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

Figures & data

Figure 1. Overview of the indoleamine biosynthetic pathway in plants as compared to animals. The primary biosynthetic pathway in animals is represented by dashed purple arrows, the primary biosynthetic pathway in plants in solid pink arrows and alternate pathways described to date in plants in solid black arrows. Enzyme names are colored to match the associated arrows. AADAC, aromatic amino acid deacetylase; AANAT, aromatic amino acid N-acetyltransferase, ASDAC, acetylserotonin-deacetylase ASMT, acetylserotonin-O-methyltransferase; COMT, caffeic acid-O-methyltransferase; HIOMT, hydroxyindole-O-methyltransferase; SNAT, serotonin-N-acetyltransferase; T-5-H, tryptamine-5-hydroxlyase; TDC, tryptophan decarboxylase; TPH, tryptophan hydroxylase.

The key intermediates in the indoleamine biosynthetic pathway are listed in order from top to bottom with chemical structures for tryptophan, serotonin and melatonin given on the left hand side. Arrows show: Flow of carbon and reactions in the main plant pathway from tryptophan to tryptamine to serotonin to N-acetylserotonin to melatonin. Flow of carbon and reactions in the animal pathway diverges at the beginning with progression from tryptophan to 5-hydroxytryptophan to tryptamine. Alternate pathways and intermediates are shown from serotonin to melatonin via 5-methoxytryptamine or from tryptamine to N-acetylserotonin as well as reverse reactions from melatonin to methoxytryptamine, and N-acetylserotonin to serotonin.

Figure 2. (a) Word cloud representing the most commonly included author keywords for original research articles included melatonin and plants published from 2021 to 2024. Word cloud was created in www.freewordcloudgenerator.com. (b) Top query terms by decade for melatonin and (c) Top query terms by decade for serotonin.

The top panel shows a word cloud in shades of blue, purple, pink and turquoise. Melatonin is the largest word in the cloud followed by terms including stress, antioxidant and photosynthesis and plant. Smaller words include species or mechanism specific terms like enzyme, phytohormone, arabidopsis. The second and third panels are stacked bar graphs with bars coloured by decade 1990s purple, 2000s pink, 2010s light blue and 2020s dark blue. Bars represent the proportion of papers mentioning each term by decade. Terms are organized from left to right in order of appearance in the literature.

Figure 3. Distribution of country of publication of articles published on melatonin and plants (top) or serotonin and plants according to a Web of Science search between 2021 and 2024. Only countries which included more than 2% of total papers published in the query are included.

Rectangular parts of the whole charts which are shaded by proportion of countries publishing papers on melatonin and serotonin are shown in shades of purple, pink and blue.

Figure 4. Phylogenetic tree of species in which melatonin (blue), serotonin (pink) or both (purple) have been studied and mapped to all plant families. Tree was constructed using PhyloT v2 and visualized in iTOL using the annotation spreadsheet editor. A list of all plant families was acquired from World Flora Online,^26,27 family and species names were confirmed using the taxonomic name resolver service.²⁸ An interactive version of this figure can be viewed at https://itol.Embl.de/export/5067182177413091708903228.

A circular phylogenetic tree is shown with the tree in black with a white background. Both the names of species are coloured and are marked by boxes in the same colour around the periphery. A greater number of species are highlighted farther down the tree as are species which have been uniquely studied in the serotonin literature.

Figure 5. Distribution of species studied in melatonin (a & c) and serotonin (b & d) by family. The line of best fit is the linear regression line of the actual number of species studied by family and the predicted number of species that would be studied if studies were proportional to the total number of species.²⁹ Dotted lines represent the 95% confidence interval. Families below the line (shaded portion) are underrepresented in the field, while those above the line are overrepresented. Panels A & B show the full plots, while c & d provide a zoomed-in view of the plots to the left. Individual points are transparent, with darker circles or squares representing a greater number of families at a given coordinate.

All four panels shown scatterplots fitted with lines of regression. The melatonin graphs are shaded under the linear regression line in purple and serotonin in light blue. Families are represented as semi-transparent circles (melatonin) or squares (serotonin) to highlight locations of highly overlapping families. Families under the line are considered underrepresented and families above the line are overpresented in the literature. Families which are far above the regression line for both melatonin and serotonin include: Asteraceae, Rubiaceae, Fabaceae Families far below the line include for melatonin: Lamiaceae, Rosaceae, Salicaceae and for serotonin: Rosaceae, Juglandaceae, Solanaceae.

Figure 6. Proportion of publications per decade mentioning quantification query terms in the abstract of title for (a) melatonin and (c) serotonin literature and the number of publications using determination approaches since 2021 for (b) melatonin and (d) serotonin. Query terms used were: UPLC or HPLC or liquid chromatography or liquid-chromatography, GC or gas chromatography or gas-chromatography, MS or mass spectrometry or mass-spectrometry, UV or PDA or DAD, Fluorescence Detect or FLD, ECD or electrochemical detect, ELISA or enzyme-linked immunosorbent assay or EIA or enzyme immunoassay, gene expression or qPCR or RT-PCR, GWAS (genome-wide association studies).

A & C are grouped bar graphs with bars shaded by decade 1990s in purple, 2000s in pink, 2010s in light blue and 2020s in dark blue. B & D are bar graphs that are coloured by theme, molecular approaches including: genomics, transcriptomics, RNA-Seq, gene expression, GWAS and microarray are shaded in pink , other -OMICS proteomics & metabolomics in turquoise and analytical approaches in purple.

Table 1. Summary of indoleamine quantification methodologies by plant species since 2021.

Plant Species	Common Name	MEL	5HT	LC	GC	MS	PDA/ UV	FL	ECD	ELISA	Reference
Brassica juncea	Mustard	X		X			X				^Citation112
Musa sapientum	Banana	X	X	X		X		X			^Citation113
Mangifera indica	Mango	X	X	X		X		X			^Citation113,Citation114
Carica papaya	Papaya	X		X				X			^Citation113
Antidesma thwaiteaianum	Makmao	X						X			^Citation113
Ananas comosus	Pineapple	X	X	X		X		X			^Citation113,Citation114
Morus alba	Mulberry	X	X	X		X					^Citation114
Solanum lycopersicum var. cerasiforme	Tomato	X	X	X		X					^Citation114
Solanum lycopersicum cv. Moneymaker	Tomato		X	X		X					^Citation115
Psidium guajava	Guava	X	X	X		X					^Citation114
Tamarindus indica	Tamarind	X	X	X		X					^Citation114
Syzygium samarangense	Rose apple	X	X	X		X					^Citation114
Citrullus lanatus	Watermelon	X	X	X		X					^Citation114
Citrus sinensis	Orange	X	X	X		X					^Citation114
Citrus reticulata	Orange	X		X				X			^Citation113
Dimocarpus longan	Longan	X	X	X		X					^Citation114
Cucumis melo var. cantalupo	Cantaloupe	X	X	X		X					^Citation114
Passiflora edulis	Red passion fruit	X	X	X		X					^Citation114
Passiflora edulis	Yellow passion fruit	X	X	X		X					^Citation114
Ziziphus jujuba	Jujube	X	X	X		X					^Citation114
Gossypium hirsutum	Cotton	X		X		x					^Citation116
Wollemia nobilis	Wollemi pine	X		X		x					^Citation117
Araucaria heterophylla	Norfolk Island Pine	X		X		X					^Citation117
Cornus mas	Cornelian cherry	X		X				X			^Citation118
Tilia tomentosa	Linden	X	X	X*			X				^Citation119
Foeniculum vulgare	Fennel	X	X	X*			X				^Citation119
Salvia officinalis	Sage	X	X	X*			X				^Citation119
Matricaria chamomilla	Chamomile	X	X	X*			X				^Citation119
Solanum lycopersicum	Tomato	X		X		X					^Citation120
Dracocephalum kotsychi		X		X*			X				^Citation121
Capsicum annuum	Sweet Pepper	X		X		X					^Citation122
Triticum aestivum	Wheat	X		X		X	X				^Citation123
Oryzae sativa	Rice	X	X	X	X	X					^Citation124,Citation125
Suaede spp.		X			X	X					^Citation126
Solanum tuberosum	Potato	X								X	^Citation127
Prunus salicina	Angeleno plum	X		X		X	X				^Citation128
Spirodela polyrhiza	Duckweed		X		X	X					^Citation129
Helianthus annuus	Sunflower**		X	X		X					^Citation130
Hippophae rhamnoides	Sea Buckthorne		X	X		X					^131
Elatostema papillosum			X		X	X					^132

*Paper specifies only ‘quantification by HPLC’ in the abstract, with no detection method.

**Wrong reference to method paper referenced.

Figure 7. Proportion of publications per decade with mentions by chemical class for (a, c) melatonin and (b, d) serotonin literature.

Figure 8. Proportion of publications per decade mentioning physiology-associated query terms in the abstract or title for (a) melatonin and (b) serotonin literature.

Figure 9. Proportion of publications per decade mentioning stress-associated query terms in the abstract or title for (a) melatonin and (b) serotonin literature.

Grouped bars graphs are formatted similar to earlier figures with bars shaded by decade, 1990s in purple, 2000s in pink, 2010s in light blue and 2020s in dark blue.

Figure 10. Summary of the interactions between and functions of serotonin in animal, microbial and plant cells and their associated microbiomes.

A Venn Diagram layout of three semi-transparent overlapping circles has animals (shaded in light red) on the left plants (shaded in light green) on the right and microbes (shaded in light blue) in the middle with smaller circles labelled Ind. for indoleamine in the centre. Arrows point at functions and interactions which are listed as black text.

Figure 11. Overview of application methods of melatonin across all studies.

A bar graph with bars shaded by x axis label in purple, pink, light blue and dark blue.

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Data availability statement

All data referenced in the manuscript may be accessed at: DOI 10.17605/OSF.IO/N3K8U.