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The Scent of Stress: Evidence From the Unique Fragrance of Agarwood (Wllapatta)

Agarwood (Aquilaria spp.) fragrance and its origin in stress make it probably the most suitable model to study stress-induced aroma. Production being confined only to certain small pockets of South and Southeast Asia, agarwood is arguably the costliest wood in the world. Formation of fragrant agarwood resin is the outcome of complex biotic, abiotic, and physical stress on the Aquilaria trees. The intricate mechanism by which some 150 odd fragrant molecules that constitute agarwood aroma is formed is still not clearly understood. The present review therefore aims to bring to focus this less known but highly valuable stress-induced aroma from Asia. Discussions on agarwood species, occurrence, distribution, formation, and products have been included as foundation. Although global trade in agarwood and its products is estimated at US$6 billion to US$8 billion, no reliable data are readily available in literature. Therefore, an effort has been made to review the current status of agarwood trade. The element of stress and its correlation to agarwood aroma is discussed in the subsequent sections. Natural agarwood formation as well as technologies and interventions in agarwood induction are stress-based (natural and artificial injury, insect and fungal attack, chemical induction). The molecular triggers are gradually coming to light as new studies are implicating jasmonate, LOX signaling, and other stress reaction routes as the source of agarwood aroma. This review therefore has strived to compile the information that is scattered across scientific as well as other authentic literature and update the reader on the current status. More information about the specific roles of other vital stressors like insects, abiotic, and genetic factors is eagerly awaited from ongoing and future research to further understand the unique fragrance of agarwood.

Introduction
Agarwood is quite unusual, since stressed, diseased, and malformed trees are preferred over healthy, luxuriant ones. In fact, the infected heartwood of agar is the most expensive wood in the world. The fervor of antiquity associated with agarwood is evident from texts and traditions of the most ancient cultures. Agarwood, aloeswood, eaglewood, agaru, and gaharu are all synonyms for the resinous, fragrant, and valuable heartwood of mostly Aquilaria spp. belonging to the family Thymelaeaceae. Owing to their widespread use in medicinal, aromatic, and religious purposes, agarwood is also known as the Wood of the Gods. Trade in agarwood is over 2,000 years old with consumer centers located mostly in the Middle East and Eastern Asia, while the supply came through traditional routes from agarwood growing zones that range from South Asia (that includes China, Northeast India, and Bangladesh) through to continental Southeast Asia and to the Indo-Malaysian archipelago (Hou and Van Steenis, 1960). After the 1970s, there has been a phenomenal increase in the demand for agarwood particularly from the Middle East. Since most of the agarwood trees grow in wild areas, concern over their sustainable utilization is justified. Due to the rampant destruction of natural habitats, most agarwood-bearing species have been relegated to the status of endangered species. In fact, all 19 known Aquilaria species are included under CITES (CITES, UNEP-WCMC Species Database: CITES-Listed Species, 2019) and the Red List of the IUCN (IUCN Red List of Threatened Species, 2019). The trade in agarwood is largely unorganized, and often fake and adulterated wood is pushed into markets as cheaper agarwood. Even under such circumstances, the price per kilogram of agarwood can range from US$100 to US$100,000 depending on the quality of the material. In the market, agarwood is available in various grades depending on the resin content, specific gravity, color, and sometimes odor. For instance, in Malaysia, Kalambak and Gaharu are the two popular grades of agarwood, while in Japan, Kanankoh (or Yara) and Jinkoh stand for the highest- and lowest-quality agarwood, respectively. Similarly, in India, four wood types, viz., True agar (black), Bantang (brown), Butha (mixture of agarwood with non-agar), and Dhum (yellow), are commercially marketable grades of agarwood, based on the decreasing intensity of dark coloration caused by resinous deposits (Naef, 2011). Formation of agarwood occurs by an intricately orchestrated stress response mechanism caused by injury due to physical, microbial or entomological activity within the wood. Secondary compounds rich in oleoresins are formed as a result of the stress and get deposited in the heartwood that turns dark and heavier (Ng et al., 1997). However, the frequency of natural infection is low and is rather a matter of chance, as seen in case of plantations where only 7–10% of the trees ultimately form resin. In fact, the phenomenon of natural agarwood formation is yet to be properly understood, and the need for proper scientific inquiry and evolution of technologies adoptable for sustainable production of agarwood is a matter of urgent concern. The present review is an attempt at analyzing this unique phenomenon of stress-induced formation of agarwood. By critically analyzing the different aspects of agarwood formation, its chemical diversity, and commercialized products, the influence of stress upon aroma comes across as a prominent inducing factor. Moreover, biotic (insect, fungus) interactions established over thousands of years of co-evolution have led to a deeper scientific interest into the origins of this unique aroma. Agarwood, therefore, can arguably be the best example of stress-induced aroma. Therefore, a review that catalogues, deliberates, promotes, and articulates the information scattered in scientific literature and other sources can provide the basis for a mechanistic understanding of such phenomena, as well as to initiate and regulate future research in this exciting field.

By - https://www.frontiersin.org/

The Scent of Stress: Evidence From the Unique Fragrance of Agarwood Agarwood (Aquilaria spp.) fragrance and its origin in stress make it probably the most suitable model to study stress-induced aroma. Production being confined only to certain small pockets of South and Southeast Asia, agarwood is arguably the costliest wood in the world. Formation of fragrant agarwo...