Melatonin (N-acetyl-5-methoxytryptamine) is a tryptophan-derived indole amine molecule
present in animals, plants, and microbes. It was first identified in animals in 1958
where it mediates the regulation of circadian and seasonal rhythms. Melatonin and
its derivatives protect animal cells by scavenging many types of reactive oxygen and
nitrogen species (ROS/RNS) and up-regulating the expression of antioxidative enzymes.
It also can act as an anti-inflammatory and immunomodulator, with the potential to
reduce the severity of disease-associated symptoms.
Discovered in plants a quarter of a century ago, phytomelatonin has been reported
from many mono- and dicotyledonous species. Research investigating the effects of
exogenous melatonin application and gene expression of melatonin-related genes indicate
that the endogenous levels of phytomelatonin can modulate many aspects of plant growth
and can have a protective role by reducing the negative effects of biotic and abiotic
stresses.
This Research Topic focuses on phytomelatonin metabolism and physiological roles in
planta and is a follow up on “Melatonin in plants.” We aim to ask how melatonin alone
or in association with other melatonin-derived compounds can modulate plant physiology
and what other role(s) they may have. This Research Topic contains one review and
12 original research studies. The articles describe findings that further elucidate
the role of phytomelatonin in biotic and abiotic stress tolerance, including some
of the molecular actors and mechanism associated to phytomelatonin biosynthesis.
The review by Murch and Erland employed a systematic approach using the PRISMA protocol
to highlight the current state of research published in plant physiology, growth,
and metabolism since its discovery in plants in 1995. An active and exciting area
of phytomelatonin research has been in enhancing understanding pathway dynamics of
melatonin biosynthesis in plants with the recent discoveries of alternate pathways
and biologically important melatonin isomers. Zheng et al. and Zhou et al. have characterized
and identified acetylserotonin-O-methlytransferase (ASMT) and serotonin-N-acetyltransferase
(SNAT) in mulberry (Morus alba L.) and St. John's wort (Hypericum perforatum L.) respectively.
In mulberry, ASMT was found to be able to catalyze production of melatonin as well
as two isomers described as melatonin isoform (MI)-1 and MI-2, with differential distribution
of these molecules across Morus species and tissues surveyed. In St. John's wort an
early model for phytomleatonin research two leaf specific HpSNAT are described which
confer salt and drought tolerance in Arabidopsis transgenic plants overexpressing
HpSNAT.
Tolerance to abiotic and biotic stresses are investigated in this Research Topic and
are key roles of melatonin in plants. Zhu et al. reported an enhanced resistance to
Botrytis cinerea infection in Arabidopsis with overexpression of SNAT and ASMT, the
final two steps in conversion of serotonin to melatonin, while susceptibility increased
when silenced. This study is of particular interest as it slows for differentiation
between serotonin and melatonin mediated effects. Interestingly jasmonic acid (JA)
content was also increased with SNAT and ASMT expression levels in transgenics following
fungal infection highlighting the role of phytomelatonin in the activation of JA signaling
pathway.
Abiotic stress responses are examined for several species. In rice, Li et al. found
that melatonin treatment improved seed germination at low temperature through activation
of gibberellin biosynthesis, by maintaining the redox homeostasis, and endogenous
melatonin biosynthesis, but prevented the buildup of abscisic acid (ABA) and hydrogen
peroxide. Moreover, the authors demonstrated that melatonin acts synergistically with
an ABI5-mediated signal involving the regulation of the rice CATALASE2. In tomato,
Ding et al. found that, while the cold stress increased JA accumulation, the biosynthesis
of melatonin was also increased via MYC2-activated SlSNAT and SlASMT therefore potentiating
cold tolerance. Yang N. et al. reported that selenite treatment at low concentration
improved the cold tolerance of cucumber seedlings and increased endogenous melatonin
content through up-regulation of key melatonin biosynthetic genes. The research by
Xing et al. on heat-resistance mediated by melatonin in chrysanthemum seedlings employed
physiological and transcript analyses to reveal the underlying associated gene regulatory
networks such as osmotic regulation, redox homeostasis, hormone signal transduction
and the chlorophyll, flavonoid, and carotenoid metabolic pathways. In tomato, Jahan
et al. found that the attenuation of heat stress induced senescence by melatonin treatment
involved enhanced gibberellic acid (GA) and endogenous melatonin content but reduced
abscisic acid, also evidenced using gene transcript levels. Furthermore, chemical
inhibition of GA and ABA synthesis failed to produce melatonin induced heat tolerance.
In tobacco, Chen, Jia et al. investigated the regulatory mechanisms by which melatonin
treatment improve dehydration-induced leaf senescence in seedlings and found that
foliar spraying decreased endogenous ABA content and oxidative damage. Moreover, metabolite
profiling and gene expression analysis showed that melatonin induces carotenoids,
inhibits chlorophyll breakdown, modulates phytohormonal biosynthesis and signaling
as well as the transcriptional network underlying leaf senescence. Yang S-J. et al.
reported that the high light stress response in Arabidopsis leaf was improved with
exogenous melatonin application by enhancing the endogenous melatonin content, protecting
the photosynthetic pigments and integrity of photosynthesis apparatus, and inhibiting
ROS accumulation. In wheat, Zhang et al. compared three winter wheat varieties with
contrasted responses for salt tolerance and reported the promotive effect of exogenous
melatonin on germination trait under salt stress. Improved wheat germination by melatonin
was found to be related to the increase in root vigor, maintenance of ion balance,
decrease in H2O2 content, regulation of soluble protein and sugar synthesis, and changes
in amino acid levels. Chen, Cao et al. found that application of melatonin on alfalfa
plants submitted to high-nitrate stress improved growth and development by regulating
the excess nitrogen and calcium intake, promoting nitrogen metabolism with enhanced
enzyme activities, reducing ATP-utilizing system and increasing ATP regeneration.
Together this Research Topic provides readers with overview of the key roles melatonin
plays in plants. It highlights innovative and growing areas in the discipline and
opens new pathways of discovery that we trust readers will find inspiring.
Author Contributions
FB wrote and revised the manuscript. HS, LE, MI, and JZ provided suggestions and revised
the manuscript. All authors approved the manuscript and the version to be published.
Funding
This research was supported by the National Key Research and Development Program of
China (2019YFD1000300) and the Starry Night Science Fund of Zhejiang University Shanghai
Institute for Advanced Study (SN-ZJU-SIAS-0011).
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial
or financial relationships that could be construed as a potential conflict of interest.
Publisher's Note
All claims expressed in this article are solely those of the authors and do not necessarily
represent those of their affiliated organizations, or those of the publisher, the
editors and the reviewers. Any product that may be evaluated in this article, or claim
that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.