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.     

Inactivation of peroxidase by hydrogen peroxide and its protection by a reductant agent

Hydrogen peroxide, the oxidant substrate of peroxidase, is also an inactivating agent of this enzyme. The reductant substrates protect the enzyme from the inactivating process. A reaction mechanism is proposed, in which two competitive routes exist for Compound I of peroxidase; one catalytic and one inactivating. The analytical solution produced at the end of the reaction supports the proposed mechanism and shows the dependence between the number of turnovers of the enzyme (r) and the ratio of both substrates.     

A kinetic study on the suicide inactivation of peroxidase by hydrogen peroxide

In the absence of reductant substrates, and with excess H2O2, peroxidase (donor: hydrogen-peroxide oxidoreductase, EC 1.11.1.7) shows the kinetic behaviour of a suicide inactivation, H2O2 being the suicide substrate. From the complex (compound I-H2O2), a competition is established between two catalytic pathways (the catalase pathway and the compound III-forming pathway), and the suicide inactivation pathway (formation of inactive enzyme). A kinetic analysis of this system allows us to obtain a value for the inactivation constant, ki = (3.92 +/- 0.06) x 10(-3) x s-1. Two partition ratios (r), defined as the number of turnovers given by one mol of enzyme before its inactivation, can be calculated: (a) one for the catalase pathway, rc = 449 +/- 47; (b) the other for the compound III-forming pathway, rCoIII = 2.00 +/- 0.07. Thus, the catalase activity of the enzyme and, also, the protective role of compound III against an H2O2-dependent peroxidase inactivation are both shown to be important.     

Melatonin: a growth-stimulating compound present in lupin tissues

Melatonin (N-acetyl-5-methoxi-tryptamine), a well-known animal hormone synthetised by the pineal gland, plays a key role in the circadian rhythm of vertebrates. An exhaustive bibliographical revision of studies on melatonin in plants published since 1990 points to very few studies (around 20), of which only 8 have a clear plant physiological focus. The data presented in this study demonstrate that melatonin plays a physiological role in plant tissues. Melatonin is seen to be a molecule that promotes vegetative growth in etiolated Lupinus albus L. hypocotyls, in a similar way to IAA. The measurements of melatonin and IAA in lupin hypocotyls by high-performance liquid chromatography with electrochemical detection, and their identification by tandem mass spectrometry, point to a different distribution of these molecules in etiolated hypocotyls.Abbreviations IAA Indole-3-acetic acid - MEL Melatonin - HPLC–EC High-performance liquid chromatography with electrochemical detection - MS/ESI+ Tandem mass spectrometry in positive electrospray ionization mode     

Catalase-like oxygen production by horseradish peroxidase must predominantly be an enzyme-catalyzed reaction

The biochemical mechanisms underlying the inhibitory effects of lycopene, the main tomato carotenoid, on the growth of cancer cells are largely unknown. It has been hypothesized that lycopene derivatives may act as ligands for a nuclear receptor in analogy to retinoic acid, the hormone derived from beta-carotene. The inhibition of human mammary cancer (MCF-7) cell growth and the transactivation of the retinoic acid receptor (RAR) reporter gene by synthetic acyclo-retinoic acid, the open chain analog of retinoic acid, was compared to the effects of lycopene and retinoic acid in the same systems. Acyclo-retinoic acid activated the DR-5 retinoic acid response element with a approximately 100-fold lower potency than retinoic acid. This effect was independent of cotransfection with the RARalpha receptor. Lycopene exhibited only very modest activity in this system. In contrast to the results from the transactivation studies, acyclo-retinoic acid, retinoic acid, and lycopene inhibited cell growth with a similar potency. Preincubation with each of the three compounds slowed down cell cycle progression from G1 to S phase. In summary, acyclo-retinoic acid inhibited cancer cell growth and interacted with RAR. However, it exhibited low affinity for RAR and a correspondingly low efficacy in activating this receptor, indicating that RAR does not mediate the growth inhibitory effect of the compound. In addition, the concentrations of acyclo-retinoic acid and of lycopene required for inducing inhibition of cell growth were similar, suggesting that acyclo-retinoic acid is unlikely to be the active metabolite of lycopene.     

Posters

Introduction  Fine needle aspiration (FNA) cytology of the thyroid is a well-established test in the clinical work-up of patients with solitary nodules of the thyroid. Thyroid FNA does however have limitations and audit of diagnostic performance is important.
Methods  The histopathology archives of the Royal Victoria Hospital were searched for all thyroid resections and the histopathological diagnosis was correlated with the pre-operative cytological diagnosis, where available. Special emphasis was placed on the accuracy of tumour diagnosis.
Results  A total of 173 cases were identified during the 2-year period, of these 93 had available pre-operative FNA. A total of 57 tumours were identified. A small number (six of 57) of significant discrepancies were identified. These included a malignant lymphoma diagnosed as Hashimoto's thyroiditis, a metastasis which the FNA had suggested was a medullary carcinoma and an insular carcinoma diagnosed as medullary carcinoma on FNA. False positives included a colloid cyst diagnosed as suspicious of malignancy and a cytological diagnosis of papillary carcinoma not confirmed on histology.
Discussion  At present, the majority of thyroid FNAs in our clinics are performed by surgeons and material is not routinely available for immunocytochemistry. In spite of these limitations, there were few major discrepancies. These might be reduced if pathologist aspirators were able to perform FNAs and collect material for further studies, where necessary. This would allow identification of medullary carcinomas and malignant lymphomas.
Conclusion  FNA of thyroid lesions is a useful investigation in our clinical setting, however, some areas of potential for improvement have been identified.     

Influence of peroxides, ascorbate and glutathione on germination and growth in Lupinus albus L.

Lupinus albus L. seeds were treated with different concentrations (from 10 μM to 50 mM) of H2O2, m-chloroperoxybenzoic acid (mCPBA), ascorbate (ASC) and glutathione (GSH). The efficiency as inhibitors on germination and on the subsequent growth of the hypocotyl was mCPBA > GSH > ASC = H2O2, which suggest that inhibitory efficiency was dependent on the compound per se rather than on its redox nature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Growth activity,rooting capacity,and tropism: three auxinic precepts fulfilled by melatonin

Plant melatonin appears to be a multiregulatory molecule with multiple functions similar to those observed in animals. It induces growth in stems and stimulates root generation. It is also able to delay senescence by protecting photosynthetic systems and related processes. One of the most studied actions of melatonin is its effect on biotic and abiotic stresses in the plant, such as that produced by drought, extreme temperatures, chemical pollution, UV radiation, etc. Recent data have demonstrated its role as a modulator of gene expression in plants. In this review, we compare studies which show that melatonin behaves in a similar way to auxin, and present data that relate the physiological responses produced by melatonin with the action of auxin, such as promoting/inhibiting growth activity and rooting capacity. In addition, for the first time, the data presented demonstrate the possible involvement of melatonin in the tropic response of roots. The possible role of melatonin as a plant regulator and its relationship with auxin action and the signaling molecule nitric oxide is presented and discussed in a hypothetical model.