Agarwood wound locations provide insight into the association between fungal diversity and volatile compounds in Aquilaria sinensis

Progressive Vertical and Horizontal Phytocompound Changes during Agarwood Formation in Aquilaria sinensis after Geotrichum candidum Injection

(1) Background: Agarwood is an aromatic resin produced by the host tree through an immunological response against biotic and abiotic stress. The aim was, first, to use the fungus Geotrichum candidum to stimulate compound changes in Aquilaria sinensis horizontally (color formation) and vertically (cutting layers) after injection with it. (2) Methods: Horizontal and vertical sections were collected and separated five months after injection with the fungal broth. Two grams of dry powder was mixed with 20 mL methanol for 3 h at room temperature, and the solution was vibrated in an ultrasonic cleaner bath at 40 °C for 1 h. After vacuum drying, a concentration of 10 mg/mL of the tested samples in methanol was prepared for reversed-phase high-performance liquid chromatography (RP-HPLC), gas chromatography/mass spectrometry (GC/MS), and thin-layer chromatography (TLC) analysis. (3) Results: The horizontal changes in the compounds and their concentrations were associated with color. Compared to the normal (N) group, G. candidum injection stimulated more compounds at RT 27-42 in the white (W) group, brown (BR) group, and black (B) group. Furthermore, a significant increase in fatty acids was observed in the W group, implying an early plant response after G. candidum injection. In the BR group, the compounds were more similar to commercial agarwood (Out group). In the B group, alkaloids were the main compounds. Vertical changes in the main compounds were not observed, although the compound level varied. A TLC analysis determined the main compounds in the BR group at 254 nm and in the B group at 365 nm. Higher fatty acid levels were found in L6 and L5 and were correlated with higher terpenoid and sesquiterpene levels, suggesting that these compounds were possibly the first stage of agarwood formation. A GC/MS analysis demonstrated that the main compound groups were almost identical to the BR parts. (4) Conclusions: The injection of G. candidum led A. sinensis to synthesize different phytochemicals horizontally, not vertically, in the BR group.

Fusarium oxysporum infection-induced formation of agarwood (FOIFA): A rapid and efficient method for inducing the production of high quality agarwood

Agarwood, a non-wood product from the endangered Aquilaria and Gyrinops tress, is highly prized for its use in fragrances and medicines. The special formation process of agarwood is closely related to external injury and fungal infection. In this study, we demonstrate that infection of Aquilaria sinensis by Fusarium oxysporum, a soilborne fungus that causes vascular wilt diseases in diverse plants, induces agarwood formation. Based on these findings, an efficient method, termed F. oxysporum infection-induced formation of agarwood (FOIFA), was developed for the rapid production of quality agarwood. The agarwood formed in response to F. oxysporum infection was similar in structure and chemical composition to wild agarwood according to TLC (Thin-layer chromatography), HPLC (high performance liquid chromatography), and GC-MS (gas chromatography-mass spectrometry) analyses, except that the contents of alcohol-soluble extract, chromones, and essential oils (mainly sesquiterpenes) were higher in the formed agarwood.

Integrating Multiple Omics Identifies Phaeoacremonium rubrigenum Acting as Aquilaria sinensis Marker Fungus to Promote Agarwood Sesquiterpene Accumulation by Inducing Plant Host Phosphorylation

The present study aimed to explore the factors that promote persistent agarwood accumulation. To this end, we first investigated the morphological changes and volatile compound distribution in five layers of "Guan Xiang" agarwood. The agarwood-normal transition layer (TL), an essential layer of persistent agarwood accumulation, showed clear metabolic differences by microscopy and GC-MS analysis. Microbiome analysis revealed that Phaeocremonium rubrigenum was the predominant biomarker fungus in the TL of "Guan Xiang" agarwood samples. Among the seven isolated fungi, P. rubrigenum exhibited a significantly heightened ability to induce the production in Aquilaria sinensis seedlings, especially for sesquiterpene. Tracing the proteome profile changes in P. rubrigenum-induced A. sinensis calli for 18 ds showed that the fungus-induced sesquiterpene biosynthesis increased mainly through the mevalonate (MVA) pathway. Specifically, the phosphorylation modification level, instead of the protein abundance of transcription factors (TFs), showed corresponding changes during sesquiterpene biosynthesis, thus indicating that induced phosphorylation is the key reason for enhanced sesquiterpene production. IMPORTANCE Agarwood is an expensive resinous portion derived from Aquilaria plants and has been widely used as medicine, incense, and perfume. The factors involved in steady agarwood accumulation remain elusive. Our current study suggests that as a TL marker fungus, P. rubrigenum could persistently promote agarwood sesquiterpene accumulation by inducing phosphorylation of the TFs-MVA network in A. sinensis. Moreover, our work provides strategies to improve agarwood industry management and sheds light on the potential molecular mechanisms of plant adaptation to native microbial conditions.

Tissue Structure Changes of Aquilaria sinensis Xylem after Fungus Induction

In this study, we analyzed the mechanism and the process of fungal-induced agarwood formation in Aquilaria sinensis and studied the functional changes in the xylem structure after the process. The microscopic structure of the white zone, transition zone, agarwood zone, and decay zone of 12-and 18-months of inoculation A. sinensis xylem was studied. The distribution of nuclei, starch grains, soluble sugars, sesquiterpenes, fungal propagules, and mycelium in xylem tissues was investigated by histochemical analysis. The results show that the process of agarwood formation was accompanied by apoptosis of parenchyma cells such as interxylary phloem, xylem rays, and axial parenchyma. Regular changes in the conversion of starch grains to soluble sugars, the production of sesquiterpenoids, and other characteristic components of agarwood in various types of parenchyma cells were also observed. The material transformation was concentrated in the interxylary phloem, providing a structural and material basis for the formation of agarwood. It is the core part of the production of sesquiterpenoids and other characteristic products of agarwood. Compared with the A. sinensis inoculated for 12 months, the xylem of the A. sinensis inoculated for 18 months was more vigorous. There were no significant differences between the 12 and 18 months of inoculation in terms of sugars and agarwood characteristic products. In production, harvesting after 12 months of inoculation can improve harvesting efficiency.

Aquilaria Species (Thymelaeaceae) Distribution, Volatile and Non-Volatile Phytochemicals, Pharmacological Uses, Agarwood Grading System, and Induction Methods

Agarwood is a highly valuable fragrant wood of Aquilaria spp. (Thymelaeaceae) which has been widely utilized in traditional medicine, religious rites, and cultural activities. This study summarizes a review on the identification of Aquilaria cultivars, volatile and non-volatile phytochemicals, pharmacological uses, and agarwood grading system to determine its quality, and different agarwood induction methods. Due to the highly demanding and depleted natural resources, the research on agarwood is still insufficient, and it has broad research and development prospects in many industries. However, due to the significant scientific nature of agarwood application, developing high-quality products and drugs from agarwood have become highly important, while no one has discussed in detail the phytochemicals uses and provided a summary until now. The main phytochemicals of agarwood include terpenoids, dominated by sesquiterpenes. For centuries, terpenoids have been used in traditional Chinese medicine and have been shown to possess various pharmacological properties, including bacteriostatic, antibacterial, sedation, analgesia, anti-inflammation, anti-asthmatic, hypoglycemic, antidepressant, and many others. Alongside biological activity screening, phytochemical advances and pharmacological research have also made certain progress. Therefore, this review discusses the research progress of agarwood in recent years and provides a reference basis for further study of Aquilaria plants and agarwood.