Endophytes: A vital source of medicine – A review

Soil and endosymbiotic microorganisms are naturally analogous to plant roots in a symbiotic and non-symbiotic manner. Several groups of plant growth-promoting bacteria and fungi have been identified as bio-fertilizers and isolated microbes have been applied to farmer's agricultural fields to enhance plant biomass and crop productivity. Overall, very few studies have been conducted on the beneficial effect of endophytic microbes on drug production in plants. This review focuses on medicinally valuable plants and their associated endophytic bacteria and fungi to develop drug molecules against human pathogens and other illnesses. Various secondary metabolites, including anticancer, antidiabetic, antibacterial, antifungal, and antiviral compounds, are present in both endophytes and their host plants. The number of secondary metabolites produced by fungal endophytes and their host plants is reported to be greater than that of endophytic bacterial species. The current proficiency of endophytic bacterial and fungal metabolites and their bioactivity against diseases are addressed in this review.


INTRODUCTION
Endophytes are microorganisms that live in intracellular locations in the plant body without causing damage to the plant. These bacteria and fungi produce a wide range of secondary metabolites that are helpful in producing drug molecules. Humans suffer due to various diseases caused by pathogenic bacteria, fungi, viruses, and other illnesses. The mortality rate of humans was reduced after the 20 th century because of the use of antibiotics and drugs in animals, which are consumed by us [1]. Although several drugs are available to cure diseases, some of them are not effective, and the development of antibiotic resistance leads to the need to identify new drugs to control pathogens.
Medicinal plants and their associated endophytes are a valuable source of crucial secondary metabolites and bioactive compounds that provide more than 80 % of the natural drugs available in the market [2]. Treatment with synthetic drugs may cause side effects; thus, low-risk therapies using traditional approaches, including synthetic drugs, herbal remedies, and biological drugs, are useful in curing diseases and illnesses [3]. Plant and microbial secondary metabolites are viable sources for producing new antibiotics and drugs for treating diseases and disorders. The present review focuses on the importance of several endophytic bacteria and fungi and their host plants' secondary metabolites for various biological effects in humans.

Endophytic bacteria
Several bacterial species living inside plant tissues depend on host plants for their survival with mutual benefits [4]. Endophytic bacteria inhibit the growth of human and plant pathogens, and their symbiotic association is beneficial to the host [5]. Endophytes produce several metabolites that are relatively similar to host plants, and the chemistry and biosynthesis of the compounds and their derivatives are also similar. The bioactivity of endophyte-produced compounds is higher than that of the respective host metabolites. Various groups of economically important secondary metabolites have been identified and recognized in endophytes [6]. Studies of endophytic bacteria and host plant associations and their active metabolite production are very limited [7]. The crosstalk between interspecies microorganisms and the similar ecological habitats of both endophytes and hosts is a reason to produce structurally similar natural compounds [6]. The utilization of natural endophytic agents to produce economically valuable compounds for pharmaceutical purposes has some advantages, including a lower production cost [8].
Antibiotics, including antifungal and antibacterial metabolites from endophytes, are considered novel sources of antibiotic production [9]. Some of the endophytes belonging to actinomycetes act as anti-diabetic agents [10]. Organic compounds, such as alkaloids, flavonoids, tannins, terpenoids, steroids, phenolic acids, chinones, quinones, and xanthones, synthesized in plants, bacteria, and fungi are considered secondary metabolites because they are not essential for their growth but protect against biotic competition and enable tolerance to adverse environmental conditions [11].

Drugs from medicinal plants and their endophytic bacteria
Medicinally valuable compounds present in plants are considered some of the most important sources of drugs used to treat human illnesses [12] (Table 1). Herbal drugs are gaining global attention due to their effective and negligible side effects. Indians use healthcare products derived from medicinal plants through homeopathic treatments [13]. Chinese have used the perennial herb Pinellia ternata (Thunb.) Berit., in traditional medicine for more than 2000 years [14], because it contains purine alkaloids [15], and it is used as an anti-emetic, analgesic, and sedative [14]. Another traditional bulbous plant, Crinum macowanii Baker, has been used to treat several ailments, including venereal disease, inflamed sores, itchy rashes, acne, backache, boils, swellings of the body, and urinary tract problems. It is also beneficial for increasing lactation in cows and women [16]. Crinum macowanii bulb extracts prevent infection by common pathogenic bacteria [17]. In addition, cytotoxic activity compounds (crinamine, bulbispermine, and lycorine) prevent human oral epidermoid carcinoma KB cells, apoptosisresistant cells, and BLS mouse melanoma cells [16]. The essential oil extracted from the leaves of Guatteria australis prevents Staphylococcus aureus and E. coli infections due to the principal antibacterial sesquiterpene compound, germacrene, present in the essential oil [18].
Some medicinal plants and their inhabiting bacteria show similar pharmacological activity. For example, Tridax procumbens L is known to heal wounds and has antimicrobial activity potential, and their endophytic bacteria also confer similar wound-healing properties and produce novel antibiotics [9]. Some Bacillus spp. and, Pseudomonas spp. (P. oryzihabitans and P. psychrotolerans) can synthesize diverse groups of economically important secondary metabolites [43].
Several reports suggest that endophytic bacteria have the potential for antibacterial, antifungal, and cytotoxic properties due to the production of alkaloids, steroids, terpenoids, flavonoids, and peptides [7] (Figure 1 and Table 2). The culture extract of Coniothyrium sp. which resides in the rhizomes of Aralia nudicaulis, contains palitantin, botrallin, craterellin C, mycosporulone, spiromassaritone, and massarigenin D, which prevents the pathogenic bacteria and fungal growth [21]. Novel antibiotics produced from microorganisms could be an alternative method for reducing antibiotic resistance and preventing pathogen growth [47].

Anticancer, anti-inflammatory, and antioxidant effects of endophytic bacteria
Plant-based alkaloids, brassinosteroids, and taxols play vital roles in preventing cancer cell growth, and their therapeutic applications are well established [30]. The endophytic bacteria Pinellia ternata shows anti-inflammation, anticancer, and anti-anxiety activity [22]. Anticancer and antiviral compounds are present in a diverse group of organisms. For example, guanosine is present in medicinal plants, [48] and inosine is present in animals and microorganisms [49]; both are reported as effectively controlling cancer and viral growth. In some cases, B. thuringiensis and B. licheniformis secrete guanosine which prevents cancer [15]. The amine compounds extracted from Acinetobacter sp. have an antitumor effect against oral squamous carcinoma cells [46]. In addition, the C. macowanii bulbs that reside in the endophytic bacterial extract have anticancer and antibacterial potential [17].

Endophytic fungi
Fungi colonize various tissues of plants without causing any negative effects; these are considered endophytic fungi. These fungi secrete antibacterial, antifungal, antiviral, insecticidal, antidiabetic, antioxidants, and anticancer compounds, along with plant-growth regulating chemicals ( Figure 2) [50]. Fungi have been integrated with several plants for 400 million years in nature by regulating the physiological process to enhance plant growth and tolerate the abiotic and biotic stress in the environments of the host plants [34]. Diversified endophytic fungi have been identified in several plants [24], and their primary and secondary bioactive compounds are beneficial to host plants and have the potential to be pharmaceutically valuable drugs against animals and human diseases [23,38,41]. Fungal alkaloids, flavonoids, phenols, benzopyranones, quinines, and quinones act as valuable drugs [51], and the structure and function of other bioactive products are also important for preparing drugs [52]. Produce economically important secondary metabolites [43,45] Acinetobacter spp. Anti-tumor activity [46] In addition, fungi-secreted amylase, cellulose, lipase, protease, laccase, and protease are useful for textiles, food processing, detergents, pharmaceuticals, and agrochemical applications [24].

Drugs from medicinal plants and their endophytic fungi
The various bioactive metabolites present in medicinal plants are resources of the cosmetic and pharmaceutical industries. The endophytes residing in plants are a good source of novel bioactive compounds, such as several alkaloids, flavonoids, terpenes, and organic acids which are involved in antiviral, antidiabetic, antilipemic, anti-inflammatory, and immunomodulatory properties ( Table 3). The endophytes associated with Anoectochilus roxburghii, edible, and medicinal plants produce kinsenoside as an immunosuppressive compound for reducing autoimmune hepatitis [23].

Fusarium tricinctum
Inhibitory effects against nitric acid production on murine macrophage cell line [42] Beauveria bassiana and Fusarium spp.

Aspergillus niger and Aspergillus awamori
Antimicrobial and antibacterial activity [56]

Aspergillus fumigatus
Anticancer activity [60] The diversified and rich sources of alkaloids, flavonoids, steroids, terpenoids, cyclopeptides, and anthraquinones are found in endophytic fungi and show their antimicrobial activity [37]. The endophytic fungus Nigrospora sphaerica isolated from Adiantum philippense produces antimicrobial compounds, such as phomalactone which controls the growth of human and plant pathogens [34]. Byssochlamys spectabilis and Alternaria spp. associated with Euphorbia prostrate have antibacterial and antiproliferative activities [35]. In addition, Dysosma versipellis produces a novel antimicrobial and anticancer compound, podophyllotoxin [36]. Solanum mauritianum resides in a number of endophytic fungi that effectively control Mycobacterium tuberculosis. Similarly, anthraquinone isolated from Talaromyces spp. and penicillic acid from Fusarium solani and other fungal endophytes are more significant for preparing drugs in the pharmaceutical industry [38]. Endophytic fungi Nigrospora spp. present in Moringa oleifera produce dechlorogriseofulvin, griseofulvin, mullein, and 8-dihydroramulosin, which act as antifungal agents [39].

Anticancer, anti-inflammatory, and antioxidant activity of endophytic fungi
An anticancer drug, taxol was initially produced from the plant Taxus brevifolia, and later their endophytic fungi were shown to produce the same metabolites in larger quantities through industrial production. Taxol synthesized from Taxomyces andreanae, camptothecin from producing Fusarium solani, and vinblastine and vincristine isolated from Fusarium oxysporum has various biological activities [38]. The endophytic fungus A. fumigatus synthesizes a novel anticancer drug, deoxypodophyllotoxin [60], and similarly, Aureobasidium pullulans secretes β-(1 → 3, 1 → 6)-glucans, which are involved in antimicrobial, immunomodulatory, anti-tumor, fungicide, and food allergy inhibition activities [38]. Colletotrichum spp., residing in Ginkgo biloba secretes flavones to control cancer and has antioxidant activity [40]. However, Das et al, [8] reported that the phenolic compounds produced by endophyte F. Chlamydosporum and plant Polygonum chinense show antimicrobial and antioxidant properties.
Beauvericin from Beauveria bassiana and Fusarium spp. plays a vital role in controlling blowflies, Colorado potato beetles, and mosquito larvae and also shows apoptotic and cytotoxic effects [50]. Antiprotozoal activity against Leishmania donovani due to the presence of purpureone and an ergochrome moiety was recorded in the endophytic fungus Purpureocilliumli lacinum [59]. The antiinflammatory compounds of depsidones and diaryl ether derivatives have been synthesized from Corynespora cassicola isolated from Gongronema latifolium [41]. A. versicolor is an endophytic fungus that produces isochromane lactones, benzolactones, versicobenzos A and B, versicoisochromanes A and B, asperfuran A, furancarboxylic derivates, ergosterol-type steroids, and asperergoster A [23]. These compounds have immunosuppressive activity. Alternaria spp. isolated from Trigonella foenumgraecum show antioxidant potential [35]. Alkaloids, such as rigidiusculamide E and (-(aoxyisohexanoyl-N-methyl-leucyl)2-), secreted from Fusarium tricinctum residing in P. notoginseng roots [42], have inhibitory effects against nitric acid production on the murine macrophage cell line.

CONCLUSION
Medicinal plants are used in specific treatments. Some medicinal plants are rarely cultivated in nature, but they contain more bioactive materials. The present review suggests that isolation, identification, and mass culture of viable drug molecules producing endophytes is a vital source of medicine and saves medicinal plants.

Funding
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Ethical approval
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Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Conflict of Interest
No conflict of interest associated with this work.

Contribution of Authors
We declare that this work was done by the authors named in this article and all liabilities pertaining to claims relating to the content of this article will be borne by the authors. Selvaraj Perumal, Ramalingam Radhakrishnan, and Ramaraj Sathasivam wrote the first draft of the review. Ramalingam Radhakrishnan and Sang Un Park conceptualized and contributed the idea and outline of the review. Ramalingam Radhakrishnan and Ramaraj Sathasivam prepared the figures. Ramalingam Radhakrishnan, Muthukrishnan Arun, and Sang Un Park proofread and edited the draft. All authors approved the review for publication.

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