function and downregulates cyclooxygenase 2. Antioxidative protection, safeguarding of mitochondrial electron flux, and in particular, neuroprotection, have been demonstrated in many experimental systems. Findings are encouraging to use melatonin as a sleep promoter and in preventing progression of neurodegenerative diseases.     

Enriched random forests

Although the random forest classification procedure works well in datasets with many features, when the number of features is huge and the percentage of truly informative features is small, such as with DNA microarray data, its performance tends to decline significantly. In such instances, the procedure can be improved by reducing the contribution of trees whose nodes are populated by non-informative features. To some extent, this can be achieved by prefiltering, but we propose a novel, yet simple, adjustment that has demonstrably superior performance: choose the eligible subsets at each node by weighted random sampling instead of simple random sampling, with the weights tilted in favor of the informative features. This results in an 'enriched random forest'. We illustrate the superior performance of this procedure in several actual microarray datasets.     

Antioxidant Capacity of Melatonin on Preimplantation Development of Fresh and Vitrified Rabbit Embryos: Morphological and Molecular Aspects

Embryo cryopreservation remains an important technique to enhance the reconstitution and distribution of animal populations with high genetic merit. One of the major detrimental factors to this technique is the damage caused by oxidative stress. Melatonin is widely known as an antioxidant with multi-faceted ways to counteract the oxidative stress. In this paper, we investigated the role of melatonin in protecting rabbit embryos during preimplantation development from the potential harmful effects of oxidative stress induced by in vitro culture or vitrification. Rabbit embryos at morula stages were cultured for 2 hr with 0 or 10⁻³ M melatonin (C or M groups). Embryos of each group were either transferred to fresh culture media (CF and MF groups) or vitrified/devitrified (CV and MV groups), then cultured in vitro for 48 hr until the blastocyst stage. The culture media were used to measure the activity of antioxidant enzymes: glutathione-s-transferase (GST) and superoxide dismutase (SOD), as well as the levels of two oxidative substrates: lipid peroxidation (LPO) and nitric oxide (NO). The blastocysts from each group were used to measure the expression of developmental-related genes (GJA1, POU5F1 and Nanog) and oxidative-stress-response-related genes (NFE2L2, SOD1 and GPX1). The data showed that melatonin promoted significantly (P<0.05) the blastocyst rate by 17% and 12% in MF and MV groups compared to their controls (CF and CV groups). The GST and SOD activity significantly increased by the treatment of melatonin in fresh or vitrified embryos, while the levels of LPO and NO decreased (P<0.05). Additionally, melatonin considerably stimulated the relative expression of GJA1, NFE2L2 and SOD1 genes in MF and MV embryos compared to CF group. Furthermore, melatonin significantly ameliorated the reduction of POU5F1 and GPX1 expression induced by vitrification. The results obtained from the current investigation provide new and clear molecular aspects regarding the mechanisms by which melatonin promotes development of both fresh and vitrified rabbit embryos.