Date Published: June 1, 2018
Publisher: Public Library of Science
Author(s): Xuyu Chen, Yangyang Liu, Yun Yang, Jian Feng, Peiwei Liu, Chun Sui, Jianhe Wei, Vijai Gupta.
Only when Aquilaria spp. or Gyrinops spp. trees are wounded, due to insect attack, or microbial invasion, agarwood can be successfully induced. In the present study, a fungus which can induce agarwood formation efficiently was isolated and a suitable method for its application to induce agarwood formation was developed. Rigidoporus vinctus was isolated from the inner layers from infectious A. sinensis trees. When the fermentation liquid of fungi inoculated back to A. sinensis tree, agarwood was found to be induced. In addition, a novel method called trunk surface agarwood-inducing technique (Agar-Sit) was developed to produce agarwood with R. vinctus. The alcohol soluble extract content of the agarwood, up to 38.9%, far higher than the requirement (10%) in Chinese Pharmacopoeia and the six characteristic compounds of agarwood used as Chinese Medicinal Materials were all detected. Their relative percentages of the sesquiterpenes in the essential oil were 22.76%. This is the first report of the Agar-Sit and also the application of R. vinctus in agarwood induction. According to the results, when the combination of Agar-Sit and R. vinctus is used agarwood can be induced with high yield and good quality.
Agarwood, the resinous wood of Aquilaria spp. or Gyrinops spp. trees [1–3], is highly valued for its extensive use in medicine, perfumes, and incense across Asia, Middle East and Europe . Agarwood is one of the famous traditional medicine for sedative, carminative, and anti-emetic effects in China, produced from Aquilaria sinensis. The wild resources of Aquilaria spp. and Gyrinops spp. are coming to be endangered and has been placed in the Appendix II list of the Convention on International Trade in Endangered Species of Wild Fauna and Flora since 2004 [5–6]. The natural formation of agarwood with high quality needs a few years or even decades after the tree is damaged by certain external factors, such as lightning strike, animal grazing, insect attack, or microbial invasion . Along with the continuously decreasing supply of wild agarwood and in order to meet market demand, much effort has been devoted to agarwood production. In 2010, an effective technique called the whole-tree agarwood-inducing technique (Agar-Wit) was developed in our laboratory . When Agar-Wit is used, Agarwood formation is mainly inside the trunk, but the surface of the trunk wood is not fully utilized. Of note, fungi have been found to play a certain role in promoting agarwood formation. Back in 1935, Burkill recognized that fungal infections to trees were the cause of agarwood formation . In 1952, Bhattacharya et al. reaffirmed there was a relationship between agarwood formation and fungal infections . In 1976, the researchers from Guangdong Institute of Botany reported for the first time that fungi infection back to A. sinensis led to the agarwood formation . In 1998, Qi et al. reported that Menanotus flavolives can accelerate agarwood formation in A. sinensis . In 2005, Subeham et al. inoculated Fusarium laseritum into the holes on the trunk of Aquilaria spp. trees and obtained agarwood one year later . In 2011, Xu reported that Fusarium sp. promoted agarwood formation one year after fungi inoculation . In 2017, the fungi of Lasiodiplodia theobromae which can promote agarwood formation was reported by the research .
In the present study, a fungal isolate which invaded deeply into the trunk of A. sinensis was first reported and was capable of promoting agarwood formation. The fungus was applied to the originally invented Agar-Sit method which induced high-quality agarwood. The discovery of the isolate of R. vinctus was due to the observation that some wild fungi could enter the inner of wood and induce agarwood formation. So the study isolated the fungi from different layers from a special sample with microbial infection characteristics. The purpose of this study was to find suspicious fungi from different layers and confirm suspicious fungi can promote agarwood formation. Fortunately, R. vinctus were isolated and confirmed that it possesses strong infection ability to induce agarwood formation. R. vinctus is a kind of wood-degrading microorganism belonging to white rot fungus, which often causes the death of crops . As already reported, white rot fungus degrades cellulose and lignin of plant cells mainly through its highly efficient enzymatic system, including the activities of lignin peroxidase (LiP), manganese peroxidase (MnP) and laccase . As agarwood formation is considered as a result of defense response when trees confront wounding, it may be assumed that R. vinctus wounds trees by biodegrading lignin, thus inducing defense responses of these trees. This agrees with the hypothesis that agarwood formation results from a tree’s defense response to diverse damages, including mechanical wound, microbial invasion, and chemical stresses .