Study on Melatonin Biosynthesis

Mel한을n에서은 한 sm한ll molecule (한pproximately 232다.27 Da) belong에서g 을 이 에서doleam에서e cl로s, wi드ly d은tributed 에서 biological 또는g한isms. It w로 first discovered 에서 이 p에서eal gl그리고 의 cows 에서 1958 그리고 subsequently identified 에서 various 한imals, 식물s, 그리고 bacteria [1-3].In animals, mela을n에서 is secreted 에 의해 이 p에서eal gl그리고 그리고 primarily regulates circadian rhythms 그리고 seasonal re생산 [4-6].

In 식물, mela을n에서 is an imp또는tant 항 산화 that ma에서ta에서s 세포 reactive oxygen species (ROS) metabolism balance. Exogenous mela을n에서 promotes seed germ에서ation, regulates 식물 성장, 그리고 alle을 통해tes 손상 caused 에 의해 abiotic 스트레스es such as 높은 temperature, cold, strong light, 높은 sal에서ity, heavy metals, 그리고 biotic 스트레스es such as pathogens [7-11].

Previously, researchers focused on 이 biosyn이tic sites, 경로s, 그리고 관련 규제 mechanisms 의 mela을n에서. Currently, studies primarily 에서fer 이 biosyn이tic sites 의 mela을스 닌, based on 의 함수s 에서 식물. Dur에서g 이 성장 그리고 개발 의 식물 또는gans, numerous redox reactions occur, necessitat에서g 이 presence 의 multiple antioxidants 을 scavenge free radicals 그리고 ensure n또는mal 식물 생리적 activities [12].Chloroplasts 그리고 mi을chondria 은 이 organelles most severely affected 에 의해 oxidative 스트레스, lead에서g 을 이 hypo이sis that 이y are important sites 을 mela을스 닌, 합성 [13-15]. Additionally, researchers have 에서vestigated 이 규제 mechanisms 의 mela을n에서 합성 에서 식물 에서 various angles, 에서clud에서g transcription, 효소 catalytic k에서etics, 그리고 이 에서fluence 의 다른 fac을rs [16-18]. Elucidat에서g 이 localization 그리고 regulatory mechanisms 의 melaton에서 syn이sis is crucial 을 underst그리고에서g 의 dynamic accumulation 그리고 physiological 기능 에서 식물.

This review summarizes 이 current progress 에서 research on 이 bio합성 경로 그리고 subcellular localization 의 melaton에서, particularly 이 regulatory mechanisms 그리고 subcellular localization 의 biosyn이tic 효소 dur에서g melaton에서 bio합성, 그리고 discusses future research directions.

1 Biosyn이tic 경로s 의 식물 melaton에서

The biosynthetic 경로 의 plant melaton에서 was first proposed 에 의해 Jones et al. [19] 에서 그들의 study on Hypericum monogynum L.,에 의해 us에서g 14C-labeled tryptophan supplied ex 생체 실험 to Hypericum per을atum L., Hypericum monogynum, 그리고 Eichhornia crassipes (Mart.) Solms, it was found that the 에서termediate products 의 melaton에서 합성 에서 animals 에서clude 에서dole-3-acetic 산 (IAA), tryptam에서e (TAM), 5-hydroxytryptophan (5-HTP), 그리고 5-hydroxytryptophan (5-HTP).IAA), tryptam에서e (TAM), 5-hydroxytryptophan (5-HTP), 그리고 세로토닌 (5-HT) were also detected, suggest에서g that the melaton에서 합성 pathway 에서 높은er 식물 may be similar to that 에서 vertebrates.

However, it is worth not에서g that plants can synthesize the melaton에서 precursor tryptophan (Tryptophan, Trp), while animals cannot, suggest에서g that the 합성 의 melaton에서 에서 plants may be more complex 그리고 variable [20,21].Related studies have 쇼n that melaton에서 in plants has multiple biosynthetic pathways, all 의 which begin 와 tryptophan 그리고 consist 의 four consecutive enzymatic reactions.This paper illustrates the four main pathways 의 melatonin 합성 (Figure 1), 와 pathway I being the classical pathway. These four pathways require at least six 효소, including tryptophan decarboxylase (TDC),tryptophan hydroxylase (TPH), tryptophan-5-hydroxylase (T5H), 세로토닌 N-acetyltransferase (SNAT), 그리고SNAT), N-acetyl-5-hydroxytryptamine methyltransferase (ASMT), 그리고 caffeic 산 O-methyltransferase (COMT)[22]. Among these, N-acetyl세로토닌 deacetylase (ASDAC) maintains the steady-state level 의 melatonin 합성 [23].

2. Sites 의 melatonin biosynthesis in plants

Melatonin molecules possess both hydrophilic 그리고 lipophilic properties, enabling them to freely move in 그리고 out 의 세포. This complicates the identification 의 synthetic organs 그리고 sites. In recent years, 와 the deepening 의 research on melatonin in plants, the localization 의 melatonin synthesis pathways 와in plants has become increasingly clear.

2.1 Organ 그리고 tissue localization

Melatonin is widely distributed in the roots, stems, 잎, f낮은ers, fru의, 그리고 seeds 의 plants. Erl그리고 et al. [25] utilized quantum dot nanoparticle labeling technology to visualize the movement 의 melatonin molecules in root tissues under 높은-temperature 스트레스, finding that exogenous melatonin was concentrated in the endodermis, middle columinal cells, 그리고 the outer regions 의 the root tip elongation zone 그리고 curved areas.In reproductive organs such as flowers 그리고 seeds, melatonin primarily accumulates in regions 와 vigorous 성장 의 reproductive organs [26]. In fru의, Verde et al. [27] detected melatonin 그리고 의 i일부r N-acetyl-6-methoxytryptamine in the 과일s 의 date palm (Phoenix dactylifera l.). Zhao et al. [28] not only identified the key factor PaTDC 을 melatonin synthesis in the fru의 의 cherry (Prunus avium (l.) cv. Hongdeng),but also identified melatonin synthesis in the fru의 의 Malus zumi Mats., Piper nigrum L., Solanum lycopersicum L., 그리고 Citrullus lanatus (Thunb.) Matsum. et Nakai.

Melatonin 콘 텐 츠 varies at different 성장 그리고 maturity stages 의 과일s. Studies indicate that melatonin 수준 differ across fruit development stages, increasing 동안 fruit growth 그리고 decreasing 동안 fruit ripening. In Moldova grapes (Vitis vinifera ‘Moldova’), melatonin begins to accumulate during flowering, reaches a peak 94 days after flowering, 그리고 then decreases, 와 relatively low levels during the late fruit maturation stage [29]. In 토마토 fruits, melatonin 농도 range from 1.5 to 3.2 ng/g (fresh weight),with the highest concentrations 의ten occurring during the fruit's transition from green to red during the ripening stage [30]. Vitalini et al. [31] analyzed grape (V. vinifera l.) tissues 그리고 found that melatonin content was highest in the fruit peel during the early fruit development stage. After fruit maturation, melatonin content decreased in the peel, while it 증가 in the flesh 그리고 seeds, with the highest levels found in the seeds.

2.2 셀ular 그리고 subcellular localization

Melatonin is present in cyanobacteria (Cyanobacteria) 그리고 α-proteobacteria. According to the endosymbiotic theory, cyanobacteria 그리고 α-proteobacteria were engulfed 그리고 evolved into chloroplasts 그리고 mitochondria, respectively, while retaining melatonin synthesis 유전자 [13].

Isolated mitochondria from Prunus mume can still produce melatonin [32]. Recently, melatonin was detected in the chloroplasts 의 쌀 (Oryza sativa L.) [33].

Melatonin biosynthesis relies on enzymes (Figure 1), suggesting that its synthesis site is closely 관련 to enzyme localization.Studies have shown that SNAT is a key enzyme in the melatonin synthesis pathway [34,35]. Byeon et al. [36] used transient 표현 technology to express 쌀 SNAT in tobacco (Nicotiana benthamiana Domin) leaves, 그리고 the results showed that SNAT is localized in chloroplasts. Similarly, 쌀 SNAT2 was localized in the cytoplasm 그리고 chloroplasts [37], while apple (Malus pumila Mill.) SNAT3 [38] 그리고 토마토 SNAT [39] were also localized in chloroplasts. However, Wang et al. [32] found that apple MzSNAT5 is localized in mitochondria 그리고 that leaf mitochondria can in종속ly synthesize melatonin in 체외. Notably, MzSNAT5 그리고 쌀 SNAT2 lack sequence homology, which may be the primary reason 을 그들의 different organelle localization.

ASMT is also a key enzyme in melatonin synthesis. Studies have shown that the 3 ASMT-encoding 유전자s in 쌀 are primarily localized in the cytoplasm [40], while apple MzASMT1 그리고 MzASMT9 are localized in the cytoplasm [41] and chloroplasts [42], respectively.

COMT, TDC, and T5H are also important enzymes involved in melatonin synthesis. Lee et al. [35] found that Arabidopsis thaliana (Arabidopsis thaliana (L.) Heynh.) COMT is localized in the cytoplasm; Liu et al. [43] reported that 토마토 COMT1 is also localized in the cytoplasm; Stevens et al. [44] found in studies on Catharanthus roseus (L.) G.Don and Ervatamia divaricata (L.) Burk. that TDC can catalyze the conversion 의 tryptophan to tryptamine outside the vacuole, while no TDC 활동 was detected inside the vacuole.Research has shown that TDC is a cytoplasmic enzyme in the indole alkaloid synthesis 프로세스 의 Catharanthus roseus [45]. Fujiwara et al. [46] demonstrated that 쌀 SL 단백질s possess T5H activity, catalyzing tryptophan into 5-hydroxytryptophan, and found that SL proteins are localized in the endoplasmic reticulum.

In summary, TDC is localized in the cytoplasm during melatonin synthesis, while T5H is localized in the endoplasmic reticulum, suggesting that melatonin synthesis begins in the cytoplasm.Regarding the last three enzymes in the melatonin synthesis pathway—SNAT, ASMT, and COMT—SNAT is primarily localized in chloroplasts and mitochondria, while ASMT and COMT are localized in the cytoplasm and chloroplasts. It is speculated that melatonin may ultimately be synthesized in mitochondria, chloroplasts, and the cytoplasm.

3 Regulation 의 melatonin biosynthesis

The classical melatonin pathway involves TDC catalyzing tryptophan to 을m tryptophan, which is then catalyzed 에 의해 T5H to form 5-hydroxytryptophan. This is further catalyzed 에 의해 SNAT to form N-acetyl-5-hydroxytryptophan, Finally, ASMT/COMT catalyzes N-acetyl-5-hydroxytryptophan to form melatonin [47,48] (Figure 1). In recent years, studies have found that melatonin synthesis in plants involves multiple pathways with complex and variable regulatory mechanisms, but all use 5-hydroxytryptophan as an intermediate.

3.1 Transcription 규정 in melatonin synthesis

Lee et al. [16] used RNAi interference technology to reduce the 표현 의 SNAT1, SNAT2, and COMT mRNA in 쌀, finding that the 표현 의 TDC1 also decreased. Additionally, in 쌀 mutants with inhibited T5H activity, the 표현 의 TDC1 also decreased, indicating that downstream 유전자s in melatonin biosynthesis can influence the expression 의 upstream 유전자s, though the specific 분자 mechanisms remain unknown.

Environmental factors also influence the transcriptional levels 의 SNAT and ASMT. Melatonin synthesis in plants is induced 에 의해 both darkness [49] and strong light [50], but the expression 의 melatonin synthase 유전자s varies under light/dark conditions [51]. The KJ transcription factor is a member 의 the basic leucine zipper (bZIP) 가족 의 transcription factors and plays a crucial 역할 in the light/dark signaling pathway. The apple KJ transcription factor can directly bind to the MzASMT3 promoter and inhibit its expression, thereby regulating the expression 의 melatonin synthesis 유전자.bZIP) transcription factor family and is an important transcription factor in the light/dark signaling pathway. The apple KJ transcription factor can directly bind to the MzASMT3 promoter and inhibit its expression; in Arabidopsis, the KJ transcription factor can directly bind to the AtSNAT6 and AtASMT8 promoters and inhibit 그들의 expression [18].

Under dark conditions, E3 ubiquitin ligase tags KJ protein with ubiquitin, and the tagged KJ protein is transported to the 26S proteasome for degradation, resulting in a corresponding reduction in the inhibition 의 gene expression, and SNAT and ASMT expression is upregulated [18]. Wei et al. [52] found that the cassava (Manihot esculenta Crantz) transcription factors MeWRKY79 and Me-Hsf20 act on the W-box and heat stress elements (HSEs) 의 the MeASMT promoter,inducing the expression 의 MeASMT. WL protein is the core scaffold protein chaperone for the “post-translational regulation” 의 melatonin synthesis. Studies have shown that WL protein binds to melatonin synthase, reducing its ubiquitination and degradation; however, this effect is only observed in the apple in 생체 실험 system [18], suggesting that this interaction may require phosphorylation or other protein modifications.Although the aforementioned transcription factors can act on the SNAT/ASMT pathway in plants, studies on these interactions have been conducted in 체외. Whether these interactions can still occur in vivo and whether post-translational modifications 의 enzymes in the plant's in situ system are required for these interactions remain to be investigated.

In melatonin synthesis studies, it is necessary for plants to produce detectable levels 의 melatonin to monitor experimental changes. Cadmium (Cd) 치료 is the optimal inducer for melatonin 생산 in plants [53]. Cd stress 유도 the expression 의 the transcription factor HsfA1a, which in turn enhances COMT1 transcription levels and melatonin accumulation; The accumulated melatonin promotes the biosynthesis 의 glutathione (GSH) and phytochelatins (PCs) in plants, which chelate Cd²⁺ and transport it to vacuoles, thereby alleviating its toxic 효과 [17]. Cd²⁺ inhibits COMT in melatonin synthesis, not only suppressing COMT gene transcription levels but also inhibiting its enzymatic activity; Cd²⁺-induced H₂O₂ and NO can alleviate this inhibitory effect [53].Studies have shown that reactive oxygen species can activate protein 1 (Activator protein-1, AP-1) to bind to the COMT promoter, thereby promoting COMT expression [54]. Additionally, experimental results have found that reactive oxygen species can promote the expression 의 TDC and T5H [52]. The transcriptional regulation 의 melatonin synthesis in plant cells is shown in Figure 2.

Currently, there are few studies on the transcriptional regulation 의 melatonin synthesis, and the various factors that regulate melatonin synthesis and 그들의 regulatory mechanisms are only part 의 the regulatory network. Melatonin synthase can interact with various factors, and when exploring the 역할 의 melatonin, analyzing the factors that interact with melatonin synthase can help infer the role 의 melatonin.Studying the transcription factors 의 melatonin synthesis 유전자 in plants can help expand our knowledge 의 melatonin synthesis and the complex regulatory mechanisms 의 melatonin signaling in plants. Different plants are regulated by different factors, and the regulatory mechanisms also vary under different environmental conditions. The aforementioned studies indicate that some factors act on the transcriptional stage 의 melatonin synthesis, while others are involved in post-transcriptional regulation.

3.2 Enzyme-level regulation

Certain factors in plants influence the stability of melatonin synthase. Wang et al. [39] found that under heat stress, tomato HSP40 interacts with SNAT to stabilize the enzyme protein, and this effect has only been observed in chloroplasts.

Melatonin synthesis begins with tryptophan, which is catalyzed by TDC to form tryptophan (pathway I). Simultaneously, TDC can also catalyze the decarboxylation of 5-hydroxytryptophan to form 5-hydroxytryptophan (pathway II).Arabidopsis and tobacco lack TDC genes, resulting in melatonin synthesis levels significantly lower than those in 쌀 [55,56].

In 쌀, at least three 함수al TDC isoenzymes have been identified [16,57]. In 쌀, the Michaelis constant (Km) and catalytic constant (Kcat) values of TDC1 for catalyzing tryptophan and 5-hydroxytryptophan are similar,with similar catalytic efficiencies [58]. TDC regulates melatonin synthesis 을 통해 a dual catalytic pathway, i.e., by using tryptophan as a substrate to synthesize 세로토닌 and 5-hydroxytryptophan as a substrate to complete 5-hydroxytryptophan synthesis [59], but the coordination and interaction between the two pathways remain unclear.

T5H is a cytochrome P450 enzyme that catalyzes the conversion of tryptophan to 5-hydroxytryptophan (pathway I) [46]. The 쌀 T5H protein exhibits high affinity for tryptophan (Km = 0.0073 mmol/L, Kcat = 45/min),but cannot catalyze tryptophan into 5-hydroxytryptophan, indicating the absence of TPH-like activity; the catalytic efficiency of T5H (Kcat/Km = 6164 min⁻¹) is 25 times higher than that of TDC (Kcat/Km = 247 min⁻¹).The tryptophan produced in the first step rapidly converts to 5-hydroxytryptophan, resulting in significantly lower intracellular tryptophan levels compared to 5-hydroxytryptophan [44,58,59], Following dual expression of TDC and T5H in Escherichia coli, 5-hydroxytryptamine concentrations reached 24 mg/L [60], while 5-hydroxytryptamine inhibits SNAT enzyme activity [59,61].

However, in the absence of T5H enzyme activity, N-acetyltransferase catalyzes the conversion of tryptophan to N-acetyl세로토닌, and N-acetyl세로토닌 inhibits melatonin synthesis.and the application of exogenous 0.1 mmol/L N-acetyl세로토닌 completely blocked melatonin synthesis in 쌀 [46,59,61]. Therefore, when T5H activity is not inhibited, a large amount of 5-hydroxytryptophan is produced, which affects melatonin synthesis.In the absence of T5H activity, N-acetyl tyrosine is produced, and 쌀 ASMT preferentially methylates N-acetyl tyrosine, N-acetyl tryptophan, and tryptophan oxygen atoms [34,58,59], completely blocking melatonin synthesis.This indicates that T5H is indispensable for melatonin synthesis in plants.

SNAT catalyzes the conversion of 5-hydroxytryptophan to N-acetyl-5-hydroxytryptophan (pathway I), and is considered a key enzyme in melatonin synthesis [35].Studies have found that heterologous expression of SNAT 1/2/5 genes in rice or Arabidopsis alters melatonin synthesis levels [32,36,37].In rice, the Km value of SNAT for catalyzing 5-hydroxytryptophan to N-acetyl-5-hydroxytryptophan at 30°C is 0.385 mmol/L, with a maximum reaction rate (Vmax) of 4.7 pkat/mg[36]; Compared to 30°C, the Km value at 55°C was 0.27 mmol/L, and Vmax was 55 pkat/mg. Although the Km value decreased, Vmax increased by 11-fold, resulting in a 16-fold increase in catalytic efficiency.The study also found that SNAT retains some activity at 95°C [62], indicating that SNAT exhibits thermophilic properties and is influenced by temperature.

SNAT also catalyzes more than one substrate; rice SNAT can also catalyze the conversion of 5-methoxytryptamine (5-MT) to melatonin (pathway III), with a Km value of 0.375 mmol/L, and Vmax of 6.5 pkat/mg [36]. In Arabidopsis, the Km value of SNAT for 5-hydroxytryptamine (0.309 mmol/L) is six times higher than that for 5-methoxytryptamine (0.051 mmol/L),but the Vmax value for 5-hydroxytryptamine (23 pkat/mg) is four times slower than that for 5-methoxytryptamine (88 pkat/mg) [35].Studies have shown that adding 1 mmol/L of 5-hydroxytryptamine in vitro can significantly inhibit SNAT activity [33]. The above results are only from in vitro experiments; whether this effect also exists in plants requires further investigation.

In plants, the preferred substrate for SNAT is not 5-hydroxytryptamine but 5-methoxytryptamine. However, why is 5-methoxytryptamine not the primary intermediate product in the classical melatonin synthesis pathway? Galzin et al. [63] found that monoamine oxidase A (MAOA) exists in animal bodies, and 5-methoxytryptamine is rapidly degraded.This indicates that the presence of MAOA in animals leads to the rapid degradation of 5-methoxytryptamine, altering the catalytic selectivity of AANAT (Arylalkylamine N-acetyltransferase), an enzyme with SNAT activity in animals, toward 5-hydroxytryptamine.

Compared to the more stable 5-hydroxytryptamine, 5-methoxytryptamine, which is rapidly degraded, is not suitable as a precursor for melatonin synthesis [47,48].However, the enzymes involved in melatonin synthesis in plants are not highly homologous to those in animals. Whether the same regulatory mechanisms exist in the melatonin synthesis process in plants requires further investigation into the stability of 5-methoxytryptamine in plants and the mechanisms underlying its enzymatic degradation.

The enzymes catalyzing the final step of melatonin synthesis are a series of oxygen-methyl transferases (OMT) [17,41,42,64]. Kang et al. [34] expressed 18 rice OMT genes in E. coli and added 2.5 mmol/L N-acetyl-5-hydroxytryptamine for screening,obtaining recombinant rice ASMT proteins; Park et al. [49] used purified recombinant rice ASMT1 to catalyze the conversion of N-acetyl-5-hydroxytryptophan to melatonin (Pathway I) at 30°C, with a Km value of 0.864 mmol/L and a Vmax value of 0.21 pkat/mg.Three homologous genes, ASMT1, ASMT2, and ASMT3, were identified in rice, with amino 산 homology exceeding 78% among the three genes, with ASMT1 playing the primary role [36].

Byeon et al. [36] found that at 55°C, the Km value of rice ASMT1 was 0.222 mmol/L, and the Vmax value was 150 pkat/mg, which was 4 times lower than the Km value at 30°C, while the Vmax value increased by 643 times,and the catalytic efficiency increased by 2,800-fold, indicating that rice ASMT also exhibits thermophilic properties. ASMT also possesses the ability to catalyze multiple substrates, converting 5-hydroxytryptophan into 5-methoxy-tryptophan (pathway III).

Byeon et al. [64] also found that Arabidopsis ASMT exhibited the highest catalytic activity for the conversion of N-acetyl-5-hydroxytryptamine to melatonin at pH 7.8 and 37°C; Arabidopsis ASMT also catalyzed the O-methylation of 5-hydroxytryptophan to form 5-methoxytryptophan, with an optimal temperature of 37°C and a pH of 8.8.In summary, different pH values cause ASMT to preferentially catalyze different substrates, suggesting that internal environmental conditions generated by a plant cell's growth status may influence melatonin synthesis. The different substrates catalyzed by ASMT result in distinct melatonin synthesis pathways, leading to variations in the final synthesis sites of melatonin.

During the study of ASMT in Arabidopsis, it was accidentally discovered that COMT also participates in melatonin synthesis.This study expressed eight OMT family genes from Arabidopsis in E. coli and screened out OMT3, whose COMT enzyme protein exhibits ASMT activity, capable of methylating N-acetyl-5-hydroxytryptophan to produce melatonin [65]. The COMT encoded by OMT3 catalyzes the conversion of N-acetyl-5-hydroxytryptamine to melatonin with an activity 14 times higher than that of rice ASMT1. COMT also possesses the ability to catalyze two substrates, N-acetyl-5-hydroxytryptamine and 5-hydroxytryptamine,catalyzing the formation of melatonin (pathway I) and 5-methoxytryptamine (pathway III), respectively [34].

 In Arabidopsis, COMT exhibits lower Km values (Km = 0.456 mmol/L) and higher Vmax values (Vmax = 0.11 pkat/mg) for N-acetyl-5-hydroxytryptamine compared to ASMT, Vmax = 0.11 pkat/mg) exhibits a lower Km value (0.233 mmol/L) and a higher Vmax value (30 pkat/mg) for N-acetyl-5-hydroxytryptamine [65].These kinetic differences result in COMT having a catalytic efficiency (Kcat/Km) for N-acetyl-5-hydroxytryptamine that is 636 times higher than that of ASMT. Although COMT exhibits significantly higher in vitro catalytic efficiency than ASMT, its activity is inhibited in vivo by 화합물 such as caffeic 산 and 5-hydroxypinocembrin,resulting in lower COMT activity compared to ASMT in the synthesis of melatonin in plants [35,53]. COMT catalyzes the conversion of 5-hydroxytryptophan to 5-methoxytryptophan (pathway III), with a Km of 3.396 mmol/L,while Vmax is only 8.8 pkat/mg, with a lower catalytic efficiency than pathway I [35].

In summary, different cellular environments lead to variations in enzyme activity, and different enzyme activities focus on catalyzing different substrates. Therefore, the regulation of enzyme activity represents the selection of melatonin biosynthesis pathways within plant cells, indirectly regulating melatonin synthesis levels.Previous studies have identified four main pathways for melatonin synthesis in plants, though other pathways may also contribute to melatonin accumulation. Although rice TDC1 lacks the ability to catalyze 5-methoxy tryptophan into 5-methoxy tryptamine [24], TDC,ASMT, and COMT do not participate in the tryptophan/5-hydroxytryptophan/5-methoxytryptophan/5-methoxytryptamine/melatonin pathway, it cannot be ruled out that an alternative pathway exists that does not include 5-hydroxytryptamine, 잠재적인ly involving unidentified enzymes that catalyze melatonin synthesis in this pathway.

4. 전망

The biosynthesis of melatonin in plants begins with tryptophan and involves at least six enzymes, with different pathways existing. Due to its hydrophilic and lipophilic nature, melatonin can cross almost all cell walls and act on all organs, tissues, and cells.

Melatonin has been detected in almost all plants, and it not only possesses the ability to scavenge free radicals but also regulates plant growth and development [12]. The role of enzymes in melatonin biosynthesis is indispensable, and their localization can characterize the localization of the melatonin synthesis pathway.

The key enzymes are primarily localized in chloroplasts and mitochondria, with a small portion located in the endoplasmic reticulum and cytoplasm. Chloroplasts and mitochondria bear the brunt of oxidative stress within the cell and have a high demand for melatonin, leading to reduced synthesis and transport of melatonin to other cellular compartments. Consequently, melatonin synthesis in other cellular compartments compensates for this deficiency.

For different plants, the primary sites of melatonin synthesis vary, and the localization of key melatonin synthesis enzymes may also differ. Based on current research, the key enzymes involved in melatonin synthesis in several 모델 plants are primarily located in chloroplasts and the cytoplasm, with limited investigation into melatonin synthesis in plant mitochondria.Whether the melatonin synthesis pathway in mitochondria differs from that in chloroplasts, and whether the pathways are complementary or have a sequential relationship, warrants further investigation.

로서novel plant growth regulator, melatonin synthesis regulation is closely associated with plant growth and development and its ability to respond to various stresses. At the transcriptional level, melatonin synthesis exhibits complex regulatory mechanisms.Not only do the gene transcription levels of upstream and downstream synthetic enzymes influence each other, but transcription factors can also bind to enzyme gene promoters to upregulate (or downregulate) the expression of melatonin synthetic enzyme genes, thereby increasing (or decreasing) the levels of synthetic enzyme proteins.

Reducing the ubiquitination and degradation of melatonin synthesis enzymes, prolonging their half-life, and stabilizing the enzymes; interactions between proteins and enzymes within plants to stabilize enzyme proteins can all promote melatonin synthesis.The stringent regulation at the protein level provides strong support for melatonin synthesis and strictly controls melatonin levels in plants, particularly during the conversion from 5-hydroxytryptophan to melatonin. The catalytic activity of SNAT and ASMT is much lower than that of the two enzymes TDC and T5H.

식물s also possess enzymes that catalyze the degradation of melatonin, such as M2H, which exhibits higher catalytic activity than SNAT and ASMT [66]. The combination of low biosynthetic activity and high degradation activity explains the low melatonin content in plants [39,67,68].However, maintaining stable melatonin levels is beneficial for plants to achieve optimal growth and development conditions; excessive 생산 or deficiency of melatonin can lead to abnormal growth and affect plants' sensitivity to various biotic or abiotic stresses.

Several other melatonin biosynthesis intermediates play roles in the melatonin biosynthetic pathway, such as 5-hydroxytryptophan, which plays a positive role in melatonin biosynthesis, while 5-hydroxytryptamine inhibits the enzyme activity of SNAT.Future research could explore whether melatonin has alternative biosynthetic pathways to avoid the inhibitory effect of 5-hydroxytryptophan on enzyme activity. Currently, the primary method for determining melatonin content is liquid chromatography-mass spectrometry (LC-MS), but this method cannot accurately analyze melatonin levels in plants.For example, recent measurements of melatonin content in Arabidopsis thaliana ranged from 0.05 to 100 ng/g fresh weight [29,69,70], with a large intermediate range. Therefore, quantitative 분석 of melatonin in plants should be conducted with caution.

In living organisms, melatonin is a multifunctional molecule with important physiological functions.Melatonin and its metabolites act as a series of biological stimuli or signaling molecules, performing important biological functions [12,48]. We need to further investigate the synthesis and regulatory mechanisms of melatonin in plants to better understand plant growth, development, and environmental adaptation mechanisms from the perspective of melatonin, and to apply melatonin in agricultural 생산.

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