The case of tryptamine and serotonin in plants: a mysterious precursor for an illustrious metabolite

Enhanced In Vitro Plant Morphogenesis of Tobacco: Unveiling Indoleamine-Modulated Adaptogenic Properties of Tulsi (Ocimum sanctum L.)

The medicinal plant tulsi (Ocimum sanctum L.) is acknowledged for its invigorating and healing properties that enhance resilience to stress in various human and animal models by modulating antioxidant compounds. While extensive research has documented these effects in humans, the adaptogenic potential of tulsi in stressful in vitro plant systems has not been explored. This study aimed to elucidate the adaptogenic properties of tulsi leaf extract on the in vitro regeneration of tobacco leaf explants through an investigation of the indoleamines at different developmental stages. Shoot regeneration from leaf explants on the medium supplemented with tulsi extract (20%) was compared to the control, and the differences in indoleamine compounds were analyzed using ultra-performance liquid chromatography. Treatment of the explants with the extract resulted in an almost two-fold increase in the number of regenerants after four weeks of culture, and 9% of the regenerants resembled somatic embryo-like structures. The occurrence of browning in the extract-treated explants stopped on day 10, shoots began to develop, and a significant concentration of tryptamine and N-acetyl-serotonin accumulated. A comparative analysis of indoleamine compounds in intact and cut tobacco leaves also revealed the pivotal role of melatonin and 2-hydroxymelatonin functioning as antioxidants during stress adaptation. This study demonstrates that tulsi is a potent adaptogen that is capable of modulating plant morphogenesis in vitro, paving the way for further investigations into the role of adaptogens in plant stress biology.

Untargeted foodomics for authenticating the organic farming of water spinach (Ipomoea aquatica)

This study aimed to conduct a comprehensive analysis of the primary and secondary metabolites of water spinach (Ipomoea aquatica) using hydrophilic interaction liquid chromatography coupled with Orbitrap high-resolution mass spectrometry (HILIC-Orbitrap-HRMS). Certified samples from two cultivars, Green stem water spinach (G) and White stem water spinach (W) cultivated using organic and conventional farming methods, were collected from the Hong Kong market. Multivariate analysis was used to differentiate water spinach of different cultivars and farming methods. We identified 12 metabolites to distinguish between G and W, 26 metabolites to identify G from organic farming and 8 metabolites to identify W from organic farming. Then, two metabolites, isorhamnetin and jasmonic acid, have been proposed to serve as biomarkers for organic farming (in both G and W). Our foodomics findings provide useful tools for improving the crop performance of water spinach under abiotic/biotic stressesand authentication of organic produce.

Nutrient composition and functional constituents of daylily from different producing areas based on widely targeted metabolomics

Daylily is a functional food with high nutritional value in China. Datong (DT) in Shanxi Province is one of the four main production areas of daylily. Therefore, Linfen (LF), Lvliang (LL), and Yangquan (YQ) in Shanxi Province have also introduced daylily from DT. However, geographical and climatic conditions and producing patterns cause variations in the daylily quality. In the present study, we found that the nutrient composition of daylilies from different producing areas of Shanxi Province varied. The key environmental factors affecting the nutrition of daylily in different regions were altitude and temperature. The widely targeted metabolomics results showed that 1642 metabolites were found in daylily. The differential metabolites between DT and YQ, LL and LF were 557, 667, and 359, respectively. Notably, 9 metabolic pathways and 59 metabolite markers were associated with daylily from different areas. This study provides a theoretical basis for the quality maintenance and health efficacy research of daylily.

Interkingdom Hormonal Regulations between Plants and Animals Provide New Insight into Food Safety

Food safety has become an attractive topic among consumers. Raw material production for food is also a focus of social attention. As hormones are widely used in agriculture and human disease control, consumers' concerns about the safety of hormone agents have never disappeared. The present review focuses on the interkingdom regulations of exogenous animal hormones in plants and phytohormones in animals, including physiology and stress resistance. We summarize these interactions to give the public, researchers, and policymakers some guidance and suggestions. Accumulated evidence demonstrates comprehensive hormonal regulation across plants and animals. Animal hormones, interacting with phytohormones, help regulate plant development and enhance environmental resistance. Correspondingly, phytohormones may also cause damage to the reproductive and urinary systems of animals. Notably, the disease-resistant role of phytohormones is revealed against neurodegenerative diseases, cardiovascular disease, cancer, and diabetes. These resistances derive from the control for abnormal cell cycle, energy balance, and activity of enzymes. Further exploration of these cross-kingdom mechanisms would surely be of greater benefit to human health and agriculture development.

Exploring the Evolvability of Plant Specialized Metabolism: Uniqueness Out Of Uniformity and Uniqueness Behind Uniformity

The huge structural diversity exhibited by plant specialized metabolites has primarily been considered to result from the catalytic specificity of their biosynthetic enzymes. Accordingly, enzyme gene multiplication and functional differentiation through spontaneous mutations have been established as the molecular mechanisms that drive metabolic evolution. Nevertheless, how plants have assembled and maintained such metabolic enzyme genes and the typical clusters that are observed in plant genomes, as well as why identical specialized metabolites often exist in phylogenetically remote lineages, is currently only poorly explained by a concept known as convergent evolution. Here, we compile recent knowledge on the co-presence of metabolic modules that are common in the plant kingdom but have evolved under specific historical and contextual constraints defined by the physicochemical properties of each plant specialized metabolite and the genetic presets of the biosynthetic genes. Furthermore, we discuss a common manner to generate uncommon metabolites (uniqueness out of uniformity) and an uncommon manner to generate common metabolites (uniqueness behind uniformity). This review describes the emerging aspects of the evolvability of plant specialized metabolism that underlie the vast structural diversity of plant specialized metabolites in nature.

Rice Mitogen-Activated Protein Kinase regulates serotonin accumulation and interacts with cell cycle regulators under prolonged UV-B exposure

Stress conditions such as UV-B exposure activates MAPKs in Arabidopsis and rice. UV-B radiation is hazardous to plant as it causes photosystem disruption, DNA damage and ROS generation. Here we report its effect on biological pathways by studying the global changes in transcript profile in rice seedling exposed to UV-B radiation for 1 h and 16 h. Short UV-B exposure (1 h) led to moderate changes, while a drastic change in transcript landscape was observed after long term UV-B exposure (16 h) in rice seedlings. Prolonged UV-B exposure negatively impacts the expression of cell cycle regulating genes and several other metabolic pathways in developing seedlings. MAP kinase signaling cascade gets activated upon UV-B exposure similar to reports in Arabidopsis indicating conservation of its function in both dicot and monocot. Expression analysis in inducible overexpression transgenic lines of MPK3 and MPK6 shows higher transcript abundance of phytoalexin biosynthesis gene like Oryzalexin D synthase and Momilactone A synthase, along with serotonin biosynthesis genes. An accumulation of serotonin was observed upon UV-B exposure and its abundance positively correlates with the MPK3 and MPK6 transcript level in the respective over-expression lines. Interestingly, multiple cell cycle inhibitor proteins including WEE1 and SMR1 interact with MPK3 and MPK6 thus, implying a major role of this pathway in cell cycle regulation under stress condition. Overall overexpression of MPK3 and MPK6 found to be detrimental for rice as overexpression lines shows higher cell death and compromised tolerance to UV-B.

Proteome of Agave angustifolia Haw.: Uncovering metabolic alterations, over-accumulation of amino acids, and compensatory pathways in chloroplast-deficient albino plantlets

Amino acids (AA) are essential molecules for plant physiology, acting as precursor molecules for proteins and other organic compounds. Chloroplasts play a vital role in AA metabolism, yet little is known about the impact on AA metabolism of albino plants' lack of chloroplasts. In this study, we conducted a quantitative proteome analysis on albino and variegated somaclonal variants of Agave angustifolia Haw. to investigate metabolic alterations in chloroplast-deficient plants, with a focus on AA metabolic pathways. We identified 82 enzymes involved in AA metabolism, with 32 showing differential accumulation between the somaclonal variants. AaCM, AaALS, AaBCAT, AaIPMS1, AaSHMT, AaAST, AaCGS, and AaMS enzymes were particularly relevant in chloroplast-deficient Agave plantlets. Both variegated and albino phenotypes exhibited excessive synthesis of AA typically associated with chloroplasts (aromatic AAs, BCAAs, Asp, Lys, Pro and Met). Consistent trends were observed for AaBCAT and AaCM at mRNA and protein levels in albino plantlets. These findings highlight the critical activation and reprogramming of AA metabolic pathways in plants lacking chloroplasts. This study contributes to unraveling the intricate relationship between AA metabolism and chloroplast absence, offering insights into survival mechanisms of albino plants.

Serotonin: A frontline player in plant growth and stress responses

Serotonin is a well-studied pineal hormone that functions as a neurotransmitter in mammals and is found in varying amounts in diverse plant species. By modulating gene and phytohormonal crosstalk, serotonin has a significant role in plant growth and stress response, including root, shoot, flowering, morphogenesis, and adaptability responses to numerous environmental signals. Despite its prevalence and importance in plant growth and development, its molecular action, regulation and signalling processes remain unknown. Here, we highlight the current knowledge of the role of serotonin-mediated regulation of plant growth and stress response. We focus on serotonin and its regulatory connections with phytohormonal crosstalk and address their possible functions in coordinating diverse phytohormonal responses during distinct developmental phases, correlating with melatonin. Additionally, we have also discussed the possible role of microRNAs (miRNAs) in the regulation of serotonin biosynthesis. In summary, serotonin may act as a node molecule to coordinate the balance between plant growth and stress response, which may shed light on finding its key regulatory pathways for uncovering its mysterious molecular network.

Advances in the microbial synthesis of the neurotransmitter serotonin

Serotonin, as a monoamine neurotransmitter, modulates the activity of the nervous system. Due to its importance in the coordination of movement and regulation of mood, impairments in the synthesis and homeostasis of serotonin are involved in numerous disorders, including depression, Parkinson's disease, and anxiety. Currently, serotonin is primarily obtained via natural extraction. But this method is time-consuming and low yield, as well as unstable supply of raw materials. With the development of synthetic biology, researchers have established the method of microbial synthesis of serotonin. Compared with natural extraction, microbial synthesis has the advantages of short production cycle, continuous production, not limited by season and source, and environment-friendly; hence, it has garnered considerable research attention. However, the yield of serotonin is still too low to industrialization. Therefore, this review provides the latest progress and examples that illustrate the synthesis pathways of serotonin as well as proposes strategies for increasing the production of serotonin. KEY POINTS: • Two biosynthesis pathways of serotonin are introduced. • L-tryptophan hydroxylation is the rate-limiting step in serotonin biosynthesis. • Effective strategies are proposed to improve serotonin production.

Identification of three key enzymes involved in the biosynthesis of tetracyclic oxindole alkaloids in Uncaria rhynchophylla

Tetracyclic oxindole alkaloids (TOAs), main active ingredients of Uncaria rhynchophylla (UR), has inspired the interest of pharmacologists and chemists because of its great potential in the treatment of the diseases of the nervous system and cardiovascular system and its special spirooxindole scaffold, but the biosynthetic pathway of this compounds is still unknown. In this work, the metabolomics and transcriptomics of hook, leaf and stem of UR were analyzed, and 31 alkaloids and 47,423 unigenes were identified, as well as the relative contents of these alkaloids were evaluated. Based on the above results and literatures, a proposal biosynthetic pathway for TOAs was devised. Furthermore, three unigenes were suggested mediating the biosynthesis of TOAs through the integrated analysis of metabolomics and transcriptomics, and three enzymes, tryptophan decarboxylase, strictosidine synthase and strictosidine-β-d-glucosidase, were identified as important catalytic enzymes for the synthesis of tryptamine, strictosidine (7) and 4,21-dehydrogeissochizine, respectively, which are considered as the important precursors of TOAs.

Indoleamines Impart Abiotic Stress Tolerance and Improve Reproductive Traits in Hazelnuts

Hazelnuts have recently gathered tremendous attention due to the expansion of the confectionary industry. However, the sourced cultivars fail to perform in initial phase of cultivation as they enter bare survival mode due to changes in climatic zones, for example, Southern Ontario, where the climate is continental, as opposed to the milder climate in Europe and Turkey. Indoleamines have been shown to counter abiotic stress and modulate vegetative and reproductive development of plants. Here, we examined the effect of indoleamines on the flowering response of the dormant stem cuttings of sourced hazelnut cultivars in controlled environment chambers. The stem cuttings were exposed to sudden summer-like conditions (abiotic stress) and the female flower development was assessed in relation to endogenous indoleamine titers. The sourced cultivars responded well to serotonin treatment by producing more flowers compared to the controls or other treatments. The probability of buds resulting in female flowers was highest in the middle region of the stem cuttings. It is interesting to note that the tryptamine titers of the locally adapted, and N-acetyl serotonin titers of native hazelnut cultivars, provided the best explanation for adaptation to the stress environment. Titers of both compounds were compromised in the sourced cultivars which resorted mostly to serotonin concentrations to counter the stress. The indoleamines tool kit identified in this study could be deployed in assessing cultivars for stress adaptation attributes.

Identification of the key metabolites and related genes network modules highly associated with the nutrients and taste components among different Pepino (Solanum muricatum) cultivars

There is considerable knowledge about plant compounds that produce flavor, scent, and aroma. Aside from the similarities, however, groups of plant-produced nutrients and taste components have little in common with each other. Network analysis holds promise for metabolic gene discovery, which is especially important in plant systems where metabolic networks are not yet fully resolved. To bridge this gap, we propose a joint model of gene regulation and metabolic reactions in two different pepino varieties. Differential metabolomics analysis is carried out for detection of eventual interaction of compound. We adopted a multi-omics approach to profile the transcriptome and metabolome analyze differences in phenolic acids, flavonoids, organic acids, lipids, alkaloids, and sugars between LOF and SRF. The two most predominant classes of metabolites are phenolic acids and lipids in pepino. Overall results show enrichment in most DEGs was carbohydrate and biosynthesis of secondary metabolites pathway. Results of DEMs predominantly comprised N-p-coumaroyl agmatine and tryptamine, and significant differences were observed in their expression between LOF and SRF. Integrated DEMs and DEGs specific networks were constructed by combining two types of networks: transcriptional regulatory networks composed of interactions between DEMs and the regulated genes, and pepino metabolite-metabolite interaction networks. Newly discovered features, such as DEGs (USPA, UBE2 and DELLA) involved in the production of secondary metabolites are found in coregulated gene clusters. Moreover, lipid metabolites were most involved in DEMs correlations by OPLS-DA while identifying a significant number of DEGs co-regulated by SENP1, HMGCS et al. These results further that the metabolite discrepancies result from characterized the nutrients and taste components between two pepino genotype. Among the possible causes of the differences between species in pepino metabolite concentrations is co-regulated by these DEGs, continue to suggest that novel features of metabolite biosynthetic pathway remain to be uncovered. Finally, the integrated metabolome and transcriptome analyses have revealed that many important metabolic pathways are regulated at the transcriptional level. The metabolites content differences observed among varieties of the same species mainly originates from different regulated genes and enzymes expression. Overall, this study provides new insights into the underlying causes of differences in the plant metabolites and suggests that genetic data can be used to improve its nutrients and taste components.

Functional characterization of a catalytically promiscuous tryptophan decarboxylase from camptothecin-producing Camptotheca acuminata

Tryptophan decarboxylases (TDCs) are a group of pyridoxal 5'-phosphate-dependent enzymes involved in the enzymatic conversion of tryptophan into tryptamine, a critical biogenic amine. We herein mined and cloned a TDC-encoding gene, CaTDC3, from camptothecin-producing plant Camptotheca acuminata. The intact CaTDC3 was heterologously overexpressed in Escherichia coli and the recombinant CaTDC3 was purified to homogeneity. High-performance liquid chromatography (HPLC)-diode array detector (DAD) and high resolution mass spectrometry (HRMS) data analyses of the CaTDC3-catalyzed reaction mixture confirmed the catalytically decarboxylative activity of CaTDC3. CaTDC3 shows strict stereoselectivity for L-tryptophan. Homology modeling and molecular docking implied CaTDC3's recognition of L-tryptophan derivatives and analogs. Substrate scope investigations revealed that the appropriate substituent groups on the indole ring, i.e., hydroxylated and halogenated L-tryptophans, could be recognized by CaTDC3 and the decarboxylation reactions generated the corresponding tryptamines. The C^β -methyl-L-tryptophans were decarboxylated by CaTDC3 efficiently. 1-Thio-L-tryptophan, the NH group of the indole ring replaced by an S atom, could be decarboxylated by CaTDC3. CaTDC3 catalyzed the decarboxylation of 7-aza-L-tryptophan, an N displacement of the C on the aromatic ring, to afford 7-aza-tryptamine. L-Kynurenine, an L-tryptophan degradation product, could be decarboxylated by CaTDC3. The present works uncover a catalytically promiscuous TDC and the TDC is a versatile decarboxylase in synthetic biology for specialized pharmaceutically important substances.

Indolamine accumulation and TDC/T5H expression profiles reveal the complex and dynamic regulation of serotonin biosynthesis in tomato (Solanum lycopersicum L.)

Tryptamine and serotonin are indolamines that fulfill diverse biological functions in all kingdoms of life. Plants convert l-tryptophan into tryptamine and then serotonin via consecutive decarboxylation and hydroxylation reactions catalyzed by the enzymes tryptophan decarboxylase (TDC) and tryptamine 5-hydroxylase (T5H). Tryptamine and serotonin accumulate to high levels in the edible fruits and seeds of many plant species, but their biological roles in reproductive organs remain unclear and the metabolic pathways have not been characterized in detail. We identified three TDC genes and a single T5H gene in tomato (Solanum lycopersicum L.) by homology-based screening and confirmed their activity by heterologous expression in Nicotiana benthamiana. The co-analysis of targeted metabolomics and gene expression data revealed complex spatiotemporal gene expression and metabolite accumulation patterns that suggest the involvement of the serotonin pathway in multiple biological processes. Our data support a model in which SlTDC1 allows tryptamine to accumulate in fruits, SlTDC2 causes serotonin to accumulate in aerial vegetative organs, and SlTDC3 works with SlT5H to convert tryptamine into serotonin in the roots and fruits.

Characterizing serotonin biosynthesis in Setaria viridis leaves and its effect on aphids

A combined transcriptomic and metabolic analysis of Setaria viridis leaves responding to aphid infestation was used to identify genes related to serotonin biosynthesis. Setaria viridis (green foxtail), a short life-cycle C4 plant in the Poaceae family, is the wild ancestor of Setaria italica (foxtail millet), a resilient crop that provides good yields in dry and marginal land. Although S. viridis has been studied extensively in the last decade, the molecular mechanisms of insect resistance in this species remain under-investigated. To address this issue, we performed a metabolic analysis of S. viridis and discovered that these plants accumulate the tryptophan-derived compounds tryptamine and serotonin. To elucidate the defensive functions of serotonin, Rhophalosiphum padi (bird cherry-oat aphids) were exposed to this compound, either by exogenous application to the plant medium or with artificial diet bioassays. In both cases, exposure to serotonin increased aphid mortality. To identify genes that are involved in serotonin biosynthesis, we conducted a transcriptome analysis and identified several predicted S. viridis tryptophan decarboxylase (TDC) and tryptamine 5-hydroxylase (T5H) genes. Two candidate genes were ectopically expressed in Nicotiana tabacum, where SvTDC1 (Sevir.6G066200) had tryptophan decarboxylase activity, and SvT5H1 (Sevir.8G219600) had tryptamine hydroxylase activity. Moreover, the function of the SvTDC1 gene was validated using virus-induced gene silencing in S. italica, which caused a reduction in serotonin levels. This study provides the first evidence of serotonin biosynthesis in Setaria leaves. The biosynthesis of serotonin may play an important role in defense responses and could prove to be useful for developing more pest-tolerant Setaria italica cultivars.

Phytochemicals, Nutrition, Metabolism, Bioavailability, and Health Benefits in Lettuce-A Comprehensive Review

Lettuce is one of the most famous leafy vegetables worldwide with lots of applications from food to other specific uses. There are different types in the lettuce group for consumers to choose from. Additionally, lettuce is an excellent source of bioactive compounds such as polyphenols, carotenoids, and chlorophyll with related health benefits. At the same time, nutrient composition and antioxidant compounds are different between lettuce varieties, especially for green and red lettuce types. The benefit of lettuce consumption depends on its composition, particularly antioxidants, which can function as nutrients. The health benefits rely on their biochemical effect when reaching the bloodstream. Some components can be released from the food matrix and altered in the digestive system. Indeed, the bioaccessibility of lettuce is measuring the quantity of these compounds released from the food matrix during digestion, which is important for health-promoting features. Extraction of bioactive compounds is one of the new trends observed in lettuce and is necessarily used for several application fields. Therefore, this review aims to demonstrate the nutritional value of lettuce and its pharmacological properties. Due to their bioaccessibility and bioavailability, the consumer will be able to comprehensively understand choosing a healthier lettuce diet. The common utilization pattern of lettuce extracted nutrients will also be summarized for further direction.

Metabolite shift in Medicago truncatula occurs in phosphorus deprivation

Symbiotic nitrogen (N) fixation entails successful interaction between legume hosts and rhizobia that occur in specialized organs called nodules. N-fixing legumes have a higher demand for phosphorus (P) than legumes grown on mineral N. Medicago truncatula is an important model plant for characterization of effects of P deficiency at the molecular level. Hence, a study was carried out to address the alteration in metabolite levels of M. truncatula grown aeroponically and subjected to 4 weeks of P stress. First, GC-MS-based untargeted metabolomics initially revealed changes in the metabolic profile of nodules, with increased levels of amino acids and sugars and a decline in amounts of organic acids. Subsequently, LC-MS/MS was used to quantify these compounds including phosphorylated metabolites in the whole plant. Our results showed a drastic reduction in levels of organic acids and phosphorylated compounds in -P leaves, with a moderate reduction in -P roots and nodules. Additionally, sugars and amino acids were elevated in the whole plant under P deprivation. These findings provide evidence that N fixation in M. truncatula is mediated through a N feedback mechanism that in parallel is related to carbon and P metabolism.

Melatonin biosynthesis pathways in nature and its production in engineered microorganisms

Melatonin is a biogenic amine that can be found in plants, animals and microorganism. The metabolic pathway of melatonin is different in various organisms, and biosynthetic endogenous melatonin acts as a molecular signal and antioxidant protection against external stress. Microbial synthesis pathways of melatonin are similar to those of animals but different from those of plants. At present, the method of using microorganism fermentation to produce melatonin is gradually prevailing, and exploring the biosynthetic pathway of melatonin to modify microorganism is becoming the mainstream, which has more advantages than traditional chemical synthesis. Here, we review recent advances in the synthesis, optimization of melatonin pathway. l-tryptophan is one of the two crucial precursors for the synthesis of melatonin, which can be produced through a four-step reaction. Enzymes involved in melatonin synthesis have low specificity and catalytic efficiency. Site-directed mutation, directed evolution or promotion of cofactor synthesis can enhance enzyme activity and increase the metabolic flow to promote microbial melatonin production. On the whole, the status and bottleneck of melatonin biosynthesis can be improved to a higher level, providing an effective reference for future microbial modification.