A new balancing act: The many roles of melatonin and serotonin in plant growth and development

Melatonin and serotonin: Mediators in the symphony of plant morphogenesis

Melatonin and serotonin are important signaling and stress mitigating molecules that play important roles across growth and development in plants. Despite many well-documented responses, a systematic investigation of the entire metabolic pathway (tryptophan, tryptamine, and N-acetylserotonin) does not exist, leaving many open questions. The objective of this study was to determine the responses of Hypericum perforatum (L.) to melatonin, serotonin, and their metabolic precursors. Two well-characterized germplasm lines (#4 and 112) created by mutation and a haploid breeding program were compared to wild type to identify specific responses. Germplasm line 4 has lower regenerative and photosynthetic capacity than either wild type or line 112, and there are documented significant differences in the chemistry and physiology of lines 4 and 112. Supplementation of the culture media with tryptophan, tryptamine, N-acetylserotonin, serotonin, or melatonin partially reversed the regenerative recalcitrance and growth impairment of the germplasm lines. Quantification of phytohormones revealed crosstalk between the indoleamines and related phytohormones including cytokinin, salicylic acid, and abscisic acid. We hypothesize that melatonin and serotonin function in coordination with their metabolites in a cascade of phytochemical responses including multiple pathways and phytohormone networks to direct morphogenesis and protect photosynthesis in H. perforatum.

Mitochondria: Central Organelles for Melatonin's Antioxidant and Anti-Aging Actions

Melatonin, along with its metabolites, have long been known to significantly reduce the oxidative stress burden of aging cells or cells exposed to toxins. Oxidative damage is a result of free radicals produced in cells, especially in mitochondria. When measured, melatonin, a potent antioxidant, was found to be in higher concentrations in mitochondria than in other organelles or subcellular locations. Recent evidence indicates that mitochondrial membranes possess transporters that aid in the rapid uptake of melatonin by these organelles against a gradient. Moreover, we predicted several years ago that, because of their origin from melatonin-producing bacteria, mitochondria likely also synthesize melatonin. Data accumulated within the last year supports this prediction. A high content of melatonin in mitochondria would be fortuitous, since these organelles produce an abundance of free radicals. Thus, melatonin is optimally positioned to scavenge the radicals and reduce the degree of oxidative damage. In light of the "free radical theory of aging", including all of its iterations, high melatonin levels in mitochondria would be expected to protect against age-related organismal decline. Also, there are many age-associated diseases that have, as a contributing factor, free radical damage. These multiple diseases may likely be deferred in their onset or progression if mitochondrial levels of melatonin can be maintained into advanced age.

Phytomelatonin: a universal abiotic stress regulator

Melatonin, a derivative of tryptophan, was first detected in plant species in 1995 and it has been shown to be a diverse regulator during plant growth and development, and in stress responses. Recently, great progress has been made towards determining the detailed functions of melatonin in plant responses to abiotic stress. Melatonin priming improves plant tolerance to cold, heat, salt, and drought stresses through regulation of genes involved in the DREB/CBF, HSF, SOS, and ABA pathways, respectively. As a scavenger of free radicals, melatonin also directly detoxifies reactive oxygen species, thus alleviating membrane oxidation. Abiotic stress-inhibited photosynthesis is partially recovered and metabolites accumulate in the presence of melatonin, leading to improved plant growth, delayed leaf senescence, and increased stress tolerance. In this review, we summarize the interactions of melatonin with phytohormones to regulate downstream gene expression, protein stabilization, and epigenetic modification in plants. Finally, we consider the need for, and approaches to, the identification of melatonin receptors and components during signaling transduction pathways.

Melatonin in plant signalling and behaviour

Melatonin is an indoleamine neurotransmitter that has recently become well established as an important multi-functional signalling molecule in plants. These signals have been found to induce several important physiological responses that may be interpreted as behaviours. The diverse processes in which melatonin has been implicated in plants have expanded far beyond the traditional roles for which it has been implicated in mammals, which include sleep, tropisms and reproduction. These functions, however, appear to also be important melatonin mediated processes in plants, though the mechanisms underlying these functions have yet to be fully elucidated. Mediation or redirection of plant physiological processes induced by melatonin can be summarised as a series of behaviours including, among others: herbivore defence, avoidance of undesirable circumstances or attraction to opportune conditions, problem solving and response to environmental stimulus. As the mechanisms of melatonin action are elucidated, its involvement in plant growth, development and behaviour is likely to expand beyond the aspects discussed in this review and hold promise for applications in diverse fundamental and applied plant sciences including conservation, cryopreservation, morphogenesis, industrial agriculture and natural health products.

Melatonin: does it have utility in the treatment of haematological neoplasms?

Melatonin, discovered in 1958 in the bovine pineal tissue, is an indoleamine that modulates circadian rhythms and has a wide variety of other functions. Haematological neoplasms are the leading cause of death in children and adolescents throughout the world. Research has demonstrated that melatonin is a low-toxicity protective molecule against experimental haematological neoplasms, but the mechanisms remain poorly defined. Here, we provide an introduction to haematological neoplasms and melatonin, especially as they relate to the actions of melatonin on haematological carcinogenesis. Secondly, we summarize what is known about the mechanisms of action of melatonin in the haematological system, including its pro-apoptotic, pro-oxidative, anti-proliferative and immunomodulatory actions. Thirdly, we discuss the advantages of melatonin in combination with other drugs against haematological malignancy, as well as its other benefits on the haematological system. Finally, we summarize the findings that are contrary to the suppressive effects of melatonin on cancers of haematological origin. We hope that this information will be helpful in the design of studies related to the therapeutic efficacy of melatonin in haematological neoplasms. LINKED ARTICLES: This article is part of a themed section on Recent Developments in Research of Melatonin and its Potential Therapeutic Applications. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.16/issuetoc.

Plant signals during beetle (Scolytus multistriatus) feeding in American elm (Ulmus americana Planch)

American Elms were devastated by an outbreak of Dutch Elm Disease is caused by the fungus Ophiostoma novo-ulmi Brasier that originated in Asia and arrived in the early 1900s. In spite of decades of study, the specific mechanisms and disease resistance in some trees is not well understood. the fungus is spread by several species of bark beetles in the genus Scolytus, during their dispersal and feeding. Our objective was to understand elm responses to beetle feeding in the absence of the fungus to identify potential resistance mechanisms. A colony of Scolytus multistriatus was established from wild-caught beetles and beetles were co-incubated with susceptible or resistant American elm varieties in a controlled environment chamber. Beetles burrowed into the auxillary meristems of the young elm shoots. The trees responded to the beetle damage by a series of spikes in the concentration of plant growth regulating compounds, melatonin, serotonin, and jasmonic acid. Spikes in melatonin and serotonin represented a 7,000-fold increase over resting levels. Spikes in jasmonic acid were about 10-fold higher than resting levels with one very large spike observed. Differences were noted between susceptible and resistant elms that provide new understanding of plant defenses.

Low-Energy Electron Interaction with Melatonin and Related Compounds

The electron attaching properties and fragmentation of temporary negative ions of melatonin and its biosynthetic precursor tryptophan are studied in vacuo using dissociative electron attachment (DEA) spectroscopy. The experimental findings are interpreted in silico with the support of Hartree-Fock and density functional theory calculations of empty orbital energies and symmetries, and evaluation of the electron affinities of the indolic molecules under investigation. The only fragment anions formed by DEA to melatonin at incident electron energies below 2 eV are associated with the elimination of a hydrogen atom (energetically favored from the NH site of the pyrrole ring, leaving the ring intact) or a CH₃^· radical from the temporary molecular negative ion. Opening of the pyrrole ring of melatonin is not detected over the whole electron energy range of 0-14 eV. The DEA spectra of l- and d-tryptophan are almost identical under the present experimental conditions. The adiabatic electron affinity of melatonin is predicted to be -0.49 eV at the B3LYP/6-31+G(d) level, indicating that the DEA mechanism in melatonin is likely to be present in most life forms given the availability of low energy electrons in living systems in both plant and animal kingdoms. In particular, H atom donation usually associated with free-radical scavenging activity can be stimulated by electron attachment and N-H bond cleavage at electron energies around 1 eV.

Melatonin Natural Health Products and Supplements: Presence of Serotonin and Significant Variability of Melatonin Content

STUDY OBJECTIVES

Melatonin is an important neurohormone, which mediates circadian rhythms and the sleep cycle. As such, it is a popular and readily available supplement for the treatment and prevention of sleep-related disorders including insomnia and jet lag. This study quantified melatonin in 30 commercial supplements, comprising different brands and forms and screened supplements for the presence of serotonin.

METHODS

A total of 31 supplements were analyzed by ultraperformance liquid chromatography with electrochemical detection for quantification of melatonin and serotonin. Presence of serotonin was confirmed through analysis by ultraperformance liquid chromatography with mass spectrometry detection.

RESULTS

Melatonin content was found to range from -83% to +478% of the labelled content. Additionally, lot-to-lot variable within a particular product varied by as much as 465%. This variability did not appear to be correlated with manufacturer or product type. Furthermore, serotonin (5-hydroxytryptamine), a related indoleamine and controlled substance used in the treatment of several neurological disorders, was identified in eight of the supplements at levels of 1 to 75 μg.

CONCLUSIONS

Melatonin content did not meet label within a 10% margin of the label claim in more than 71% of supplements and an additional 26% were found to contain serotonin. It is important that clinicians and patients have confidence in the quality of supplements used in the treatment of sleep disorders. To address this, manufacturers require increased controls to ensure melatonin supplements meet both their label claim, and also are free from contaminants, such as serotonin.

COMMENTARY

A commentary on this article appears in this issue on page 163.

Melatonin in Plants and Plant Culture Systems: Variability, Stability and Efficient Quantification

Despite growing evidence of the importance of melatonin and serotonin in the plant life, there is still much debate over the stability of melatonin, with extraction and analysis methods varying greatly from lab to lab with respect to time, temperature, light levels, extraction solvents, and mechanical disruption. The variability in methodology has created conflicting results that confound the comparison of studies to determine the role of melatonin in plant physiology. We here describe a fully validated method for the quantification of melatonin, serotonin and their biosynthetic precursors: tryptophan, tryptamine and N-acetylserotonin by liquid chromatography single quadrupole mass spectrometry (LC-MS) in diverse plant species and tissues. This method can be performed on a simple and inexpensive platform, and is both rapid and simple to implement. The method has excellent reproducibility and acceptable sensitivity with percent relative standard deviation (%RSD) in all matrices between 1 and 10% and recovery values of 82-113% for all analytes. Instrument detection limits were 24.4 ng/mL, 6.10 ng/mL, 1.52 ng/mL, 6.10 ng/mL, and 95.3 pg/mL, for serotonin, tryptophan, tryptamine, N-acetylserotonin and melatonin respectively. Method detection limits were 1.62 μg/g, 0.407 μg/g, 0.101 μg/g, 0.407 μg/g, and 6.17 ng/g respectively. The optimized method was then utilized to examine the issue of variable stability of melatonin in plant tissue culture systems. Media composition (Murashige and Skoog, Driver and Kuniyuki walnut or Lloyd and McCown's woody plant medium) and light (16 h photoperiod or dark) were found to have no effect on melatonin or serotonin content. A Youden trial suggested temperature as a major factor leading to degradation of melatonin. Both melatonin and serotonin appeared to be stable across the first 10 days in media, melatonin losses reached a mean minimum degradation at 28 days of approximately 90%; serotonin reached a mean minimum value of approximately 60% at 28 days. These results suggest that melatonin and serotonin show considerable stability in plant systems and these indoleamines and related compounds can be used for investigations that span over 3 weeks.

Serotonin: An ancient molecule and an important regulator of plant processes

Serotonin is an ancient indoleamine that was presumably part of the life cycle of the first prokaryotic life forms on Earth millions of years ago where it functioned as a powerful antioxidant to combat the increasingly oxygen rich atmosphere. First identified as a neurotransmitter signaling molecule in mammals, it is ubiquitous across all forms of life. Serotonin was discovered in plants many years after its discovery in mammals; however, it has now been confirmed in almost all plant families, where it plays important roles in plant growth and development, including functions in energy acquisition, seasonal cycles, modulation of reproductive development, control of root and shoot organogenesis, maintenance of plant tissues, delay of senescence, and responses to biotic and abiotic stresses. Despite its widespread presence and activity, there are many questions which remain unanswered about the role of serotonin in plants including the mode of signaling and receptor identity as well as the mechanisms of action of this important molecule. This review provides an overview of the role of serotonin in plant life and their ability to adapt.

Constitutive photomorphogenesis protein 1 (COP1) and COP9 signalosome, evolutionarily conserved photomorphogenic proteins as possible targets of melatonin

The ubiquitin proteasome system has been proposed as a possible mechanism involved in the multiple actions of melatonin. COP1 (constitutive photomorphogenesis protein 1), a RING finger-type ubiquitin E3 ligase formerly identified in Arabidopsis, is a central switch for the transition from plant growth underground in darkness (etiolation) to growth under light exposure (photomorphogenesis). In darkness, COP1 binds to photomorphogenic transcription factors driving its degradation via the 26S proteasome; blue light, detected by cryptochromes, and red and far-red light detected by phytochromes, negatively regulate COP1. Homologues of plant COP1 containing all the structural features present in Arabidopsis as well as E3 ubiquitin ligase activity have been identified in mice and humans. Substrates for mammalian (m) COP1 include p53, AP-1 and c-Jun, p27(Kip1) , ETV1, MVP, 14-3-3σ, C/EBPα, MTA1, PEA3, ACC, TORC2 and FOXO1. This mCOP1 target suggests functions related to tumorigenesis, gluconeogenesis, and lipid metabolism. The role of mCOP1 in tumorigenesis (either as a tumor promoter or tumor suppressor), as well as in glucose metabolism (inhibition of gluconeogenesis) and lipid metabolism (inhibition of fatty acid synthesis), has been previously demonstrated. COP1, along with numerous other ubiquitin ligases, is regulated by the COP9 signalosome; this protein complex is associated with the oxidative stress sensor Keap1 and the deubiquitinase USP15. The objective of this review was to provide new information on the possible role of COP1 and COP9 as melatonin targets. The hypothesis is based on common functional aspects of melatonin and COP1 and COP9, including their dependence on light, regulation of the metabolism, and their control of tumor growth.