Abstract

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.

Introduction

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. Y. et al., 2017). The formation of agarwood is generally associated with the wounding and fungal infection of the Aquilaria trees (Liu Y. et al., 2013; Mohamed et al., 2014). 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, 2013).

Agarwood has been widely used as therapeutic perfumes, traditional medicine, religious purposes and aromatic food ingredient (Liu Y. et al., 2013). Some of the earliest known uses of agarwood were recorded in ancient literatures, religious scriptures and medical texts. The word “aloes” which means agarwood was found occurring in the Sanskrit poet, Kâlidâsa that can be dated back to c. 4th–5th century CE (Lee and Mohamed, 2016). 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., 2016; Liu Y. Y. et al., 2017).

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., 2010). As the wealth of the consumer countries has gradually increased in the recent decades, the market’s demand for agarwood started to exceed its supply. Global agarwood prices can be ranging from US$ 20 – 6,000 per kilogram for the wood chips depending on its quality or US$ 10,000 per kilogram for the wood itself (Abdin, 2014). In addition, the value of agarwood essential oil can be as high as US$ 30,000 per kilogram. The annual global market for agarwood has been estimated to be in the range of US$ 6 – 8 billion (Akter et al., 2013), yet a large number of the trades have not been recorded.

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, 2016). The destructive exploitation of agarwood, however, has badly affected the wild population of all Aquilaria species. As a consequence, the genus is now listed as endangered species and protected under Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) regulation due to a drastic declination of the species in the wild (Convention On International Trade In Endangered Species [CITES], 2004; Lee and Mohamed, 2016).

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 300 C.E. in the Chinese history, where it was recorded that resin deposition accompanied with color changes of internal tissues can happen within a year by injuring the trees (López-Sampson and Page, 2018). Besides mechanical wounding approach, the use of chemical, insect and pathogen-inducing techniques is increasingly common in the agarwood industry nowadays (Liu Y. et al., 2013; Mohamed et al., 2014; Kalita, 2015).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.

Agarwood Induction Approaches

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., 2018). 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, 2016; Wu et al., 2017). 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., 2010; Azren et al., 2018). 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.

Strengths and weaknesses of different types of agarwood inducing methods.

The Main Constituents of Agarwood

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. Y. et al. (2017). 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., 2015; Liu Y. Y. et al., 2017).

Many studies have been conducted to clarify the metabolite composition of agarwood obtained either from wild or artificially induced methods (Chen et al., 2012; Gao X. et al., 2014; Hashim et al., 2014). It was concluded that the composition of agarwood resin is mainly composed of the mixtures of sesquiterpenes and 2-(2-phenylethyl) chromones (PECs) (Naef, 2011; Chen et al., 2012; Subasinghe and Hettiarachchi, 2015; Figure 1). Meanwhile, the constituents of agarwood essential oil were shown primarily to be sesquiterpenoids (Fazila and Halim, 2012; Hashim et al., 2014; Jayachandran et al., 2014). Together, all of these major compounds and some low abundant volatile aromatic metabolites form the unique and fragrant-smelling property of agarwood.

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, 2012; Jong et al., 2014; Pasaribu et al., 2015).

Nonetheless, there are over 150 compounds as reviewed by Naef (2011) 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, 2011).

Several sesquiterpenes were observed to be more frequently present in agarwood from different studies, including aromadendrene, agarospirol, β-agarofuran, guaiol and (-)-aristolene (Fazila and Halim, 2012; Liu Y. et al., 2013; Jayachandran et al., 2014; Jong et al., 2014; Figure 2). Some sesquiterpenes are reported to be species-specific, such as jinkoh-eremol and epi-γ-eudesmol that only present in A. malaccensis, while baimuxinal only exists in A. crassna and A. sinensis (Naef, 2011; Liu Y. et al., 2013; Jong et al., 2014; Hashim et al., 2016).

It is worth mentioning that in the study of Pasaribu et al. (2015), the content of aromadendrene was found to be greater in higher grade agarwood and therefore it was suggested as an effective chemical marker for agarwood grading. Besides aromadendrene, Jayachandran et al. (2014) later has proposed an additional marker valencene which can be important in the grading of agarwood oil.

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, 2013; Jong et al., 2014).

In comparison to the sesquiterpene constituents in agarwood, the types of PECs being determined by GC-MS are relatively limited. Structural studies revealed that all previously reported PECs in agarwood own the same basic skeleton (molecular weight: 250) and similar substituents, i.e., either hydroxy or methoxy groups (Mei et al., 2013). The percentage of 2(2-phenylethyl) chromone and 2-(2-4-methoxy-phenylethyl) chromone in the high grade agarwood such as kanankoh can be as high as 66.47 %, which is overwhelmingly higher than the lower-quality agarwood jinkoh that has only 1.5% (Ishihara et al., 1993).

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., 1982). There are 17 types of chromone derivatives which are agarwood specific and potential marker for the purpose of authentication (Naef, 2011). 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., 1986).

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 (2011) regarding the major constituents of agarwood, new compounds continue to be discovered in the later studies (Wu et al., 2012a; Yang et al., 2014b; Wang et al., 2015). The number of discovered compounds in agarwood will certainly be further increased in the future.

Source: Scientific report on Agarwood by the authors:

• Yoswathana, 2013;

• Jong et al., 2014

• Ishihara et al, 1993

• Shimada et al., 1982

• Naef, 2011

• Shimada et al., 1986

• Wu et al, 2012a;

• Yang et al., 2014;

• Wang et al., 2015

• Pasaribu et al., 2015;

• Liu Y. Y. et al., 2017;

• Chen et al., 2012;

• Gao X. et al., 2014;

• Hashim et al., 2014

• Rasool & Mohamed, 2016;

• Wu et al., 2017

• Azren et al., 2018

• Rasool & Mohamed, 2016;

• Fazila & Halim, 2012;

• Jayachandran et al ., 2014;

• NCBI (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6374618/)

Please see more:

• Agarwood Essential Oil Uses
• Agarwood Perfume Extrait
• The Biosynthesis Pathways of Agarwood Constituents