Medicinal plants refer to plants used in medicine to prevent and treat diseases [1]. All or part of medicinal plants are used in medicine and will also be used as raw materials for the pharmaceutical industry, which have a wide range of medicinal and economic uses [2]. Especially in the field of traditional medicine represented by traditional Chinese medicine and Indian folk medicine, medicinal plants, as the main source of natural drugs, provide very important health care services for the population of developing countries ^[3][4][5][3,4,5]. With the rapid development of modern medicine, many clinical drugs still come from natural products extracted from medicinal plants [2]. Although many kinds of medicinal plants have been used in clinical treatment, due to environmental stress, overexploitation, low reproductive capacity, and other factors, some rare, high-demand, and wild medicinal plant resources cannot meet the market demand, so how to improve the germplasm resources of medicinal plants has become an urgent problem to be solved.

In recent years, researchers have gradually realized that endophytes can play an important role in affecting the quality and yield of medicinal plants through special microbe-plant interactions ^[6][7][6,7]. Plant endophytes are microbial groups that widely exist in healthy medicinal plant tissues, coexist harmoniously with host plants, and do not cause significant damage to hosts [8]. They are also an important part of theplant micro-ecosystem, which is rich in species, mainly including endophytic fungi, endophytic bacteria, and endophytic actinomycetes ^[7][9][10][7,9,10]. At present, endophytes have been isolated from a variety of medicinal plants, and many endophytes have been verified to secrete plant hormones, growth factors, etc., which are conducive to plant growth and development and can also regulate the accumulation and production of active ingredients in medicinal plants ^[10][11][10,11]. They increase the active ingredients of the host by producing the same or similar active products as the active ingredients in the host ^[11][12][13][11,12,13]. The most interesting property of endophytes is that they can convert the original active ingredients of the plant into new compounds. In 1993, Stierle et al. isolated an endophytic fungus from Taxus brevifolia and found that it can produce paclitaxel, an anti-tumor substance similar to the host plant, which inspired researchers to find bioactive substances from endophytes of medicinal plants [14]. Endophytes provide more resources of new bioactive metabolites, especially alkaloids, saponins, quinones, flavonoids, terpenoids, etc., which have a lot of biological activities and have also become research hotspots in the composition and production of natural drugs [8].

2. Medicinal Plants and Their Cultivable Endophyte Resources

Endophytes in medicinal plants mainly include endophytic fungi, endophytic bacteria, and endophytic actinomycetes, and they are rich in species diversity ^[6][7][6,7]. It is found that the biological functions of these endophytes have a great influence on medicinal plants ^[10][11][10,11], so obtaining more microbial resources, especially those with biological activity, can greatly promote the development of the medicinal plant industry.

2.1. Culturable Endophytic Bacteria Diversity in Medicinal Plants

Atractylodes macrocephala Koidz., called Baizhu in Chinese, is a medicinal plant used in traditional Chinese medicine theoretical systems to treat gastrointestinal dysfunction, cancer, osteoporosis, obesity and other symptoms, and has various pharmacological activities [15]. Wu et al. [16] explored the cultivable endophytic bacteria in the stems, leaves, roots, and rhizomes of Atractylodes macrocephala Koidz. in four different regions and their potential correlation with plant bioactives. A total of 118 endophytic bacteria belonging to 3 phyla, 5 classes, 11 orders, 26 families and 48 genera were identified from four Atractylodes macrocephala Koidz. tissues. Among them, Bacillus sp. is the most widely distributed. Dendrobium is one of the largest genera of Orchidaceae, with more than 1500 species distributed all over the world [17]. As a medicinal plant, dendrobium has greatly. contributed to the medical industry with its anticancer, antifatigue, and gastrointestinal protective effects [18]. In addition, there are also many microbial resources in dendrobium. Wang et al. [19] isolated and cultured endophytic bacteria from Dendrobium officinale samples of six different sources and cultivars. A total of 165 cultivable endophytic bacteria were isolated from sterilized Dendrobium officinale stems and classified into 43 species based on 16S rRNA gene sequence analysis, of which 14 strains had anti-plant-pathogenic activity. Mulberry, which belongs to the genus Morus of the Moraceae family, is an aggregated berry that is oval-shaped, palatable, and also rich in nutrients; it is regarded as a very important medicinal and edible plant due to its rich, effective chemical composition and wide range of biological activities ^[20][21][22][20,21,22]. Xu et al. [23] isolated a total of 608 endophytic bacteria from four mulberry cultivars, belonging to 4 phyla and 36 genera. Bacteria, as the largest group of plant endophytes, have been isolated from many kinds of medicinal plants and widely studied due to their biocontrol functions ^{[24][25][26][27][28][29][30][31][32]}[24,25,26,27,28,29,30,31,32]. By reviewing the recent literature on most of the endophytic bacteria of medicinal plants including Bacillus sp., Pseudomonas sp., Enterobacter sp., Agrobacterium sp., etc., and a large number of endophytic bacteria in the roots, stems, and leaves, rwesearchers collated some of the relevant data of the isolated endophytic bacteria in Table 1. ^][39][38,39]. A total of 328 fungal isolates were found in leaf, stem and root tissues of plants by Hafeez et al., and 12 endophytic fungal species were identified by molecular characterization [39]. Crocus sativus L. (family Iridaceae) has been widely used as an antimicrobial, antidepressant, digestive, anticancer, and anticonvulsant medicine due to its abundant natural products as well as antioxidant activity ^[40][41][40,41]. Lu et al. [42] isolated endophytic fungi from five different locations in Crocus sativus tissues (corm, scape, leaf, petal, and stigma) and identified a total of 32 endophytic fungal groups, assigned to seven orders within four classes. Wang et al. [43] isolated 34 endophytic fungi from Salvia miltiorrhiza, a traditional Chinese medicine, belonging to 10 genera and 16 species, and Epicoccus sp. SX19 and Colletotrichum gloeosporioids showed strong inhibitory effects on five pathogens. Ogbe et al. [30] isolated a total of 11 endophytic fungi from the roots and leaves of a drought tolerant mint species Endostemon obtusifolius. Similarly, five endophytic fungi were isolated from the leaf segments of wild Dendrobium nobile and identified as Colletotrichum tropicicola, Fusarium keratoplasticum, Fusarium oxysporum, Fusarium solani, and Trichoderma longibrachiatum [44]. Codonopsis pilosula, as a famous medicinal and food homologous plant, has functions such as strengthening the spleen, tonifying the lungs, and engendering liquid in traditional Chinese medicine [45]. Fan et al. [46] obtained 205 strains of endophytic fungi from the roots of Codonopsis pilosula, collected from six regions in Gansu Province, China, of which Fusarium sp., Aspergillus sp., Alternaria sp., Penicillium sp., and Plectosphaerella sp. were the dominant genera. Vernonia anthelmintica (L.) Willd has a long history in the treatment of several diseases related to skin, central nervous system, kidney, gynecology, gastrointestinal, metabolism, and general health [47]. Researchers have isolated more than 30 types of endophytic fungi from Vernonia anthelmintica. [48] From the research in recent years, it can be seen that Fusarium sp., Aspergillus sp., and Penicillium sp. can be isolated from most medicinal plants, and because of their many biological functions, they are regarded as the key research objects of endophytic fungi in medicinal plants. The recent research results are summarized in Table 2.

2.3. Culturable Endophytic Actinomycetes Diversity in Medicinal Plants

Dioscorea has powerful medicinal functions and is a potential source of bioactive substances for combating various diseases [49]. Zhou et al. [50] isolated 116 actinomycetes from the tissues of Dioscorea opposita Thunb. and found a new Streptomyces sp. with strong biocontrol function. As a traditional Chinese medicine, Eucommia ulmoides Oliv. has been used to treat various diseases since ancient times [51]. The research group led by Mo et al. ^[52][53][52,53] isolated two new species of Nocardia sp. from the leaves and roots of Eucommia ulmoides Oliv. Thymus roseus schipcz is one of the traditional Chinese herbs belonging to Lamiaceae and has been proven to have anti-inflammatory, antioxidant, anti-cancer and other functions [54]. Musa et al. [54] isolated 128 strains from the roots, stems and leaves of Thymus roseus schipcz, with a predominance of Streptomyces sp., followed by Nocardiopsis sp., Micrococcus sp., Kocuria sp., and others. Viola odorata grows in the high altitude area of the Himalayas and is used as a natural medicine because of its antidiabetes, anti-inflammatory, and other functions ^[55][56][55,56]. Salwan et al. [56] isolated a Streptomyces strain with antioxidant and antibacterial activity from the medicinal plant Viola odorata collected in the Himalayas, which has the potential to produce antibacterial and antioxidant components. Xanthium sibiricum is a well-known Chinese herbal medicine commonly used to treat autoimmune and inflammatory diseases [57]. Hu et al. [58] isolated two new Streptomyces strains from healthy leaves and seeds of Xanthium sibiricum. According to the research on endophytic actinomycetes of medicinal plants in recent years, actinomycetes are mainly distributed in the roots of medicinal plants, and their number is greater than that in other tissues of plants. Streptomyces sp. is the main research object of actinomycetes, and Streptomyces sp. has received extensive attention because of its strong biological activity [59]. Actinomycetes of other genera can also be isolated from medicinal plants, but the number is relatively small compared with Streptomyces. Many studies have isolated new species of bioactive endophytic actinomycetes from medicinal plants, which greatly expanded microbial resources and laid a foundation for the industrial application of actinomycetes ^{[52][53][58][60][61]}[52,53,58,60,61]. The results of endophyte isolation from medicinal plants in recent years are shown in Table 3.

2.2. Culturable Endophytic Fungal Diversity in Medicinal Plants

Aconitum heterophyllum is an alkaloid-rich medicinal plant which is widely used in traditional Chinese medicine clinics ^[38