INTRODUCTION:

Secondary metabolites considered them as organic compounds that are generally produced by microbes (i.e. bacteria, fungi). The most familiar examples of secondary metabolites are alkaloids, antibiotics, terpenoids, flavonoids and peptides along with growth factors^{[1, 2]}. Most of the secondary metabolites are toxic or repellant to herbivores including microbes and also help them pertaining to defend these endophytic bacteria producing plants^[3,4]. In other words, secondary metabolites are mainly classified into four major classes i.e. terpenoids/flavonoids/phenolic compounds/alkaloids and sulphur-containing compounds^[1-4].

Secondary metabolites are believed as one of the most useful natural medicinal plant products and also considered them as reservoir of phytochemicals^{[2, 5]}.

These phytochemicals mainly protecting as well as safeguarding these medicinal plant products against various environmental constraints. So, these secondary metabolites may directly link to the induction of morphological differentiation and maturation during plant growth^[5,6]. In addition, secondary metabolites should be extracted from plant products and reported various medicinal properties. These secondary metabolites should be used for human nutrition and also act as first line of defense against oxidative damage induced through different environmental (including high temperature) constraints^[2,5,6]. But these plant based secondary metabolites have not been fully focused as well as studied with respect to their endophytes and their proper relevant functions in plants^[7]. The objective of this study is to understand the concept and applications of secondary metabolites and studied its correlation with endophytic bacterial and fungal production.

ENDOPHYTES:

Microorganisms played an important as well as crucial role in the environment. Endophytes mean those microbes; mainly colonize especially on living especially internal tissues of plants without causing any immediate or any negative effects. The term endophyte was first coined by De Bary in 1866 and this can be used for a wide spectrum of potential hosts inhabitants, e.g. bacteria, fungi, etc. Various forms of microorganisms that have been recovered as endophytes include bacteria, fungi, actinomycetes and mycoplasma^[7-10]. In contrast, some of the compounds produced by endophytic microbes as shown in Table 1.

Table 1. Functions of derived compounds produced from endophytic microbes

Endophytic microbes derived compounds (Fig.1.)	Function
Taxol ^[11]	World’s first billion-dollar anti-cancer drug
Cryptocin ^[12]	Antifungal agent
Cryptocandin ^[13]	Molecule with potent anti-fungal properties
Jesterone ^[14]	Antifungal agent
Oocydin ^[15]	Antifungal agent
Isopestacin ^[16]	Antioxidant
Pseudomycins ^[17]	Antifungal agents for use in humans
Ambuic acid ^[18]	Antifungal agent

Endophytes is an endosymbiont (microbes that lives inside the body or cell), frequently a bacterium or fungi, that lives inside a plant for in any event part of its life cycle without causing clear sickness. In other words, there is existence of bacteria reported in healthy plants particularly internal tissues as mentioned in the literature. Unfortunately, people still ignored in the past but now researchers reported as well as isolated various micro-organisms from medicinal plant products especially roots, stem and leaves including seeds. These endophytes may upgrade their development, supplement procurement and improve the plant's capacity to endure abiotic stresses, for example, dry spell, and abatement biotic worries by upgrading plant protection from creepy crawlies, pathogens and herbivores. In general, endophytes are pervasive and have been found in all types of plants; nonetheless, a large portion of the endophyte/plant connections are not surely known.

Endophytic microorganisms have been found in practically every plant considered, where they colonize the inside tissues of their host plant and can shape a scope of various connections including advantageous, mutualistic, commensalistic and trophobiotic. Most endophytes seem to start from the rhizosphere or phyllosphere; notwithstanding, some might be transmitted through the seed. Endophytic microbes can advance plant development and yield and can go about as biocontrol specialists. Endophytes can likewise be advantageous to their host by creating a scope of characteristic items that could be saddled for potential use in medication, farming or industry. Moreover, it has been appeared that they can possibly expel soil contaminants by upgrading phytoremediation and may assume a job in soil ripeness through phosphate solubilization and nitrogen obsession. There is expanding enthusiasm for building up the potential biotechnological uses of endophytes for improving phytoremediation and the practical generation of nonfood crops for biomass and biofuel creation. In view of this, we focused on endophytic bacteria that are reported in various medicinal plant products and these are responsible for producing secondary metabolites.

Secondary metabolites and endophytes:

According to the literature, endophytes synthesize various bioactive metabolites and these are very effective plant-based candidates and used as therapeutic agents against numerous diseases. In general, endophytes are mainly responsible for producing host plant based secondary metabolites with therapeutic value e.g. paclitaxel (Taxol); podophyllotoxin, camptothecin and structural analogs, hypericin and emodin and azadirachtin. Exciting possibilities exist for exploiting these endophytic microbes for the production of novel biologically active secondary metabolites. In view of this, we reported some medicinal plants where endophytic microorganisms were reported and also showed various immunopharmacological activities as shown below-

· In papaya leaves, secondary metabolites are reported in endophytic bacteria (five isolates i.e. EC-1 to EC-5). In literature, water extract from papaya leaves showed antioxidant and antimicrobial (inhibit gram-positive bacteria i.e. B. subtilis, P. aeruginosa and S. aureus and gram-negative i.e. E.coli, S. typhi and K. pneumonia) activity. In addition, endophytic bacterial isolates symbiotes with papaya leaves may be able to produce secondary metabolite (i.e. alkaloid, flavonoids, saponins, tannins and triterpenoids) based compounds that behave like antioxidant and antibacterial agents. In contrast, fungi (Colletotrichum, Fusarium, Aspergillus, Phoma, Cladosporium, Phomopsis, Penicillium, Rhizopus, are also associated with C. papaya diseases andspoilage. Some of the fungi also reported as endophytes in C. papaya without causing any disease symptoms. In other words, endophytes originated from bacteria and fungus is capable of producing various extracellular enzymes and its various biotechnological applications are reported. So, these fungal endophytes from C. papaya leaves reported several bioactive molecules and also showed cytotoxic effect against mouse lymphoma cells^[19,20].

· In Cosmos caudatus, fifteen isolates of endophytic bacteria were reported from leaves using ethyl acetate as solvent system. So, this extract showed the presence (qualitatively and quantitatively) of only 4 isolates [(PKM 1 (Serratia sp.), PKM 2 (Neisseria sp.), PKM 17 (Actinobacter sp.), and PKM 19 (Yersinia sp.)] produced flavonoid and determined through thin-layer chromatography method and also showing its anticancer (brine shrimp lethality test, BSLT) and antimicrobial (paper disc method) activity^[21,22,23].

· Aralia elata (Chinese/Japanese/Korean angelica-tree; family Araliaceae), woody plant reported 96 strains of endophytic fungi and these were identified on the basis of polymerase chain reaction (PCR). One of the secondary metabolites i.e. saponin was measured in culture filtrate of respective strains of fungi and also examined its antimicrobial activity. For confirmation of saponins, high-performance liquid chromatography (HPLC) technique was applied pertaining to identify the saponins produced by representative strains. Fungal species (Alternaria, Botryosphaeria, Camarosporium, Cryptosporiopsis, Diaporthe, Dictyochaeta, Penicillium, Fusarium, Nectria, Peniophora, Schizophyllum, Cladosporium and Trichoderma) were isolated and determined the saponin content. From these studies, authors claimed that isolates (>25%) belongs to Diaporthe followed byAlternaria (12.5%) and also confirmed its antibacterial activity against Staphylococcus aureus^[24].

· Endophytic fungi (38 strains) of Panax ginseng were reported and showing its ability to produce saponin which is confirmed through Polymerase chain reaction (PCR) and identified its sequencing of isolates. In addition, antimicrobial activity of saponin produced by strains of endophytic fungi was measured using agar diffusion method and also analyzed ginsenosides of particular endophytic fungal strain was measured through HPLC. Fungal strains (Nectria, Aspergillus, Fusarium, Verticillium, Engyodontium, Plectosphaerella, Penicillium, Cladosporium, and Ascomycete) were isolated. From these studies, authors claimed the presence of isolates (18.4%) belonged to Nectria (Nectria haematococca) followed by Aspergillus (13.2%) and Penicillium (10.5 %). In contrast, these isolates exhibited its antibacterial activity Staphylococcus aureus^[25].

· In Ginkgo biloba, fungal strain (ST22 and SX10, members of Aspergillus) from twigs were reported and also showing its ability to produce various phenolic and flavonoid compounds. These strains were examined on the basis of morphological (light microscope and scanning electron microscope) characteristics and also measuring phenolic and flavonoid content using UV-spectrophotometry. On the basis of morphological strain studies revealed that ST22 identified as Aspergillus nidulans and SX10 as Aspergillus oryzae. In other words, strains from Ginkgo bilobamay have potential as sources of natural medicines^[26].

· In Fritillaria unibracteata var. wabuensis using bulbs where fungal (53) endophytes were reported. Out of these, 49 fungal strains were screened and grouped into different taxa (17 in number) primarily from the Fusarium genus. In these strains, most of the fungal endophytes showed antioxidant activity (using petroleum ether, ethyl acetate, n-butyl alcohol and ethanol fractions; out of these last three fractions showed more potent antioxidant activity than the first fraction) and also produced multiple secondary metabolites (i.e. phenolic, flavonoid and saponin) including some common active compounds, which played the crucial roles on the antioxidant activities. These fungal endophytes showed the potential to remove reactive oxygen species (ROS) from within the host plant and are good resources for obtaining novel or reported antioxidants. In short, phenolics, flavonoids and saponins are crucial bioactive constituents in these abundant fungal endophytes and can be viewed as new potential antioxidant resources^[27].

· There are multiple and valuable endophytic microbes that have been studied in one of the medicinal plants i.e. Plectranthus tenuiflorus. Plectranthus, genus which possess 300 species and these are extensively applied and used in treating various diseases especially on skin, digestive system, respiratory tract etc. In contrast, extraction of oil from Plectranthus barbatus gives anti allergic response whereas P. incanus is active against Staphylococcus aureus. Some of these endophytes are mainly involved in symbiotic relationship with the host plant bacteria and excreting various antimicrobial compounds from different plant part namely- root, stems, leaves. Among 28 of the bacterial endophytes 8 has been isolated by the partial sequencing of their 16s RNA gene. In addition, antimicrobial activity of this medicinal plant is so effective against human pathogenic microbes (S. aureus, E. coli, K. pneumoniae, S. agalactiae, P. mirabilis) using the crude extracts of the isolated endophytic strains^[28].

· Camptothecine (CPT), quinoline alkaloid, considered as one of the most powerful inhibitor of cancer. Due to its extensive demand in market led to the harvesting of two extensively producing CPT plant species i.e. Campotheca acuminate (China), Nothapodytes nimmmoniana (India). Few semi-synthetic drugs of CPT like Hycamtin (topotecan) and camptostar (CPT 11 or irinotecan) are reported and used in clinical studies but it is effective against ovarian and small lung disease. Investigations have detailed the creation of CPT by pathogenic endophytic partners of a portion of these plant species. The microscopic organisms was found out by Gram recoloring and 16s rRNA sequencing.

CONCLUSION:

Diversity of endophytic microbes especially bacteria and fungi are reported as well as isolated from various medicinal plant products. Its major role in plants where these endophytic microbes contributes more on plant physiology along with its growth which may help in restoration of available nutrient and phytoremediation. In view of this, researcher’s exploiting various endophytic microorganisms for improvement of soil quality along with its plant growth and development. So, these are key molecules pertaining to replace heavy agrochemical input in agricultural systems. In short, understanding the process, composition and functioning of plant associated microbial communities has a large potential of enhancing plant growth and restoration of soil quality.

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Received on 22.11.2019 Modified on 09.01.2020

Research J. Pharm. and Tech 2020; 13(9):4479-4483.

DOI: 10.5958/0974-360X.2020.00790.8