We'd like to understand how you use our websites in order to improve them. Register your interest. Gyrinops walla Gaertn. Although agarwood resin induction and extraction from Aquilaria species of the same family have been practised for many decades in Southeast Asian region, the ability of producing agarwood resins in G. Since previous studies were on agarwood resins formed due to natural causes, the present study was conducted to identify the potential fungal species that are capable of artificially inducing agarwood resin formation in G.
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Agarwood is a resinous part of the non-timber Aquilaria tree, which is a highly valuable product for medicine and fragrance purposes. To protect the endangered Aquilaria species, mass plantation of Aquilaria trees has become a sustainable way in Asian countries to obtain the highly valuable agarwood.
As only physiologically triggered Aquilaria tree can produce agarwood, effective induction methods are long sought in the agarwood industry. In this paper, we attempt to provide an overview for the past efforts toward the understanding of agarwood formation, the evolvement of induction methods and their further development prospects by integrating it with high-throughput omics approaches.
Agarwood also known as gaharu in the South East Asia, oud in the Middle East, chen xiang in China, jinkoh in Japan and agar in India is a highly valuable aromatic dark resinous heartwood of Aquilaria species Liu Y. The formation of agarwood is generally associated with the wounding and fungal infection of the Aquilaria trees Liu Y. The resin is secreted by the trees as defense reaction and deposited around the wounds over the years following the injury, where the accumulation of the volatile compounds eventually forms agarwood Subasinghe and Hettiarachchi, Agarwood has been widely used as therapeutic perfumes, traditional medicine, religious purposes and aromatic food ingredient Liu Y.
Some of the earliest known uses of agarwood were recorded in ancient literatures, religious scriptures and medical texts.
Meanwhile, the use of agarwood in the prescription of traditional Chinese medicine of the same period had also been recorded. The Chinese medicine uses it as a natural sedative, pain reliever, digestive aid and carminative Ye et al.
Agarwood has high demand throughout the world as a raw material for incense, perfume and medicine purposes, with Middle East and East Asia as the two major regions of consumption Antonopoulou et al.
Aquilaria belongs to the Thymelaeaceae family of angiosperms, which is endemic to the Indomalayan realm. To date, there is a total of 21 Aquilaria species which have been documented and 13 of them are recognized as the agarwood-producing species Lee and Mohamed, The destructive exploitation of agarwood, however, has badly affected the wild population of all Aquilaria species. High demand of quality agarwood in conjunction with the depletion of the wild Aquilaria trees implied that the price of the agarwood will continue to soar.
As an alternative, mass cultivation and large plantation of Aquilaria trees which serve as a sustainable source to obtain agarwood have greatly resolved the shortage of agarwood supply in the global market. Since healthy Aquilaria tree does not form agarwood, leaving it worth next to nothing, the scarcity of naturally occurring agarwood has prompted the development of artificial agarwood-inducing methods.
Efforts to artificially induce the agarwood formation can be traced back to as early as C. Besides mechanical wounding approach, the use of chemical, insect and pathogen-inducing techniques is increasingly common in the agarwood industry nowadays Liu Y.
All of these induction techniques in any case mimic the natural processes of agarwood formation, which have their own strengths and weaknesses. In this article, we endeavor to provide a more comprehensive coverage of existing induction methods and their development prospects using the advancement of biotechnology. To better understand the agarwood formation process, the molecular mechanism of secondary metabolite biosynthetic pathways underlying the resin production will also be elaborated.
The indiscriminate harvesting of agarwood from natural habitats has seriously hampered natural regeneration of Aquilaria trees, thus threatening the survival of the species in the wild. In order to meet the high market demand yet to protect the species from extinction, mass plantations of Aquilaria trees have been established across the Asian countries to allow sustainable agarwood production Azren et al.
Since agarwood formation in natural environment is a very long process which can take up to 10 years, the development of effective induction technology has received a great attention as it is extremely crucial to ensure the stability of agarwood yield from the domesticated Aquilaria trees. Naturally, agarwood formation is often linked to the physical wounding or damage of Aquilaria trees caused by thunder strike, animal grazing, pest and disease infestations Rasool and Mohamed, ; Wu et al.
These events expose the inner part of the trees toward pathogenic microbes, which elicit the defense mechanism of Aquilaria to initiate the resin production. This natural formation process of agarwood has greatly inspired the development of diverse artificial induction methods Table 1.
For example, many traditional induction approaches like nail in setting, holing, burning, trunk breaking and bark removal have adopted the concept of physically wound the trees Mohamed et al. Although it is cost effective and requires only personnel with little or no scientific knowledge on agarwood, but these induction methods usually result in inferior quality and uncertain yield of agarwood.
Table 1. Strengths and weaknesses of different types of agarwood inducing methods. With more understanding on Aquilaria -fungal interactions in promoting the agarwood formation, the induction methods gradually shifted from sole mechanical wounding into deliberate wounding coupled with the application of biological inoculum Jong et al. Many pure-culture strains of fungi isolated from natural agarwood were found to be effective biological agents to induce agarwood formation in healthy Aquilaria trees Cui et al.
The fungal infected Aquilaria trees were reported to deposit agarwood resin around the infected sites as barrier to prevent further fungal intrusion Cui et al.
One obvious advantage of using fungal inoculum is that it is generally believed to be safe for handling and eco-friendly.
However, fungal inoculation will normally give rise to localized and inconsistent quality of agarwood due to the different fungal consortium used. As a solution, laborious holing process and long incubation time is required to maximize the colonized surface area on the tree to produce better quality of agarwood Mohamed et al.
Instead of relying on external stimuli to trigger plant responses, either by mechanical wounding or biological inoculum, some induction approaches have been developed to introduce signaling molecules directly and specifically into Aquilaria trees to initiate agarwood resin biosynthesis pathways Liu Y. Chemical inducers normally comprise of phytohormones, salts, minerals and biological-derived substances Zhang et al.
Besides, suitable delivery method is often developed together with the chemical formulations to ease the large-scale induction process, such as vessel equipped with transfusion needle Yang et al. To date, several induction approaches have been developed based on the chemical induction concept such as cultivated agarwood kit CA-kit , the whole-tree agarwood inducing technique Agar-Wit and biologically agarwood-inducing technique Agar-bit. CA-kit is a combined method based on physical wounding and chemical induction, where the inducing agent is applied into the Aquilaria tree via an aeration device inserted into the wound Blanchette and Heuveling, This method results in satisfying yield and quality, but the procedures are in some way conventional.
On the other hands, Agar-Wit is a transpiration-assisted chemical treatment to form an overall wound in the tree, where the preloaded inducer in a transfusion set is distributed via plant transpiration Liu Y. Through this method, a larger agarwood coverage area can be achieved, but unfortunately produces more decayed tissues. Similarly, Agar-bit method adopts the idea of distributing the inducing reagent by plant transpiration, except that the reagents are injected directly into the stems of the tree Wu et al.
Through chemical induction approach, the time-consuming holing process can be minimized as less induction sites are needed to deliver the inducers throughout the plants via transpiration process. Properly formulated inducer was shown to be able to produce artificial agarwood with quality closely resembled to those obtained from natural source Liu Y.
In spite of the fast results and high yields, the application of chemical inducers still poses skepticism of toxicity on both human and environment. More assessments on chemical inducers are required to test its effectiveness on fields and also to popularize its use. Chemical inducers are undoubtedly more suitable for mass production with easier quality control than biological inoculum, which is highly potential to substitute conventional induction methods and the use of biological inoculum in agarwood industry.
The main attraction of the agarwood industry is its extremely high market value. Yet, the price of agarwood is largely determined by its quality which is graded solely based on human experience from the age-old practices of each country.
The unavailability of standard quality grading system can be due to the intricate appearance of the traded agarwood and personal interest. The currently adopted agarwood quality assessment in the market has been extensively reviewed by Liu Y.
Recently, the metabolite analysis of agarwood has gained increasing attention as some studies showed that there is correlation of agarwood quality to its resin yield and metabolite constituents Pasaribu et al. Many studies have been conducted to clarify the metabolite composition of agarwood obtained either from wild or artificially induced methods Chen et al.
It was concluded that the composition of agarwood resin is mainly composed of the mixtures of sesquiterpenes and 2- 2-phenylethyl chromones PECs Naef, ; Chen et al.
Meanwhile, the constituents of agarwood essential oil were shown primarily to be sesquiterpenoids Fazila and Halim, ; Hashim et al. Together, all of these major compounds and some low abundant volatile aromatic metabolites form the unique and fragrant-smelling property of agarwood.
Figure 1. The basic molecular skeleton of sesquiterpenes A and 2- 2-phenylethyl chromones B. The number and types of agarwood metabolite constituents of each reported studies vary depending on the agarwood source, extraction methods and analysis approaches used Fazila and Halim, ; Jong et al. Nonetheless, there are over compounds as reviewed by Naef have been identified thus far in agarwood from different sources.
Among these compounds, there are 70 sesquiterpenes and about 40 types of PECs which have been recognized in agarwood and their structures have been elucidated Naef, It is worth mentioning that in the study of Pasaribu et al. Besides aromadendrene, Jayachandran et al. Figure 2. Chemical structures of sesquitepene compounds that commonly exist in agarwood resin. The PEC derivatives, as other major fragrance constituents of agarwood are the important contributors to the sweet, fruity and long lasting scent of agarwood when it is burnt.
These compounds can only be detected by supercritical carbon dioxide and solvent extraction methods but never present in the extract of hydrodistillation Yoswathana, ; Jong et al. Structural studies revealed that all previously reported PECs in agarwood own the same basic skeleton molecular weight: and similar substituents, i.
The percentage of 2 2-phenylethyl chromone and 2- methoxy-phenylethyl chromone in the high grade agarwood such as kanankoh can be as high as Furthermore, the presence of certain PEC derivatives in agarwood was proposed to be useful in the evaluation of the grading of agarwood products Shimada et al.
There are 17 types of chromone derivatives which are agarwood specific and potential marker for the purpose of authentication Naef, The substituted chromones, such as agarotetrol and isoagarotetrol Figure 3 , were shown to have positive correlation with the quality of agarwood obtained in the market with some exceptions Shimada et al.
Figure 3. Chemical structures of 2- 2-phenylethyl chromone derivatives commonly present in agarwood resin. The types and derivatives of major compounds in agarwood are extremely wide and diverse, indicating the miscellaneous fragrance properties of agarwood from different species and regional sources. The better insight of agarwood metabolites will definitely facilitate the identification of universally accepted biomarkers for agarwood grading.
Since the publication of the comprehensive review of Naef regarding the major constituents of agarwood, new compounds continue to be discovered in the later studies Wu et al. The number of discovered compounds in agarwood will certainly be further increased in the future.
Agarwood formation can be related to the self-defense mechanism of Aquilaria trees in response to biotic and abiotic stresses Gao et al. Stresses trigger the defense responses of Aquilaria species which in turn initiate the secondary metabolite biosynthesis and the accumulation of agarwood resin. Previously, we have mentioned that sesquiterpenes and PEC derivatives are the main constituents in agarwood.
Hence, it is crucially important to understand the metabolic pathway for the regulation and biosynthesis of sesquiterpenes and chromone derivatives in Aquilaria species to effectively induce the agarwood formation. In plants, the isoprenoid precursors for the biosynthesis of sesquiterpenes, triterpenes and sterols has generally been assumed to be provided from the mevalonic acid MVA pathway in cytosol.
In plastids, the 1-deoxy-D-xylulosephosphate DXP or known as methylerythritol phosphate MEP pathway provides precursors for the production of monoterpenes, diterpenes, and carotenoids Rohmer, ; Dong et al.
These two pathways biosynthesise C5 homoallylic isoprenoid precursor, that is isopentenyl pyrophosphate IPP and its electrophilic allylic isomer dimethylallyl pyrophosphate DMAPP. The genes encode for these enzymes have been identified from Aquilaria species through transcriptome sequencing analysis Xu et al.
The FPS is one of the key-limiting enzymes responsible for the sesquiterpene biosynthesis Gaffe et al. The transcript level of AsFPS1 was reported to be higher in stem and roots than the leaves, suggesting that sesquiterpene synthesis in Aquilaria species tends to be tissue-specific. This provides clues for the artificial induction of agarwood formation via exogenous chemically induced approaches by triggering the sesquiterpene biosynthetic pathway in Aquilaria trees.
Figure 4. Schematic relationships between the wound-induced signal transduction mechanisms for the sesquiterpene biosynthesis and regulation in Aquilaria species for the agarwood production. Direct and indirect interactions are shown as solid and dotted lines, respectively. In the final stage of sesquiterpenes production, the enzymes accountable for the diversification of sesquiterpene mainly come from the classes of sesquiterpene synthases SesTPs and cytochrome P dependent mono-oxygenases Ps.
Agarwood Induction: Current Developments and Future Perspectives
Agarwood , aloeswood, eaglewood, or gharuwood is a fragrant dark resinous wood used in incense, perfume, and small carvings. It is formed in the heartwood of aquilaria trees when they become infected with a type of mold Phialophora parasitica. Prior to infection, the heartwood is odourless, relatively light and pale coloured; however, as the infection progresses, the tree produces a dark aromatic resin, called aloes not to be confused with Aloe ferox , the succulent known as bitter aloes or agar not to be confused with the edible, algae-derived agar as well as gaharu , jinko , oud , or oodh aguru not to be confused with bukhoor , in response to the attack, which results in a very dense, dark, resin-embedded heartwood. The resin-embedded wood is valued in Indian - North Eastern culture for its distinctive fragrance, and thus is used for incense and perfumes. The aromatic qualities of agarwood are influenced by the species, geographic location, its branch, trunk and root origin, length of time since infection, and methods of harvesting and processing. One of the main reasons for the relative rarity and high cost of agarwood is the depletion of the wild resource. The odour of agarwood is complex and pleasing,  with few or no similar natural analogues.
Agarwood is a resinous part of the non-timber Aquilaria tree, which is a highly valuable product for medicine and fragrance purposes. To protect the endangered Aquilaria species, mass plantation of Aquilaria trees has become a sustainable way in Asian countries to obtain the highly valuable agarwood. As only physiologically triggered Aquilaria tree can produce agarwood, effective induction methods are long sought in the agarwood industry. In this paper, we attempt to provide an overview for the past efforts toward the understanding of agarwood formation, the evolvement of induction methods and their further development prospects by integrating it with high-throughput omics approaches. Agarwood also known as gaharu in the South East Asia, oud in the Middle East, chen xiang in China, jinkoh in Japan and agar in India is a highly valuable aromatic dark resinous heartwood of Aquilaria species Liu Y. The formation of agarwood is generally associated with the wounding and fungal infection of the Aquilaria trees Liu Y. The resin is secreted by the trees as defense reaction and deposited around the wounds over the years following the injury, where the accumulation of the volatile compounds eventually forms agarwood Subasinghe and Hettiarachchi,