**1. Introduction**

*Anoectochilus roxburghii* (Wall.) Lindl., also called "Jin Xianlian" and "Jin Xianlan" is a perennial herb of the genus *Anoectochilus* of the Orchidaceae family. It is widespread in southern China and considered a famous drug in the provinces of Jiangxi, Taiwan, Guangdong, Guizhou, Zhejiang, and Fujian which is its main markets [1]. The herb is a valuable Chinese medicinal material that is known as the "King Medicine", the "Golden Grass", and the "Bird Ginseng" by countryfolk [2]. The whole plant is used as a medicine and has the efficacy to clear the heat, cool the blood, eliminate dampness, and detoxification. Many pharmacological studies have demonstrated its wide range of pharmacological effects including its antioxidant [3,4], hepatoprotective [3], anti-diabetic [5,6], anti-rheumatoid arthritis [7], anti-hyperglycemic [4,8], anti-inflammatory [9] and many other properties. Due to the fact of its high medical values, *A. roxburghii* is not only widely used in medicine and health care but also in beauty and drinking products with an increasing market demand. The average annual demand in South Korea and Japan is more than 1000 t, 70% of which depend on imports [10]. The vigorous market demand results in excessive harvesting and exploitation of the plant, leading to resource depletion. Thus, tissue culture has become the main source of commercial *A. roxburghii*.

Most orchids form mycorrhizae with mycorrhizal fungi [11]. The fungal hyphae form coiled structures termed "peloton" inside the cells of the plant roots which play a key role in the nutrients' exchange and absorption between the orchid and its mycorrhizal fungi [12]. The process is unique and complex involving various processes related to growth and development, such as colonization, increasing the survival rate and morphological growth. Moreover, the mycorrhizal fungal elicitor can rapidly induce the expression of relative genes that are related to secondary metabolic pathways which result in a significant accumulation of active ingredients in the host plant [13]; this was demonstrated in orchid by a few studies. The symbiosis between *Dendrobium nobile* and *Mycena* sp. MF23 caused the accumulation of dendrobine and polysaccharide [14,15]. *Mycena* sp. MF23 could also stimulate the accumulation of flavonoid and kinsenoside in *A. formosanus* [16]. For *A. roxburghii*, the major bioactive components include polysaccharides, kinsenosides, steroids, triterpenes, amino acids, alkaloids, and flavonoids that have been regarded as the quality standard of *A. roxburghii* [10]. However, *A. roxburghii* contains a very limited number of flavonoids, which limits the development and utilization of *A. roxburghii*-based medicines. In order to improve the content of flavonoid in *A. roxburghii* and to avoid excessive exploitation, many methods have recently been put forward by researchers [17–19]; the role of mycorrhizal fungi in the accumulation of flavonoid in *A. roxburghii* has been gradually recognized. Wang et al. reported that flavonoid accumulated significantly in *A. roxburghii* growth for 8 weeks treated with different fungi such as *Rhizoctonia* sp. cw-6 and cw-13, *Exophila pisciphila* (cw-8), *Nemania* sp. (cw-10), and *Umbelopsis* sp. (cw-1) [20]. However, few studies investigated the effects of mycorrhizal fungi on flavonoid accumulation in *A. roxburghii* during different growth stages.

The flavonoid biosynthetic pathway has been well characterized in some medicinal plants such as *Gnetum parvifolium* [21], *Chrysanthemum morifolium* [22], *Lotus japonicus* [23]. Its biosynthesis can be divided into two stages: phenylpropanoid and flavonoid pathways. Phenylalanine ammonia-lyase (PAL) is the first enzyme of the phenylpropanoid pathway which can convert phenylalanine into cinnamic acid [24]. Cinnamic acid is then converted into p-coumaric acid by trans-cinnamate 4-hydroxylase (C4H). Next, 4-coumarate CoA ligase (4CL) converts coumaric acid into its CoA ester. 4CL is one of the key branch point enzymes in the phenylpropanoid pathway and its products are subsequently used by various oxygenases, reductases, and transferases for the biosynthesis of lignin, flavonoids, anthocyanins, aurones, stilbenes, coumarins, suberin, cutin, and sporopollenin [25]. Chalcone synthase (CHS) and chalcone isomerase (CHI) are involved in two step condensation reaction, producing naringenin chalcone and naringenin, respectively. Then, flavanone is catalyzed by flavonoid 3 -hydroxylase (F3 H) and other enzymes. Subsequently, flavanone produces the branches of flavone and dihydroflavonol under the catalysis of flavone synthase (FNS) and flavanone 3-hydroxylase (F3H), respectively. Next, flavonol synthase (FLS) catalyzes C-3 hydroxylation in the structure of dihydroflavonols to form various flavonols, and flavonol-glycosides are formed by flavonoid 3-O-glucosyltransferase (GT) and rhamnosyltransferase (RT) or GT.

With the rapid development of high-throughput sequencing technology and systems biology, multi-omics technology has become an indispensable research method in the field of life science [26,27]. It can provide the dynamic changes of the plant's growth and development from the cell to the individual level. The metabolome is a powerful approach to qualitatively and quantitatively analysis all the small-molecule metabolites (mass ≤ 1000 Da) in the cells or tissues of an organism at any

physiological period using different analysis technologies including nuclear magnetic resonance (NMR) spectroscopy, liquid chromatograph-mass spectrometer (LC-MS), and gas chromatography-mass spectrometer (GC-MS) [28]. In other words, this method could provide the global metabolic changes. Similarly, the transcriptome means the detection of all RNA transcripts in a sample and reflects gene expression differences between different treatments [29]. Integrated transcriptome and metabolome analyses have been successfully applied to study the metabolic pathways of some substances [30,31], the color formation of vegetables, fruits, and flowers [32,33], the stress resistance mechanisms [34,35], and the growth and development mechanisms of the plants [36,37]. The combination can not only elucidate changes in the content of a series of metabolites, but it can also analyze the corresponding differentially expressed genes.

To investigate the changes of metabolites in *A. roxburghii* that is infected with mycorrhizal fungus, we performed metabolome and transcriptome analyses on six-month growth data of the mycorrhizal *A. roxburghii* (M) and non-mycorrhizal ones (NM). The results indicated that AR2 significantly promoted flavonoid biosynthesis in the plant. During the growth stage, the flavonoid content (two flavones: nobiletin and tangeretin; two flavonols: quercetin and isorhamnetin; five flavonol-glycosides: narcissin, rutin, isorhamnetin-3-O-beta-d-glucoside, quercetin 7-o-glucoside, and kaempferol 3-O-glucoside) in different metabolites and the expression of the genes that were related to the biosynthesis of flavonoid were further tested over a period of six months using high-performance liquid chromatography coupled with tandem mass spectrometry (HPLC-MS/MS) then quantitative real-time polymerase chain reaction (qRT-PCR). These will provide valuable information to reveal the effects of *Ceratobasidium* sp. AR2 on the flavonoid accumulation in *A. roxburghii*.
