Nanmu (Phoebe zhennan) is an extremely valuable tree plant that is the main source of famous “golden-thread nanmu” wood. The potential metabolites and gene regulation mechanisms involved in golden thread formation are poorly understood, even though the color change from sapwood to heartwood has been investigated in several tree plants. Here, five radial tissues from sapwood to heartwood were compared via integrative metabolomic and transcriptomic analysis to reveal the secondary metabolites and molecular mechanisms involved in golden thread formation. During heartwood formation, gradual starch grain loss is accompanied by the cell lumen deposition of lipids and color-related extractives. Extractives of 20 phenylpropanoids accumulated in heartwood, including cinnamic acids and derivatives, coumarin acid derivatives, and flavonoids, which were identified as being closely related to the golden thread. Phenylpropanoids co-occurring with abundant accumulated metabolites of prenol lipids, fatty acyls, steroids, and steroid derivatives may greatly contribute to the characteristics of golden thread formation. Additionally, the expression of nine genes whose products catalyze phenylpropanoid and flavonoids biosynthesis was upregulated in the transition zone, then accumulated and used to color the heartwood. The expression levels of transcription factors (e.g., MYB, bHLH, and WRKY) that act as the major regulatory factors in the synthesis and deposition of phenylpropanoid and flavonoids responsible for golden thread formation were also higher than in sapwood. Our results not only explain golden thread formation in nanmu, but also broaden current knowledge of special wood color formation mechanisms. This work provides a framework for future research focused on improving wood color.

Walnut oil is a high-value oil product. Investigation of the variation and the main climatic factors affecting the oil’s chemical composition is vital for breeding and oil quality improvement. Therefore, the fatty acid, micronutrients, and secondary metabolites compositions and contents in walnut oil were determined in three species: Juglans regia L. (common walnut), J. sigillata Dode (iron walnut), and their hybrids (Juglans sigillata Dode × J. regia L.), which were cultivated aat different sites. The major fatty acids were linoleic (51.39–63.12%), oleic (18.40–33.56%), and linolenic acid (6.52–11.69%). High variation in the contents of fatty acids, micronutrients, and secondary metabolites was found between both species and sites. Interestingly, myristic, margaric, and margaroleic acid were only detected in the hybrids’ walnut oil, yet α-tocopherol was only detected in common and iron walnut oil. Climatic factors significantly affected the composition and content of fatty acid, whereas δ-tocopherol was mostly dependent on the genetic factors. The average relative humidity explained the most variation in the fatty acids, micronutrients, and secondary metabolites, which showed a significant positive and negative effect on the monounsaturated fatty acids and polyunsaturated fatty acids, respectively. These findings contribute to the provision of better guidance in matching sites with walnut trees, and improvement of the nutritional value of walnut oil.

Phoebe hui is an extremely valuable tree that is the main source of the fragrant golden-thread nanmu wood. Although the fragrance of wood has been investigated in several trees, the potential substances and gene regulation mechanisms that are involved in fragrance formation are poorly understood. Here, three radial tissues, sapwood (SW), heartwood (HW), and the transition zone (TZ) in between them, were compared via integrative physiological, volatile-metabolomic, and transcriptomic analyses to identify the key metabolites and regulatory mechanisms involved in fragrance formation. During heartwood formation, gradual starch grain loss was accompanied by the deposition of lipids and extractives in the cell lumen. Extracts of terpenoids were synthesized and accumulated in the heartwood, including monoterpenoids (limonene and p-cymene) and sesquiterpenes (cubebene and guaiadiene); these were identified as being closely related to the special fragrance of the wood. Additionally, the expression of transcripts showed that the genes related to primary metabolism were specifically upregulated in the SW, whereas genes annotated in terpenoid biosynthesis were specifically upregulated in the HW. Therefore, we speculated that terpenoid biosynthesis occurs in situ in the HW via the HW formation model of Type-III (Santalum) using the precursors that were produced by primary metabolism in the SW. The expression levels of transcription factors (e.g., MYB, WRKY, and C2H2) acted as the major regulatory factors in the synthesis of terpenoids. Our results explain the special fragrance in P. hui and broaden the current knowledge of the regulatory mechanisms of fragrance formation. This work provides a framework for future research that is focused on improving wood quality and value.

Wood essential oil and wood products with special fragrances are high value-added forest products. Despite the availability of essential oil and volatile organic compounds (VOCs) from Phoebe zhennan wood, their variation and dependence on tree age have not been examined. After essential oil extraction and wood processing, the yields and compositions of essential oils and VOCs in wood from P. zhennan trees of different ages (10a, 30a, and 80a) were determined. The yield of essential oil from 30a wood was significantly greater than that from 10a and 80a wood. Liquid chromatography–mass spectrometry (LC−MS) and gas chromatography−mass spectrometry (GC−MS) revealed 672 and 41 volatile compounds, respectively, in the essential oil and wood, the majority of which exhibited large fluctuations in relative content and composition depending on tree age. Sesquiterpenoids, fatty acids and conjugates may greatly contribute to the main components of essential oil from wood. Almost all major sesquiterpenoid compounds, such as caryophyllene α-oxide, eudesmo, and cubebene, were identified in the essential oils from the 30a and 80a wood, and their relative contents were much greater than those in the 10a wood. The main components of the wood fragrance were sesquiterpenoids. The types and relative contents of sesquiterpenoids from wood increased with tree age. These results suggest that choosing wood from trees of a suitable age will significantly improve the efficiency of wood utilization.

Nanmu (Phoebe zhennan) has a unique fragrance and is a high-quality tree species for forest conservation. The types and contents of volatile compounds in different tissues of nanmu wood are different, and the study of its volatile metabolites can help us to understand the source of its fragrance and functions. In order to explore the metabolites related to the wood fragrance of nanmu and to find out the unique volatile substances in the heartwood, gas chromatography–mass spectrometry (GC-MS) was performed to analyze the non-targeted metabolomics in five radial tissues from the sapwood to the heartwood of nanmu. A total of 53 volatile metabolites belonging to 11 classes were detected in all tissues, including terpenes, aromatic hydrocarbons, organoheterocyclics, phenols, esters, organic acids, alcohols, alkaloids, alkane, indoles derivatives, and others. And most of the volatile metabolites were identified for the first time in nanmu wood. Among them, terpenes and aromatic hydrocarbons were the main volatile components. In addition, 22 differential metabolites were screened from HW and SW, HW, and TZ via metabolomic analysis. Among these DAMs, three volatile metabolites (cadinene, a sesquiterpenoid; p-cymene, a monoterpenoid; 1,3,5-triisopropylbenzene, an aromatic hydrocarbon) contributed heavily to the characteristic fragrance of the heartwood. Additionally, the expression of transcripts showed that the unigenes in the terpenoid biosynthesis pathway were especially up-regulated in the SW. Therefore, we speculated that fragrance-related metabolites were synthesized in SW and then deposited in heartwood during sapwood transformed to heartwood. The expression levels of transcription factors (e.g., WRKY, C2H2, NAC) acted as the major regulatory factors in the synthesis of terpenoid. The results lay the foundations for further studies on the formation mechanism of fragrance components in nanmu wood and also provide a reference for the further development and utilization of nanmu wood.