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Review

Advances in Biosynthesis and Pharmacological Effects of Cinnamomum camphora (L.) Presl Essential Oil

by
Yuqing Du
1,†,
Hua Zhou
2,†,
Liying Yang
3,
Luyuan Jiang
1,
Duanfen Chen
4,
Deyou Qiu
1 and
Yanfang Yang
1,*
1
State Key Laboratory of Tree Genetics and Breeding, Key Laboratory of Tree Breeding and Cultivation of State Forestry Administration, Research Institute of Forestry, Chinese Academy of Forestry, Beijing 100091, China
2
The Key Laboratory of Horticultural Plant Genetic and Improvement of Jiangxi Province, Institute of Biological Resources, Jiangxi Academy of Sciences, Nanchang 330096, China
3
China Flower Association, Beijing 100102, China
4
College of Horticulture, Hebei Agricultural University, Baoding 071001, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Forests 2022, 13(7), 1020; https://doi.org/10.3390/f13071020
Submission received: 31 May 2022 / Revised: 20 June 2022 / Accepted: 26 June 2022 / Published: 28 June 2022
(This article belongs to the Special Issue Molecular Mechanism of Secondary Metabolic Pathways in Forest Trees)
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Abstract

:
Cinnamomum camphora (L.) Presl essential oil (CCEO) is a volatile oil with aroma and is extracted from various tissues of Cinnamomum camphora. It is traditionally used as a spice, preservative, as an anti-inflammatory and for sterilization. Terpenoids are the main active components in CCEO. Based on currently available research, considerable effort is still needed to study the biosynthesis and regulation of terpenoids in CCEO. In this review, the research progress related to terpenoid biosynthesis and bioactivity in CCEO in recent years is summarized, with the data compiled and presented mainly from online resources such as PubMed, Scopus and CNKI in China up to May 2022. The research advances related to key enzymes in the terpenoid biosynthesis pathway are mainly discussed. Previous studies have isolated some genes encoding key enzymes involved in terpenoid biosynthesis; however, among these genes, only a few TPS genes have been verified to catalyze the production of terpenoid synthases at the protein level. Most genes encoding key enzymes have been cloned and isolated, but no transgenic experiments have been carried out to verify gene function. In-depth study of the biosynthesis of terpenoids in CCEO may contribute to a better understanding of the differential accumulation of terpenoids in different types of C. camphora and provide reference for improving terpenoid content in CCEO.

1. Introduction

Essential oils are a kind of secondary metabolite in aromatic plants that consist of a variety of components with wide sources and especially intense aromas, easy extraction and easy volatilization without residue. Cinnamomum camphora (L.) Presl essential oil (CCEO), which is extracted from roots, stems, leaves, seeds and fruits, is the general name of volatile oil-like substances with a certain odor in C. camphora; it is mainly used in fragrances, health care, the chemical industry, food and other fields [1].
In recent years, with the in-depth study of CCEO, many researchers have confirmed that CCEO possesses a variety of biological activities, such as anticancer [2], antioxidant [3], anti-inflammatory [3], antibacterial [4], analgesic [5] and antidiabetic [3]. Related research and reports on the chemical constituents of CCEO have focused principally on terpenoids, mainly monoterpenoids linalool, camphor, eucalyptol, citral, phellandrene and sesquiterpenoids nerolidol, β-caryophyllene, elemene, methylisoeugenol, etc. [6,7]. Moreover, with the rapid development of genomics, transcriptomics, proteomics and metabolomics, omics technologies have been applied to the study of the biosynthesis of CCEO. Moreover, the genes encoding the key enzymes in the biosynthesis of CCEO have been successfully isolated and cloned, and the protein structures, functions and mechanisms of more and more key enzymes have been studied [8,9,10,11,12]. These works lay a theoretical foundation for further research on the biosynthesis of CCEO.
In this paper, the research progress regarding CCEO’s chemical constituents, isolation and cloning of key genes in the biosynthesis pathway of terpenoids, and pharmacological activities are reviewed, aiming to provide reference for further research and utilization of CCEO.

2. Chemical Constituents of CCEO

There are some distinctions in the chemical composition of CCEO due to differences in species of C. camphora, provenance, climate in the growing area, genetic factors, etc. At present, the essential oil chemical constituents of a total of 18 species of Cinnamomum have been identified, including C. camphora, C. camphora var. linaloolifera Fujita, C. longepaniculatum (Gamble) N. Chao ex H. W. Li, Cinnamomum parthenoxylon (Jack) Meisner, C. micranthum (Hay.) Hay, C. bodinieri Lévl. and C. chartophyllum H. W. Li, and about 330 kinds of compounds in CCEO have been identified [13].
Analysis of the main chemical constituents of CCEO shows that the main compositions are monoterpenes and sesquiterpenes (Table 1). The main monoterpenoids in the chemical constituents include linalool, eucalyptol, camphor, safrole, citral, phellandrene, α-terpineol, etc. (Figure 1); the main sesquiterpenoids are iso-nerolidol, β-caryophyllene, elemenol, methylisoeugenol, etc. (Figure 2). Most terpenoids have an aromatic smell and are widely used in perfume, cosmetics, flavoring agents and so on. In addition, terpenoids have important medicinal value and biological activity. Linalool, the main component of CCEO, has analgesic, anti-inflammatory [14] and antitumor effects [15] and is widely used in medical drugs and reagents. Camphor is detumescence, analgesic, antibacterial and insecticidal, and is extensively used in the pharmaceutical industry [16]. Caryophyllene can be used for local anesthesia, as an anti-inflammatory, etc., and has great potential value in drug synthesis [17].

3. Biosynthesis Pathway of CCEO

CCEO is the secondary metabolite produced in the physiological metabolism of C. camphora. These secondary metabolites play an important role in the growth and development of C. camphora [22,23,24] and in resisting the harsh external environment. For instance, essential oil content is related to tree height, basal diameter and biomass of each part of C. camphora, with complex correlativity [23]. Besides, there are many factors affecting the content of essential oils in different aromatic plants, including genotype, environmental factors, development stage and tissue parts. Among the Cinnamomum camphora, discrepancy of essential oil composition into different chemical types is due to the interaction between genetic factors and the environment and is related to developmental stage [25]. Therefore, the biosynthesis mechanism of these secondary metabolites in plants has always been an important subject.
Biosynthesis of terpenoids is a process in which small molecular reactants in plants react to produce various complex and diverse terpenoids under the catalysis of a series of related enzymes and can be divided into three main stages (Figure 3): (1) Synthesis of the general precursors, terpenoids isopentenyl diphosphate (IPP) and its double-bonded isomer dimethylallyl diphosphate (DMAPP); namely, the C5 precursor formation stage; (2) Formation of direct precursor substances such as farnesyl diphosphate (FPP); (3) Formation and modification of terpenoids [26].
There are usually two pathways for the formation of the C5 precursor. The first is the mevalonate pathway (MVA), which occurs in the cytoplasm; the second is the 2-C-methyl-D-erythritol-4-phosphate pathway (MEP) occurring in the plastids, also referred to as the deoxyxylulose-5-phosphate pathway (DXP). Under the action of isoprenyl diphosphate isomerase, the IPP generated by the two pathways is partially converted into a double-bonded isomer, DMPAA. IPP and DMAPP are the basic units for the synthesis of all terpenoids in nature. In plants, different proportions of IPP and DAMPP reactants generate direct precursors of geraniol diphosphate (GPP), farnesyl diphosphate (FPP) and geraniol diphosphate (GGPP) under the action of geraniol diphosphate synthase (GPPS), farnesyl diphosphate synthase (FPPS) and geraniol geraniol diphosphate synthase (GGPPS), respectively [27]. The first two stages are the sharing stage of terpenoids synthesized by plants in nature. The third stage is the most important for the synthesis of terpenoids, as it determines the complexity and diversity of terpenoids. There are complex and diverse terpene synthases and various modifying enzymes in this stage, resulting in a wide variety of terpene compounds in plant essential oil components [28,29,30]. Direct precursors are catalyzed by different types of TPS and modifying enzymes to produce, respectively, monoterpenes and sesquiterpenes.

4. Genes Involved in CCEO Biosynthesis Pathway

The biosynthesis pathway of terpenoids in natural plants is basically straightforward, and the studies of cloning, expression and regulation of the genes encoding the enzymes involved in biosynthesis are important research fields. Terpenoids are rich in the Lauraceae family, where several genes involved in biosynthesis of terpenoid in CCEO have been successfully identified and functionally characterized.

4.1. Genes Encoding Key Enzymes in MVA Pathway

The 3-hydroxy-3-methylglutaryl CoA reductase (HMGR), one of the important rate-limiting enzymes in the MVA pathway, is a vital regulatory site in the terpene biosynthesis pathway in the cytoplasm [31]. In C. camphora, two HMGR genes, CcHMGR1 and CcHMGR2, have been cloned based on transcriptome data with five chemical types of C. camphora as experimental materials for the first time. Real-time PCR indicated that the expression level of CcHMGRs was the highest in cineol-type C. camphora with the highest content of essential oils [32]. Meanwhile, the gene CcPMK, encoding 5-phosphomevalonate kinase (PMK), which is another key rate-limiting enzyme in the MVA pathway, has also been cloned and analyzed by the same methods as for CcHMGEs, and the results confirmed that its expression level was highest in borneol-type C. camphora, which has a high content of principal component borneol among five chemical types of C. camphora [33]. Therefore, it can be speculated that the expression levels of HMGR and PMK genes in C. camphora are related to the contents of terpenoids, and by increasing the expression levels of key genes in the upstream terpenoid synthesis pathway, terpenoids content may be increased.
Mevalonate kinase (MK), which catalyzes the transfer of phosphate groups on adenosine triphosphate to the fifth hydroxyl group of mevalonate to form mevalonate-5-phosphate, is a key rate-limiting enzyme in the MVA pathway [34]. The CcMK gene was successfully cloned by designing specific primers, and it was found that expression of CcMK in C. camphora leaves was the highest, so it was presumed that biosynthesis of terpenoids occurs in the leaves of C. camphora [35].
A series of enzyme-related genes in the MVA pathway were cloned by Wang et al. [36]. The expression patterns of these genes in different tissues and leaves from different chemical types of C. camphora were detected by real-time PCR. The results showed that HMGS2, HMGR2 and IDI1 genes had similar expression patterns, which were significantly upregulated in sesquiterpene-rich tissues (leaves, petioles, flowers, pedicels), and the three genes’ expression levels in leaf tissues of sesquiterpene were 3–6 times that found in the leaves of other chemical types. Further analysis showed that HMGS2, HMGR2 and IDI1 had co-expression patterns, and the three genes may play important roles in regulating the formation and distribution of sesquiterpenoid. It is a pity that this presumption lacks evidence at the protein level. Therefore, more in-depth research is needed to analyze and compare the promoter sequences of various genes in the MVA pathway of C. camphora and to find regulatory transcription factors and transformation model plants for gene function identification.

4.2. Genes Encoding Key Enzymes in MEP Pathway

In the MEP pathway, the key rate-limiting enzymes that have been widely studied include 1-deoxy-D-xylulose-5-phosphate synthase (DXS) and 1-deoxy-D-xylulose-5-phosphate reductionoisomeras (DXR). At present, DXS and DXR genes in C. camphora have been isolated and cloned, and their expression levels have been analyzed [37,38]. It was found that both DXS and DXR are highly expressed in leaves. They may have tissue-specific expression patterns in C. camphora, and they may mainly play biological functions in leaves. Meanwhile, the expression level of CcDXR1 was the highest in borneol-type C. camphora among five different chemical types. The borneol-type C. camphora also has the highest content of CCEO among the five chemical types [24]. These results indicate that the expression of DXR gene was positively correlated with the content of essential oil in C. camphora. Therefore, it can be speculated that upregulation of DXR gene expression in C. camphora may increase terpenoid content.
The CcDXS1-5, CcDXR and CcHDR genes in the MEP pathway from C. camphora were identified by homology comparison [39]. Meanwhile, under treatment with methyl linoleate, the expression of CcDXS1-3 was significantly upregulated; however, the increase of CcDXS4 expression was not obvious, indicating the function of the paralogs gene may be divergent; under treatment with gibberellin, the expression levels of CcDXS1-4, CcDXR, and CcHDR increased by various degrees; under treatment with indoleacetic acid, the expression of the gene did not show a clear upward trend. These results show that different plant hormones have different effects on the expression of these three types of genes, and their regulatory pathways may also be different and further studies are needed in the future. Overall, these results show that the MEP pathway plays an important role in regulating CCEO synthesis in C. camphora at the transcriptional level.
The gene CcCMK1 encoding 4-diphosphocytidyl-2-C-methyl-D-erythritol kinase (CMK) was isolated by using transcriptome data. Real-time PCR showed that the CcCMK1 gene had the highest expression level in C. longepaniculatum among the five chemotypes of C. camphora [40]. Some genes, for example the gene encoding (+)-linalool synthase, had higher expression in the linalool type and lower expression in the cineole type [41]. It can be inferred that in different chemical types of C. camphora, the content of these compounds is closely related to the expression of genes in the MEP pathway. However, another report found that most genes in the MVA and MEP pathways were expressed at similar levels in the leaves of borneol-type and linalool-type C. camphora. Moreover, in the same study, there were seven TPS genes differentially expressed in the different chemotypes, suggesting that the different expression levels of TPS genes in the two chemotypes may be the reason for their accumulation of different terpenoids. Therefore, the downstream genes also play a special and important role in the accumulation of CCEO, in addition to the genes in the MVA and MEP pathways.

4.3. Genes Encoding the Prenyltransferase (PTS)

PTS is one of the vital enzymes in the synthesis of terpenoids; it acts on the IPP and DMPAA generated in the first stage and catalyzes the formation of direct precursors. According to the different intermediates, PTS can be divided into GGPPS, FPPS or GPPS. There is too little research about PTS in C. camphora. Although GGPPS and FPPS genes have been cloned and the characteristics of their encoded proteins analyzed by bioinformatics methods [42,43], identification of protein activity and gene function have no related reports. As described above, in-depth study of downstream genes such as PTS will help improve CCEO content.

4.4. Genes Encoding Terpenoid Synthase (TPS)

Terpene synthase, the key enzyme to synthesize the final terpenoid products, is located downstream in the terpenoid biosynthesis pathway. A large number of different TPSs and the fact that some TPSs produce multiple products are the chief reasons for the variety in terpenes [44]. Terpenoid compounds in plants can also be further modified by modification enzymes, including dehydrogenase, reductase, glycosyltransferase, acyltransferase, methyltransferase and cytochrome P450 monooxygenase [45]. It has been mentioned that terpenoid yield can be improved by enhancing the expression of upstream genes in the MVA and MEP pathways. However, analysis and utilization of key enzyme genes downstream in the terpenoid biosynthesis pathway would also be of great significance to improve terpenoid content.
According to their products, TPSs can be divided into monoterpene synthase, sesquiterpene synthase, diterpene synthase, etc. More than 200 kinds of terpenoid biosynthase genes have been successfully cloned from hundreds of plants, most of which are involved in secondary metabolism in plants. The linalool synthase gene has been isolated from C. osmophloeum Kanehira and expressed in E. coli to collect the recombinant proteins. Linalool and iso-nerolidol have been generated in vitro by adding GPP or FPP to the recombinant protein as substrate [46].
Three TPS genes involved in monoterpene biosynthesis have been identified, and their expression levels in borneol-type camphor trees were higher than those in linalool-type camphor trees [47], which led to speculation that the three TPS genes may play an important role in terpenoid accumulation in borneol-type C. camphora. The specific functions of these three genes remains to be further verified.
Moreover, three TPS genes have been cloned from three different chemical types (eucalyptol, linalool and camphor), and it was speculated that the three genes are related to the biosynthesis of camphor, eucalyptol and linalool, respectively [48]. After overexpression in the woody model plant poplar 84K (Populus alba × P. glandulosa), the three genes were detected at the transcriptional level, but linalool, camphor and eucalyptol were not detected. It can be seen that there are too many unsolved mysteries, and the synthesis of CCEO with plants as the chassis has a long way to go.

5. Pharmacological Activity of CCEO

C. camphora has always been an important spice, widely used by people in medicine, disinfection and sterilization, daily spices, cosmetics, food flavoring and brewing [49]. The active substances extracted from CCEO, such as linalool and camphor, have precious medicinal value, but the mechanism of most active substances is still in the stage of research and exploration.

5.1. Antibacterial and Bacteriostatic Activity

CCEO contains chemical components with broad-spectrum antimicrobial characteristics, such as linalool, eucalyptus oil, etc. The antibacterial and bacteriostatic mechanisms of CCEO are still in the stage of research and exploration. The gas-phase bactericidal mechanism of CCEO against E. coli has been studied. It was found that CCEO had obvious gas-phase antibacterial activity, which was related to the relative hydrophilicity and volatility of CCEO [19]. Moreover, antibacterial tests showed that CCEO had different degrees of inhibitory effects on Gram-negative bacteria (E. coli, Pseudomonas aeruginosa) and Gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis), and the latter had stronger resistance to CCEO than the former [50].
Besides, CCEO also has certain bacteriostatic effects on fungi, and its bacteriostatic effect on fungi is stronger than on bacteria [51]. It has been reported that C. longepaniculatum essential oil had a strong inhibitory effect on the mycelial growth of Pyricularia oryzae Cav., Pestalotia funereal, Rhizoctonia solani Kuha, Curvularia lunata (walk) Boed, Gibberelle zeae (Schw.) Petch. and Collectotrichum gloeosporioides (Perz.) sacc. [52]. In addition, the latest research found that CCEO had stronger anti-Candida albicans activity than grapefruit, lemon and lavender essential oil, this activity coming from α-terpineol, terpinolene, α-terpinene, terpinene-4-ol, sabinene and 1,8-cineole in CCEO [53].

5.2. Anti-Inflammatory Action

The 1,8-cineole is a component of CCEO that may reduce the number of inflammatory cells and proinflammatory factor content, thus exhibiting certain anti-inflammatory properties [54]. Additionally, relevant experiments have indicated that extracts from C. camphora and C. subavenium Miq. exert an obvious immunomodulatory effect on a variety of inflammatory responses at the transcriptional level [3,55]. According to the presented findings, it can be speculated that the effects might be caused through inhibiting the expression of proinflammatory genes, such as those for cytokines and cytotoxic-molecule-generating enzymes, including inducible nitric oxide synthase and cyclooxygenase, via affecting the nuclear factor kappa B pathway. Furthermore, it has been reported that CCEO extracted from seeds could reduce the levels of inflammatory markers (tumor necrosis factor-α, interleukin-6 and P65), probably through upregulating peroxisome proliferator-activated receptor [56]. Moreover, another study reported that 80% ethanol extract of C. camphora leaves exhibits anti-inflammatory effects by phosphorylation of signal transducer, activator of transcription 1 and extracellular signal-regulated kinase 1/2 [57]. However, further experimental investigations should be carried out related to how these active substances, such as borneol, β-caryophyllene and camphor, play a role in treatment of inflammation [58].

5.3. Anti-Tumor Effect

The polyphenols and flavonoids present in Cinnamomum extracts and the safrole exracted from C. longepaniculatum essential oil have an anticancer effect [59,60]. The effects of ethanol extracts from C. camphora leaves on the proliferation of human lung cancer 95-D cells, human oral epidermal carcinoma KB cells and liver cancer HepG2 cells were studied, and it found that the ethanol extracts of C. camphora leaves had obvious anticancer effects in vitro [2]. In addition, relevant studies have identified that camphor white oil (CWO), which is produced by steam distillation of wood from the camphor laurel tree (C. camphora), and its derivatives can induce the regression of keratinocyte-derived malignancies. The components of CWO were analyzed by GC-MS, and α-pinene and d,l-Limonene were identified as active compounds to promote tumor regression. However, it is still necessary to continue to explore whether these components act synergistically or separately on the molecular pathway [61].

5.4. Antioxidant Activity

CCEO has a certain antioxidant capacity, which increases with essential oil concentration [62]. It has been reported that CCEO can reduce the number of reactive oxygen-free radicals via improving the activities of antioxidant enzymes, so as to have an antioxidant effect [56]. In addition, a high concentration of C. osmophloeum Kanehira extract has been proven to have an obvious free-radical scavenging capability [63]. Earlier studies reported that ethanolic extract of C. curvifolium (Lam.) Nees, C. burmanni, C. cassia, C. verum and C. tamala possessed important antioxidant activities, and positive correlations existed between total antioxidant activity and total phenolic content [64]. Moreover, C. zeylanicum Blume essential oil has antioxidant effects, which may attributed to the presence of phenolic compounds [65]. Taken together, it can be speculated that the antioxidant activity of CCEO may be derived from phenolic compounds in essential oil. However, some studies have shown that the antioxidant capacity of essential oils is due to the existence of several monoterpenes, such as α-pinene, 1,8-cineole [66] and some sesquiterpenes [67]. Further exploration of possible antioxidant mechanisms and antioxidant active components in CCEO needs to be done.

5.5. Parasite Control and Insecticidal Activity

Certain components in CCEO repel and kill insects. Camphor has a certain repellent effect on mosquitoes, and its effective time is up to 6 h. Linalool, derived from C. camphora, possesses properties against parasites (protozoa and helminths) and insects [19]. Moreover, CCEO from leaves, which mainly contains camphor, camphene and limonene, has insecticidal activity against midges (Chaoborus plumicornis), butterfly larvae (Pieris rapae Linnaeus), fruit flies (Drosophila melanogaster Meigen) and fire ants (Solenopsis invicta × richteri hybrid) [68]. The ethanolic extract of C. camphora exhibited significant acaricidal activity against the mite Tetranychus cinnabarinus (Boisduval), and the most effective components in the extract were 2,4-di-tert-butylphenol and ethyloleate [69].

6. Discussion

C. camphora is an important timber and ornamental tree. In addition, the root, stem, leaf and peel of C. camphora contain essential oil of very high economic value, as it can be used in industrial production and medicine. However, there are many offspring with sexual propagation of C. camphora species, which makes it difficult to maintain the high essential oil properties of the mother tree, as the essential oil content varies greatly. This has seriously affected the production of essential oils. Previous studies have speculated that this is jointly determined by ecological environment and genetic factors. However, a detailed molecular mechanism is not very clear. The development of molecular biology technology will be helpful to improve the yield of CCEO by understanding the synthesis mechanism of C. camphora. This review summarized the past decades’ research progress regarding CCEO. Although the biosynthesis mechanism of CCEO has been developed, it needs to be further analyzed. This paper provides direction for further analysis of the molecular mechanism of CCEO synthesis to finally improve production.
CCEO has a long history of application in the world, and its rich chemical components are complex, diverse and precious. It also has great application value and development potential. With the development of molecular biology technology and omics technology, many achievements have been made in the biosynthesis of CCEO, the role of key enzymes in the synthesis process, the cloning and analyzing of related enzyme genes and so on. Several genes involved in the essential oil biosynthesis pathway of C. camphora have been cloned and analyzed, and the characteristics, chemical properties and other biological information of these genes and enzymes have been explored and studied by molecular biology techniques. However, more genes should be isolated and their functions studied to understand the biosynthesis mechanism of CCEO and to improve production in the future.
Several aspects highlighted below need to be considered and investigated further in C. camphora. Only a few genes of key enzymes in the MVA and MEP pathways have been cloned and studied; studies of TPS in the third stage of the CCEO biosynthesis pathway are fewer and are not deep enough. Based on functional analysis of the reported terpene synthase genes, other uninvestigated terpene synthase genes may be viewed as promising genes with important regulatory functions. Thus, future research should consider TPS genes in the biosynthesis pathway of CCEO.
More research is also needed to explore the biological activity of CCEO. Most studies on the biological activity of the chemical components of CCEO are still in the experimental stage. The action mechanism of most active substances in essential oils are still in the stage of exploration and research. There are no reported clinical studies to assess the efficacy and safety of the reported bioactivities in C. camphora essential oil. Clinical studies are indispensable to adequately characterize these aspects.
Collectively, CCEO has broad prospects in scientific research, development and utilization, but there are also some problems that should be appreciated at this stage. Therefore, the research, development and innovation of CCEO still need to be continuously deepened to promote maximum utilization of CCEO.

Author Contributions

D.C. and D.Q. contributed suggestions. Y.Y., Y.D. and H.Z. wrote the manuscript. L.J. and L.Y. performed literature collection. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Key Laboratory of Horticultural Plant Genetic and Improvement of Jiangxi Province (2020KFJJ003) and the Key Research and Development Program of Jiangxi Province (20203BBFL63057).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The paper is a review and does not present original data. All data can be found in the cited references.

Conflicts of Interest

The authors declare no conflict of interest.

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Forests 13 01020 g001 550
Figure 1. Structural formulas of monoterpene compounds.
Figure 1. Structural formulas of monoterpene compounds.
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Figure 2. Structural formulas of sesquiterpene compounds.
Figure 2. Structural formulas of sesquiterpene compounds.
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Figure 3. There are two biosynthesis pathways of terpenoids in plants. MEP pathway in the cytoplasm is showed in blue, and MVA pathway in the cytoplasm is showed in green. The C5 precursor formation in the first stage is represented in blue and green fonts. The generation of direct precursors in the second stage is represented in blue and green to black fonts. The generation of terpene compounds in the third stage is represented in black to purple fonts. The dashed-line arrow marks multiple steps, and the solid-line arrow indicates transportation of products in cells. Purple fonts express the terpenoid compounds catalyzed by terpenoid synthases and modifying enzymes. CDP-ME, 4-(cytidine 5’-diphospho)-2-C-methyl-D-erythritol; CDP-MEP, CDP-ME 2-phosphate; DXP, 1-deoxy-D-xylulose-5-phosphate; HBMPP, 1-hydroxy-2-methyl-2-butenyl-4-diphosphate; HMG-CoA, 3-hydroxy-3-methylglutaryl CoA; MEP, 2-C-methyl-D-erythritol 4-phosphate; MEcPP, 2-C-methyl-D-erythritol 2,4-cyclodiphosphate; MVA, mevalonic acid; MVP, 5-phosphomevalonate; MVPP, 5-diphosphomevalonate; AACT, acetoacetyl-CoA thiolase; HMGS, 3-hydroxy-3-methylglutaryl CoA synthase; HMGR, 3-hydroxy-3-methylglutaryl CoA reductase; MVK, mevalonate kinase; PMK, 5-phosphomevalonate kinase; MVD, 5-diphosphomevalonate decarboxylase; DXS, 1-deoxy-D-xylulose 5-phosphate synthase; DXR, 1-deoxy-D-xylulose 5-phosphate reductoisomerase; MCT, 2-C-methyl-D-erythritol 4-phosphate cytidyltransferase; CMK, 4-(cytidine 5’-diphospho)-2-C-methyl-D-erythritol kinase; MDS, 2-C-methyl-D-erythritol 2,4-cyclodiphosphate synthase; HDS, 1-hydroxy-2-methyl-2-butenyl 4-diphosphate synthase; HDR, 1-hydroxy-2-methyl-2-butenyl 4-diphosphate reductase; IDI, isopentenyl diphosphate isomerase.
Figure 3. There are two biosynthesis pathways of terpenoids in plants. MEP pathway in the cytoplasm is showed in blue, and MVA pathway in the cytoplasm is showed in green. The C5 precursor formation in the first stage is represented in blue and green fonts. The generation of direct precursors in the second stage is represented in blue and green to black fonts. The generation of terpene compounds in the third stage is represented in black to purple fonts. The dashed-line arrow marks multiple steps, and the solid-line arrow indicates transportation of products in cells. Purple fonts express the terpenoid compounds catalyzed by terpenoid synthases and modifying enzymes. CDP-ME, 4-(cytidine 5’-diphospho)-2-C-methyl-D-erythritol; CDP-MEP, CDP-ME 2-phosphate; DXP, 1-deoxy-D-xylulose-5-phosphate; HBMPP, 1-hydroxy-2-methyl-2-butenyl-4-diphosphate; HMG-CoA, 3-hydroxy-3-methylglutaryl CoA; MEP, 2-C-methyl-D-erythritol 4-phosphate; MEcPP, 2-C-methyl-D-erythritol 2,4-cyclodiphosphate; MVA, mevalonic acid; MVP, 5-phosphomevalonate; MVPP, 5-diphosphomevalonate; AACT, acetoacetyl-CoA thiolase; HMGS, 3-hydroxy-3-methylglutaryl CoA synthase; HMGR, 3-hydroxy-3-methylglutaryl CoA reductase; MVK, mevalonate kinase; PMK, 5-phosphomevalonate kinase; MVD, 5-diphosphomevalonate decarboxylase; DXS, 1-deoxy-D-xylulose 5-phosphate synthase; DXR, 1-deoxy-D-xylulose 5-phosphate reductoisomerase; MCT, 2-C-methyl-D-erythritol 4-phosphate cytidyltransferase; CMK, 4-(cytidine 5’-diphospho)-2-C-methyl-D-erythritol kinase; MDS, 2-C-methyl-D-erythritol 2,4-cyclodiphosphate synthase; HDS, 1-hydroxy-2-methyl-2-butenyl 4-diphosphate synthase; HDR, 1-hydroxy-2-methyl-2-butenyl 4-diphosphate reductase; IDI, isopentenyl diphosphate isomerase.
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Table
Table 1. Common monoterpenes and sesquiterpenes in essential oil of Cinnamomum camphora (L.) Presl.
Table 1. Common monoterpenes and sesquiterpenes in essential oil of Cinnamomum camphora (L.) Presl.
TypeCompound NameMolecular FormulaReferences Bibliography
monoterpeneOxygenated monoterpenescitralC10H16O[18]
borneolC10H18O[18]
linaloolC10H18O[12,19,20]
camphorC10H16O[19,20,21]
eucalyptolC10H18O[12,19,20,21]
α-terpineolC10H18O[7,12,19,20,21]
safroleC10H10O2[19,21]
Monoterpene hydrocarbonsphellandreneC10H16[12]
α-PineneC10H16[7,12,20,21]
β-PineneC10H16[7,12,20,21]
sabineneC10H16[7,20]
myrceneC10H16[6]
limoneneC10H16[21]
campheneC10H16[6]
sesquiterpeneOxygenated sesquiterpenenerolidolC15H26O[12,20]
iso-nerlidolC15H26O[18]
spathulenolC15H24O[12,19]
Sesquiterpene hydrocarbonsgermacreneC15H24[12]
β-caryophylleneC15H24[12,18,20]
elemeneC15H24[9]
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Du, Y.; Zhou, H.; Yang, L.; Jiang, L.; Chen, D.; Qiu, D.; Yang, Y. Advances in Biosynthesis and Pharmacological Effects of Cinnamomum camphora (L.) Presl Essential Oil. Forests 2022, 13, 1020. https://doi.org/10.3390/f13071020

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Du Y, Zhou H, Yang L, Jiang L, Chen D, Qiu D, Yang Y. Advances in Biosynthesis and Pharmacological Effects of Cinnamomum camphora (L.) Presl Essential Oil. Forests. 2022; 13(7):1020. https://doi.org/10.3390/f13071020

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Du, Yuqing, Hua Zhou, Liying Yang, Luyuan Jiang, Duanfen Chen, Deyou Qiu, and Yanfang Yang. 2022. "Advances in Biosynthesis and Pharmacological Effects of Cinnamomum camphora (L.) Presl Essential Oil" Forests 13, no. 7: 1020. https://doi.org/10.3390/f13071020

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