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

Views and perspectives on the indoleamines serotonin and melatonin in plants: past, present and future

In the decades since their discovery in plants in the mid-to-late 1900s, melatonin (N-acetyl-5-methoxytryptamine) and serotonin (5-methoxytryptamine) have been established as their own class of phytohormone and have become popular targets for examination and study as stress ameliorating compounds. The indoleamines play roles across the plant life cycle from reproduction to morphogenesis and plant environmental perception. There is growing interest in harnessing the power of these plant neurotransmitters in applied and agricultural settings, particularly as we face increasingly volatile climates for food production; however, there is still a lot to learn about the mechanisms of indoleamine action in plants. A recent explosion of interest in these compounds has led to exponential growth in the field of melatonin research in particular. This concept paper aims to summarize the current status of indoleamine research and highlight some emerging trends.

Melatonin: The Multifaceted Molecule in Plant Growth and Defense

Melatonin (MEL), a hormone primarily known for its role in regulating sleep and circadian rhythms in animals, has emerged as a multifaceted molecule in plants. Recent research has shed light on its diverse functions in plant growth and defense mechanisms. This review explores the intricate roles of MEL in plant growth and defense responses. MEL is involved in plant growth owing to its influence on hormone regulation. MEL promotes root elongation and lateral root formation and enhances photosynthesis, thereby promoting overall plant growth and productivity. Additionally, MEL is implicated in regulating the circadian rhythm of plants, affecting key physiological processes that influence plant growth patterns. MEL also exhibits antioxidant properties and scavenges reactive oxygen species, thereby mitigating oxidative stress. Furthermore, it activates defense pathways against various biotic stressors. MEL also enhances the production of secondary metabolites that contribute to plant resistance against environmental changes. MEL's ability to modulate plant response to abiotic stresses has also been extensively studied. It regulates stomatal closure, conserves water, and enhances stress tolerance by activating stress-responsive genes and modulating signaling pathways. Moreover, MEL and nitric oxide cooperate in stress responses, antioxidant defense, and plant growth. Understanding the mechanisms underlying MEL's actions in plants will provide new insights into the development of innovative strategies for enhancing crop productivity, improving stress tolerance, and combating plant diseases. Further research in this area will deepen our knowledge of MEL's intricate functions and its potential applications in sustainable agriculture.

Insight into melatonin-mediated response and signaling in the regulation of plant defense under biotic stress

Melatonin plays a crucial role in the mitigation of plant biotic stress through induced defense responses and pathogen attenuation. Utilizing the current knowledge of signaling and associated mechanism of this phytoprotectant will be invaluable in sustainable plant disease management. Biotic stress in plants involves complex regulatory networks of various sensory and signaling molecules. In this context, the polyfunctional, ubiquitous-signaling molecule melatonin has shown a regulatory role in biotic stress mitigation in plants. The present review conceptualized the current knowledge concerning the melatonin-mediated activation of the defense signaling network that leads to the resistant or tolerant phenotype of the infected plants. Fundamentals of signaling networks involved in melatonin-induced reactive oxygen species (ROS) or reactive nitrogen species (RNS) scavenging through enzymatic and non-enzymatic antioxidants have also been discussed. Increasing evidence has suggested that melatonin acts upstream of mitogen-activated proteinase kinases in activation of defense-related genes and heat shock proteins that provide immunity against pathogen attack. Besides, the direct application of melatonin on virulent fungi and bacteria showed disrupted spore morphology, destabilization of cell ultrastructure, reduced biofilm formation, and enhanced mortality that led to attenuate disease symptoms on melatonin-treated plants. The transcriptome analysis has revealed the down-regulation of pathogenicity genes, metabolism-related genes, and up-regulation of fungicide susceptibility genes in melatonin-treated pathogens. The activation of melatonin-mediated systemic acquired resistance (SAR) through cross-talk with salicylic acid (SA), jasmonic acid (JA) has been essential for viral disease management. The high endogenous melatonin concentration has also been correlated with the up-regulation of genes involved in pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI). The present review highlights the versatile functions of melatonin towards direct inhibition of pathogen propagule along with active participation in mediating oxidative burst and simulating PTI, ETI and SAR responses. The hormonal cross-talk involving melatonin mediated biotic stress tolerance through defense signaling network suggests its suitability in a sustainable plant protection system.

Mammalian Melatonin Agonist Pharmaceuticals Stimulate Rhomboid Proteins in Plants

Melatonin is a human neurotransmitter and plant signalling metabolite that perceives and directs plant metabolism. The mechanisms of melatonin action in plants remain undefined. We hypothesized that roots have a melatonin-specific receptor and/or transporter that can respond to melatonin-mediating pharmaceuticals. To test this hypothesis Arabidopsis seedlings were grown with melatonin pharmaceutical receptor agonists: ramelteon and tasimelteon, and/or antagonists: luzindole and 4-P-PDOT. Ramelteon was found both to mimic and competitively inhibit melatonin metabolism in plants. Due to the higher selectivity of ramelteon for the MT1 receptor type in humans, a sequence homology search for MT1 in Arabidopsis identified the rhomboid-like protein 7 (RBL7). In physiological studies, Arabidopsis rbl7 mutants were less responsive to ramelteon and melatonin. Quantum dot visualizations of the effects of ramelteon on melatonin binding to root cell membranes revealed a potential mechanism. We propose that RBL7 is a melatonin-interacting protein that directs root architecture and growth in a mechanism that is responsive to environmental factors.

A Systematic Review of Melatonin in Plants: An Example of Evolution of Literature

Melatonin (N-acetyl-5-methoxy-tryptamine) is a mammalian neurohormone, antioxidant and signaling molecule that was first discovered in plants in 1995. The first studies investigated plant melatonin from a human perspective quantifying melatonin in foods and medicinal plants and questioning whether its presence could explain the activity of some plants as medicines. Starting with these first handful of studies in the late 1990s, plant melatonin research has blossomed into a vibrant and active area of investigation and melatonin has been found to play critical roles in mediating plant responses and development at every stage of the plant life cycle from pollen and embryo development through seed germination, vegetative growth and stress response. Here we have utilized a systematic approach in accordance with the preferred reporting items for systematic reviews and meta-analyses (PRISMA) protocols to reduce bias in our assessment of the literature and provide an overview of the current state of melatonin research in plants, covering 1995-2021. This review provides an overview of the biosynthesis and metabolism of melatonin as well as identifying key themes including: abiotic stress responses, root development, light responses, interkingdom communication, phytohormone and plant signaling. Additionally, potential biases in the literature are investigated and a birefringence in the literature between researchers from plant and medical based which has helped to shape the current state of melatonin research. Several exciting new opportunities for future areas of melatonin research are also identified including investigation of non-crop and non-medicinal species as well as characterization of melatonin signaling networks in plants.

Metabolomics and hormonomics to crack the code of filbert growth

INTRODUCTION

Plants respond to changes in their environments through hormonal activation of a physiological cascade that redirects metabolic resources and growth. In filberts (Corylus sp.), chelated iron promotes the growth of new shoots but the mechanism(s) are not understood.

OBJECTIVES

To use untargeted metabolomics and hormonomics approaches to generate novel hypotheses for the morphoregulatory role of ferric ethylenediamine-N,N'-di-(ortho-hydroxyphenyl) acetic acid (Fe-EDDHA) in filbert shoot organogenesis in vitro.

METHODS

Data were generated using previously optimized standardized untargeted metabolomics protocols with time of flight mass spectrometry. Multivariate statistical tools (principal component and partial least squares discriminant analysis) did not detect significant differences. Discovery tools Significance Analysis of Microarrays (SAM), multiple linear regression analysis, Bayesian analysis, logical algorithms, machine learning, synthetic biotransformations, targeted hormonomics, and online resources including MetaboAnalyst were used.

RESULTS

Starch/sucrose metabolism and shikimate pathway metabolites were increased. Dose dependent decreases were found in polyphenol metabolism, specifically ellagic acid and its methylated derivative 3,4,3'-tri-O-methylellagic acid. Hormonomics analysis revealed significant differences in phytohormones and their conjugates. FeEDDHA treatment reduced indole-3-acetic acid, abscisic acid, salicylic acid, jasmonic acid conjugates (JA-Trp, JA-Ile, OH-JA) and dihydrozeatinglucoside in regenerating explants. Serotonin (5HT) was decreased in FeEDDHA-treated regenerating tissues while the related metabolite melatonin was increased. Eight phenolic conjugates of 5HT and eight catabolites were affected by FeEDDHA indicating that metabolism to sequester, deactivate and metabolize 5HT was induced by Fe(III). Tryptophan was metabolized through kynurenine but not anthranilate.

CONCLUSION

Seven novel hypotheses were generated to guide future studies to understand the regulatory control(s) of shoot organogenesis.

Auxin driven indoleamine biosynthesis and the role of tryptophan as an inductive signal in Hypericum perforatum (L.)

In the 60 years since Skoog and Miller first reported the chemical redirection of plant growth the underlying biochemical mechanisms are still poorly understood, with one challenge being the capacity for applied growth regulators to act indirectly or be metabolized to active phytohormones. We hypothesized that tryptophan is metabolized to auxin, melatonin or serotonin inducing organogenesis in St. John's wort (Hypericum perforatum L.). Root explants from two germplasm lines of St. John's wort with altered melatonin metabolism and wildtype were incubated with auxin or tryptophan for 24, 48 or 72 h to induce regeneration. In wildtype, tryptophan had little effect on the indoleamine pathway, and was found to promote primary growth, suggesting excess tryptophan moved quickly through various secondary metabolite pathways and protein synthesis. In lines 4 and 112 tryptophan was associated with modified morphogenesis, indoleamine and auxin levels. Incubation with tryptophan increased shoot organogenesis while incubation with auxin led to root regeneration. The established paradigm of thought views tryptophan primarily as a precursor for auxin and indoleamines, among other metabolites, and mediation of auxin action by the indoleamines as a one-way interaction. We propose that these processes run in both directions with auxin modifying indoleamine biosynthesis and the melatonin:serotonin balance contributing to its effects on plant morphogenesis, and that tryptophan also functions as an inductive signal to mediate diverse phytochemical and morphogenetic pathways.

A virus-induced gene-silencing system for functional genetics in a betalainic species, Amaranthus tricolor (Amaranthaceae)

PREMISE OF THE STUDY

Research in Amaranthaceae could be accelerated by developing methods for targeted gene silencing. Most amaranths, including Amaranthus tricolor, produce betalains. However, the physiological and ecological roles of these pigments are uncertain. We sought to establish a virus-induced gene-silencing (VIGS) method for amaranths, using silencing of betalain pigments as a proof-of-principle.

METHODS

We targeted AtriCYP76AD1, a putative cytochrome P450 component of the betalain biosynthetic pathway, using VIGS, and compared two different methods of introducing the VIGS construct into plants. We measured transcript abundance and concentrations of betalains and their l-DOPA precursor in VIGS-treated plants, and compared these to controls.

RESULTS

We observed that when AtriCYP76AD1 was targeted by VIGS in normally red plants, AtriCYP76AD1 and the related genes AtriCYP76AD6 and AtriCYP76AD5 had diminished transcript abundance. Furthermore, newly emergent petioles and leaves of VIGS-treated plants appeared green, betacyanin accumulation was strongly reduced, and l-DOPA accumulation was increased. No betaxanthin could be detected in this variety of A. tricolor, either before or after VIGS treatment.

DISCUSSION

These results help to establish the genetic basis of betalain synthesis in amaranths. Furthermore, this is the first report of VIGS in amaranths and demonstrates the potential of this technique for basic and applied research in these species.

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.