Towards further understanding the applications of endophytes: enriched source of bioactive compounds and bio factories for nanoparticles

The most significant issues that humans face today include a growing population, an altering climate, an growing reliance on pesticides, the appearance of novel infectious agents, and an accumulation of industrial waste. The production of agricultural goods has also been subject to a great number of significant shifts, often known as agricultural revolutions, which have been influenced by the progression of civilization, technology, and general human advancement. Sustainable measures that can be applied in agriculture, the environment, medicine, and industry are needed to lessen the harmful effects of the aforementioned problems. Endophytes, which might be bacterial or fungal, could be a successful solution. They protect plants and promote growth by producing phytohormones and by providing biotic and abiotic stress tolerance. Endophytes produce the diverse type of bioactive compounds such as alkaloids, saponins, flavonoids, tannins, terpenoids, quinones, chinones, phenolic acids etc. and are known for various therapeutic advantages such as anticancer, antitumor, antidiabetic, antifungal, antiviral, antimicrobial, antimalarial, antioxidant activity. Proteases, pectinases, amylases, cellulases, xylanases, laccases, lipases, and other types of enzymes that are vital for many different industries can also be produced by endophytes. Due to the presence of all these bioactive compounds in endophytes, they have preferred sources for the green synthesis of nanoparticles. This review aims to comprehend the contributions and uses of endophytes in agriculture, medicinal, industrial sectors and bio-nanotechnology with their mechanism of action.


Introduction
According to the report published by the IPCC (2018), the probability of limiting the effects of global warming to 1.5°C is majorly determined by the cumulative emission of carbon dioxide (CO 2 ) and future non-CO 2 radiative forcing. The devastating impact of climate change can be observed in sustainable agriculture systems and overall agriculture productivity (Verma et al., 2022). Agricultural activities in the 21 st century are largely depending on the extensive use of fertilizers, pesticides, fungicides etc. and also sometimes involve the over-irrigation and use of high-yielding crop varieties (Hegazy et al., 2017). Such practices have a negative impact on the environment and lead to low fertility of the soil by decreasing the symbiotic association of fungal and bacterial communities in the soil. Such practices also lead to groundwater pollution due to the leaching out of nitrogen and phosphorus from the soil into the groundwater (Rafi et al., 2019). In a similar vein, many biotic factors, such as bacteria, fungi, viruses, weeds, insects, and nematodes, are major constraints of stress that tend to increase the reactive oxygen species that affect the physiological and molecular functioning of plants and also lead to the decrease in crop productivity . In addition, the increased temperatures, atmospheric CO 2 levels, and precipitation patterns also affect agricultural production and insect infestations (Skendzǐćet al., 2021). In order to reduce the negative impact of such practices and to maintain the fertility of the soil, various eco-friendly farming techniques are employed such as the inculcation of microorganisms as fertilizers encompassing nutrient mobilizing capacity (Gamez et al., 2019). The symbiotic relationship between plants and microorganisms exerts various benefits on plants such as an increase in height and weight, the high nutritional value of plants etc. It results in increasing crop yield, nutrient cycling and fertility of the soil (Card et al., 2021). A single plant is colonised by a large number of microbes, these microbes can be termed epiphytes and endophytes. Endophytes, symbiotic bacterial and fungal communities, are present in intercellular and intracellular spaces of plant parts, such as stems, roots, leaves, etc. (Oukala et al., 2021). Endophytes are also accommodated by weeds inflorescences, petioles, buds, and dead and hollow hyaline cells of plants, fruits and seeds (Lonkar and Bodade, 2021). Either the full or some part of the life cycle of endophyte microbes occur inside the host plant, without causing any negative impact on the plant (Joo et al., 2021). Endophytic associations are reported in various types of plants such as soybeans, chickpeas, cowpeas, sunflower, pearl millet, rice, maize, mustard, sugarcane, cotton, tomatoes, etc (Fadiji and Babalola, 2020). Endophytes are classified on the basis of their biological nature, mode of transmission, and diversity into two classes: transient endophytes and true endophytes (Sharma et al., 2021). In addition, on the basis of their association with host plants, endophytes are classified as obligate and facultative endophytes. Endophytes that spread among plants vertically and completely rely on plant metabolism for survival are referred to as "obligate endophytes," whereas endophytes that enter plants from neighbouring soil or environment and only partially rely on the host plant, completing only some part of the host plant's lifecycle, are referred as "facultative endophytes" (Khare et al., 2018). The endophytic world has gained popularity among researchers due to its significant contributions as it produces various types of bioactive compounds that play important roles in various industries such as agricultural, pharmaceutical, medical, and biotechnological industries ( Figure 1) (Latz et al., 2018;Tiwari et al., 2023).
Endophytes have a significant impact on the health of their hosts, including their ability to absorb nutrients, produce phytohormones, and reduce the damage caused by pathogens through antibiosis, the generation of lytic enzymes, the activation of secondary metabolites, and the activation of hormones . However, the complete elucidation of total metabolites produced by endophytes, their functions, protein-protein interactions, and the factors which influence the interaction between fungal, and bacterial endophytes with different plants is still in inferencing.
Nowadays, both fungal and bacterial endophytes are important in the industrial, pharmacological, and biotechnological sectors. This is because they produce different types of metabolites that are used as antitumor, antiviral, and antimicrobial agents, plant growth promoters, bio-control agents, stress tolerance of plants, and immunosuppressants. They also produce different types of compounds that make them good antibiotic, anti-diabetic, and antioxidant agents (Gouda et al., 2016). Several researchers have demonstrated the potential of these endophytes in the synthesis of novel nanomaterials and the role of microbial endophytes in agriculture , (Dhara et al., 2023).
In the present review, the authors have emphasized the importance of both fungal and bacterial endophytes in industries, pharmaceuticals and biotechnology. This study addressed recent research on endophytes in order to address a gap in the field and give a detailed application of the metabolites or bioactive components that have been isolated from endophytes and their potential application to bio-nanotechnology. In addition, this review has emphasised on the importance of endophytes-mediated synthesis of nanoparticles as well as their applicability in a variety of sectors.

Various approaches for screening bioactive compounds in endophytic culture
From the various pieces of literature, it has been proven that there are various approaches for the screening of bioactive compounds from the culture of endophytes like axenic culture, OSMAC approach, and elicitors (Gakuubi et al., 2021). All these three approaches play an important role in the activation of cryptic gene clusters in the endophyte's genome. In addition to this, there are several instrument-based approaches like solid-phase microextraction-gas chromatography-mass spectroscopy (SPME GCMS), high-performance liquid chromatography high-resolution mass spectroscopy (HPLC-HRMS) and matrix-associated laser desorption ionization-HRMS (MALDI-HRMS). SPGE-GC-MS helps in the screening of volatile compounds especially signalling compounds secreted by the endophytes. HPLC-HRMS helps in the screening for bioactive compounds mainly antimicrobial and anticancerous. MALDI-HRMS, the technique could help in the screening of the distribution and release of target compounds in host plants' apoplast (Mishra et al., 2022). Figure 2 is showing a schematic diagram for various approaches for screening bioactive compounds from endophytic culture.
3 The mechanism employed by endophytes in plant growth promotion and protection directly promote plant cell elongation and proliferation by producing phytohormones, indole-3-acetic acid (IAA), siderophores, 1-aminocyclopropane-1-carboxylic acid, phosphate and potassium solubilization antibiosis, and by suppressing the pathogens, etc.
In addition, endophytes promote the capacity to convert atmospheric nitrogen into ammonia, which is required for the synthesis of proteins and nucleic acids and provides tolerance against salt and drought by synthesizing sugar molecules (Muthu Narayanan et al., 2022). Several studies have highlighted the role of endophytic fungi in mitigating biotic and abiotic stresses, making them an essential component of climate-smart and sustainable agriculture (Verma et al., 2022), (Tyagi et al., 2022). The endophytes are known to produce various types of siderophores such as carboxylate, catecholate, phenolates, and hydroxamates. These siderophores perform a variety of functions, including the biocontrol of phytopathogens by limiting the pathogens' ability to absorb iron, the reduction of heavy metal toxicity, and the induction of induced systemic resistance (ISR) , (Goḿez-Godıńez et al., 2023). Several microbial species such as Rhodococcus spp, Bacillus spp, Enterobacter spp, Methylobacterium spp, Pseudomonas fluorescens, Pseudomonas putida, Pantoea ananatis and Pantoea agglomerans, etc. are positively shown to produce siderophores (Karuppiah et al., 2022;Singh et al., 2022).
Endophytes emit different organic acids such as malic, gluconic, and citric acids that convert insoluble soil phosphate (apatite, fluorapatite and hydroxyapatite) into soluble orthophosphates by chelating cations attached to the phosphate (Fadiji and Babalola, 2020), (Yadav et al., 2018). Various bacterial and fungal endophytes have been reported to date for phosphate solubilisation activity, phosphate solubilisation, phytohormone production and nitrogen fixation activity (Supplementary Table 1). Endophytic associative nitrogen-fixing microbes are superior to rhizosphere microorganisms in terms of their ability to enable plant life to flourish in nitrogen-deficient soil and to support the overall health and growth of plants (Afzal et al., 2019). Curvularia geniculata, a dark septate root endophytic fungus isolated from the roots of Parthenium hysterophorus, is known to stimulate the growth of plants by solubilising phosphorus (P) and producing phytohormones (Mehta et al., 2019). Colonisation by the endophytic fungus Serendipita indicia enhances nutrient uptake and helps maintain ionic homeostasis by limiting the passage of sodium (Na + ) and potassium (K + ) ions in plants and enhancing gene transcription, both of which play important roles in Na+ and K + homeostasis (Tyagi et al., 2022). Endophytes like Colletotrichum, Pseudomonas, Bacillus. Herbaspirillum, Alcaligenes, Streptomyces, Piriformospora indica, Sebacina vermifera, and Penicillium have gained particular interest amongst others because of the propensity Schematic diagram for screening of various bioactive compounds from endophytic culture adopted from (Mishra et al., 2022). Choudhary et al. 10.3389/fpls.2023.1193573 Frontiers in Plant Science frontiersin.org to produce phytohormones such as auxins, gibberellins (GA), cytokinins and ethylene that favour improved plant development under harsh conditions (Burragoni and Jeon, 2021), (Omomowo et al., 2023). It has been found that the pestalotiopsis microspore produces pestalotin analogue, a metabolite with gibberellin activity that promotes faster germination . Likewise, Cladosporium sphaerospermum, an endophyte of Glycine max, is responsible for the production of gibberellic acid, which is known to encourage the growth of rice and soybean plants (Omomowo et al., 2023). The indirect mechanism adopted by endophytes promotes the plant growth by enhancing the plant defence system using various mechanisms like plant resistance induction, environmental stress tolerance, predation and hyperparasite, stimulation of secondary metabolites in plants etc (Tian et al., 2008). When a plant is under attack from a biotrophic pathogen, signalling molecules like salicylic acid (SA) and associated pathogenesis-related (PR) proteins act to induce "systemic acquired resistance" (SAR), which in turn triggers "local resistance" by producing a hypersensitivity reaction (HR) in the infected and surrounding areas of the plant (Muthu Narayanan et al., 2022). For instance, pre-treatment of Pisum sativum seeds with Pseudomonas fluorescens (OKC) and Trichoderma asperellum (T42) prevents powdery mildew disease by stimulating the defence response by upregulation of phytohormone, SA, and PR-1 protein (Patel and Saraf, 2017).
Induced system resistance (ISR) is the second defence mechanism plants use to fend against infections (Qin et al., 2021). By triggering the release and transport of signalling molecules like JA and associated PR proteins to the affected areas, it helps plants defend themselves against necrotrophic diseases. Neither the pathogenic virus nor its replication is directly hindered by the ISR approach. On the contrary, it reinforces the plants' inherent physical or chemical defences (Muthu Narayanan et al., 2022). For example, the modulation of signalling pathways by JA and its product JA-isoleucine (JA-Ile) hormone is often achieved by the utilisation of abscisic acid (ABA) or ethylene (necrotrophic pathogens defender) (Muthu Narayanan et al., 2022). ISR is induced by Bacillus subtilis PTA-271 and Pseudomonas fluorescens PTACT2 to prevent canker and grey mould disease caused by Pseudomonas syringae Pst DC3000 and Botrytis cinerea, respectively, in the Arabidopsis plants. Infected plant leaves provide evidence of their antagonistic impact through an increase in JA and ABA (Nguyen and Nguyen, 2020).
In antibiosis, various secondary metabolites such as lipopeptides antibiotics, amino acid-rich peptides (neomycin), and cyclic cationic lipopeptides are produced by different endophytes that serve as biocontrol agents as they exhibit antifungal, antibacterial, and nematocidal activities against phytopathogens (Muthu Narayanan et al., 2022). For instance, to protect the leaves of the W. somnifera plant from infection by A. alternata, endophytic bacteria like B. amyloliquefaciens and P. fluorescens enhance callose deposition in guard cells . Epichloe is a monophyletic genus of filamentous fungi that develop everlasting symbioses with cool-season grasses (Card et al., 2021). Epichloe fungal endophytes not only boost plant immunity against chewing insects by creating protective alkaloids, but they also promote plant immunity by increasing endogenous defence responses mediated by the jasmonic acid (JA) pathway (Card et al., 2021). The foliar endophytic fungus Colletotrichum tropicale isolated from T. cocoa protects the cocoa tree from black pod disease caused by Phytophthora spp. by upregulating genes related to cellulose and lignin deposition and host cell wall hardening (Sadoral and Cumagun, 2021). Bacillus atrophaeus and Bacillus mojavensis, isolated from the Glycyrrhiza uralensis (Licorice) plant, have antifungal activity due to the presence of various compounds such as 1,2-bezenedicarboxyl acid, methyl ester, decanodioic acid, and bis (2-ethylhexyl) ester (Mohamad et al., 2018). The antibacterial activity of Aspergillus sp., endophyte, isolated from the Bauhinia guianensis plant has been reported due to the presence of fumigaclavine C and pseurtotin C (Wen et al., 2022). In another study, the protein bacillomycin D was produced by B. amyloliquefaciens, which was shown to have antagonistic action against the fungus F. graminearum (Gu et al., 2017).

Endophytic bioactive components of medicinal significance
The ecosystem is a treasure trove of medicinal plants that contain chemicals that have the potential to serve as a substitute for medications produced synthetically (Michel et al., 2020). The major problem with collecting these compounds from medicinal plants is that there aren't very many of them, while the demand is high. This leads to overexploitation, which in turn reduces plant population (Palanichamy et al., 2018). In order to fulfil the largescale production of these compounds, alternative approaches like tissue culture, semi-synthesis, exploitation of endophytes for these compound syntheses, heterologous production, etc. are adopted (Sandargo et al., 2019). Endophytes present in plants are considered a treasure house of various bioactive compounds such as tannins, alkaloids, terpenoids, benzopyranones, quinones, polyketides, chinones, saponins, flavonoids, phenolic acids, steroids, xanthones etc. (Figure 3) (Kumari et al., 2020;Vaid et al., 2020). These bioactive compounds are known to have various uses in the medical sector.
Taxol (Paclitaxel) is a diterpene alkaloid which is produced in Taxus sp. by different endophytes such as-Taxomyces andreanae, Fusarium solani, Metarhizium anisopliae and many others which exhibit anticancer (antitumor) activity (Table 1). Taxol acts as a mitotic inhibitor and causing microtubules to break down at the time of cell division. Whereas other bioactive compounds such as-Brefeldin-A, Phomopsin-A/B/C, Cytosporone-B/C, Terpene, and cryptocandin produced in different plants Quercus variabilis Blume, Excoecaria agallocha L., Allamanda cathartica L. and Tvipterigeum wilfordii Hook. f. by different endophytes such as Cladosporium sp., Phomopsis sp., and Cryptosporiopsis quercina are known to have antimicrobial, antifungal, antibacterial and antimycotic activities respectively. Similarly, Lovastatin is produced in the Solanum xanthocarpum plant by Phomopsis vexans endophyte known to have blood cholesterol-lowering properties (Parthasarathy and Sathiyabama, 2015). Lovastatin inhibits the HMG-CoA reductase enzyme (hydroxyl methylglutaryl coenzyme A reductase), which plays an important role in the regulation of the rate-limiting step of cholesterol biosynthesis as this enzyme catalyses the conversion of HMG-CoA to mevalonate, in a competitive manner (Bhargavi et al., 2018). Ligustrazine which is also known as TMP (Tetra methylpyrazine) can stimulate neuronal differentiation by controlling Topoisomerase IIb epigenetic activity (Lin et al., 2022). It safeguards against the oxygen-glucose deprivationinduced degeneration of neurons, encourages the migration of brain progenitor/precursor cells and also inhibits H 2 O 2 -induced apoptosis in bone marrow-derived mesenchymal stem cells by controlling the PI3K/Akt and ERK1/2 signalling pathways. 3-Nitropropionic acid and tenuazonic acid exhibit strong antitubercular effects on M. tuberculosis H37Ra by disrupting the isocitrate lyase enzyme pathways required for the metabolism and virulence of the pathogen (Adeleke and Babalola, 2021). HupA (huperzine A) acts as a cholinesterase inhibitor (ChEI) which function to decrease the breakdown of acetylcholine and is used in dementia and Alzheimer's treatment (Dou et al., 2018). Likewise; DPT (Deoxypodophyllotoxin) cyclolignan compound, isolated from various plants accompanied by endophytes (Table 1). The anti-cancer effect of DPT on colorectal cancer cells through induction of apoptosis, by destabilization of microtubules, activation of mitochondrial apoptotic pathway via regulation of B-cell lymphoma 2 (Bcl-2) family proteins, (decreasing Bcl-xL and increasing Bcl-2 associated X (BAX)) and suppression of tumorigenesis have been reported recently (Gamage et al., 2019). In addition, bilobalide, sesquiterpene tri-lactone, obtained from Ginkgo biloba, could be a potential therapeutic agent for brain ischemia and neurodegeneration due to its upregulation of mitochondrial DNA-encoded COX III subunit of cytochrome c oxidase and the ND1 subunit of NADH dehydrogenase genes. Both genes are involved in neuroprotection through the preservation of mitochondrial functions and hindrance in apoptosis. Nowadays the prodrug approach is often used to combat pharmacokinetic, pharmaceutical, and thermodynamic barriers that limit the inculcation of new drugs. The influenza virus is a deadly virus causing severe damage to human beings. Neuraminidase (NA) inhibitor drugs nowadays are used to treat influenza infections. But due to its antigenic drift and antigenic shift, the influenza virus is continuously evolving and may become resistant to previous drugs. Recently, cyclosporine A (CsA) and its analogues have been reported for antiviral activity against influenza A and B strains (Ma et al., 2016). Cyclosporine is a natural product and can be produced by endophytes (Supplementary Table 1). Likewise, Human cytomegalovirus (hCMV) encodes a 256 amino acid serine protease which is responsible for capsid assembly, an essential process for herpes virus production. Cytonic acids A and B, protease inhibitors, obtained from endophytic fungi Cytonaema sp., prevent the development of infectious herpes viruses by blocking the assembly (Tiwari et al., 2023).

Morchella tomentosa (leaves) Trichoderma longibrachiatum, Syncephalastrum racemosum
Antagonistic effect (group of fungi) and ACA (Ibrahim et al., 2017) Euphorbia prostata Byssochlamys spectabilis Alternaria sp. ABA (Khiralla et al., 2016) (Continued) employed most often to promote the growth of plants (Burragoni and Jeon, 2021). Biopesticides and biofertilizers are becoming formulated with Trichoderma species like T. hamatum, T. harzianum, T. polysporum, and T. virideare because of their ability to colonise root tissues and interact with the host plant via molecular crosstalk, thereby improving nutrient and water uptake, inducing disease resistance, degrading toxic compounds, and ultimately promoting plant growth (Topolovec-Pintaric, 2019). Endophytes defend plants from environmental stresses such as salinity, drought, and others through a variety of mechanisms. One of these mechanisms is the increased production of abscisic acid (ABA), which in turn produces proteins that assist plants in reducing the amount of water lost through transpiration and oxidative stress (Sah et al., 2016). In addition, increased tryptophan production leads to the production of IAA, which is the plant growth hormone auxin and promotes plant growth and rooting (Khan et al., 2019). In a similar fashion, 1-Aminocyclopropane-1-carboxylase (ACC) deaminase hydrolyses the ACC and reduces the production of ethylene, which is responsible for the senescence of the plant, while promoting the production of ammonia and alpha ketobutyrate, which are potential plant growth promoters (Nascimento et al., 2018). Endophytes confer drought tolerance on their hosts by increasing tissue solute accumulation, decreasing water conduction through the leaf, lowering transpiration rates, or thickening the cuticle of the leaf and through osmoregulation for instance, endophytic Neotyphodium spp. improves grass plant drought tolerance (Chhipa and Deshmukh, 2019). Secretion of antioxidant metabolites such as ascorbate and glutathione by endophytes reduces host tissue reactive oxygen species and promotes salt stress tolerance (He et al., 2017). These mechanisms work together to improve plant growth under abiotic and biotic stress conditions by increasing root length and density, increasing nutrient supply to plants, suppressing phytopathogens (Supplementary Table 1), and improving relative water content, osmotic adjustment, and antioxidant property (Khan et al., 2019). Investigators have shown that fungal endophytes have an important role in the host plant, especially in Phyto-stimulation, phytoremediation, phyto-immobilization, phytotransformation and biological control. In addition to this such fungal endophytes produce secondary metabolites which play a role in the reduction of heavy metal toxicity (Radziemska et al., 2021), (Anamika et al., 2018). These boost the plant's antioxidative mechanism, leading to detoxification and allowing it to grow in polluted soil, thereby increasing the plant's resistance to heavy metals (Radziemska et al., 2021). Besides this, fungal endophytes have also several beneficial effect on the host plant which is shown below in Figure 4. Hydrophobic, organic molecules with a low molecular weight (300 Da) and a high vapour pressure (0.01 kPa at 20°C) are known as volatile organic compounds (VOCs). Most of these chemicals are derivatives of amino acids, benzenoid compounds, fatty acids, phenylpropanoids, or terpenoids (Kaddes et al., 2019). Endophytic bacteria and fungi produce volatile organic compounds (VOCs) that effectively prevent plant diseases caused by phytopathogens (Kaddes et al., 2019), (Etminani and Harighi, 2018;Etminani et al., 2022). The endophytic fungus Trichoderma harzianum, isolated from the tomato plant Solanum lycopersicum, produces the volatile organic compound diterpene. This compound inhibits the growth of the phytopathogen Botrytis cinerea by inducing the expression of tomato defence genes related to salicylic acid (SA) (Faucon et al., 2017). Grapevine endophytic bacteria such as Pantoea sp. Sa14, Pseudomonas sp. Sn48, Pseudomonas sp. Ou22, Pseudomonas sp. Ba35, Serratia sp. Ba10, and Enterobacter sp. Ou80 all produce volatile organic compounds (VOCs) that impede the growth of Agrobacterium tumefaciens in a number of ways, such as inhibiting the chemotaxis, motility, biofilm growth, and root attachment (Etminani and Harighi, 2018;Etminani et al., 2022) Antiherbivore defences in grasses are bolstered by the presence of the endophytic fungus Epichloe, both through alkaloid-dependent and-independent pathways (Bastias et al., 2017). Epichloe endophytes not only defend host plants against herbivores but also from several pathogens. For instance, the presence of an Epichloe endophyte within plants reduced the symptoms of plant diseases caused by the biotrophic fungal infections Blumeria graminis, Claviceps purpurea, Ustilago bullata, and Laetisaria fuciformis (Kou et al., 2021).

Endophytes of industrial significance
Bacterial and fungal endophytes are the greatest sources of enzyme production which can be used in several industries , (Hirata et al., 2018). Endophyte species produce several enzymes, including proteases, pectinases, amylases, cellulases, xylanases, laccases, lipases, and others, which are significant in many industrial industries (Table 2) (Zaferanloo et al., 2014). Additionally, some enzymes generated by endophytic species play crucial roles in a wide range of industries, including the production of biofuels in the energy sector, which is used as an alternative source of AMA (Yang et al., 2015) Bauhinia forficata Acremonium curvulum, Asp. ochraceus, Gibberella fujikuroi, Myrothecium verrucaria and Trichoderma piluliferum, Penicillium glabrum AMA (Bezerra et al., 2015) conventional fuel; the development of pigments for the food industry; the manufacturing of enzymes to degrade polyurethane; and many more (Mengistu, 2020;Singh et al., 2023). Endophytes Phomopsis, Cephalosporium, Microsphaeropsis, and Nigrospora were isolated from plants Taxus chinensis var. mairei Mast, Cupressus torulosa D. Don, Keteleeria davidiana varchienpeii, Sabina chinensis cv. Kaizuca and Keteleeria evelyniana Mast, are known to synthesize enzymes which aid in the extraction of biofuels (Tiwari et al., 2023). In place of toxic chemicals, the textile industry uses a combination of pectinase with amylase, lipase, cellulase, and hemicellulase to digest cotton and remove sizing agents. Pectinase has been investigated extensively in oil extraction from several sources, such as flaxseed, dates, and olives (Haile and Ayele, 2022). Currently, immobilised lipases are used in a wide range of commercial processes, including the manufacture of biosensors, biodiesel and cleansers as well as the organic synthesis of various substances, including cosmetics, meals, medications, fragrances, and tastes (Ismail and Baek, 2020). Xylanase is used as a pre-bleaching agent in the paper and pulp industries. It also has biotechnological applications in the biofuel, food, textile, and feed industries . Chemical, beverage, textile, food, biofuel, and paper sectors are just a few of the many that rely on the starch-digesting enzyme amylase. It is widely used in the pharmaceutical industry to hydrolyse starch to create various sugars including glucose and maltose, which have a variety of applications. The starch industry uses amylases most frequently to hydrolyze starch during the starch liquefaction process, which turns starch into fructose and glucose syrups (Mehta and Satyanarayana, 2016). Proteases are essential industrial enzymes with numerous uses in chemical and biological reactions. Proteases are also utilised in many other industries, including the production of detergents, the food industry, the tanning of leather, the manufacturing of paper, the recovery of silver from photographic films, the manufacturing of paper, bioremediation procedures and employed therapeutically to cure inflammation and dangerous lesions (Abdel Wahab and Ahmed, 2018;Othman et al., 2018). Cellulases are in great demand across many different industries, including the food and beverage industry, the paper and pulp businesses the manufacturing of textiles, the pharmaceutical field, the cleaning products industry, and the biofuels sector (Raghav et al., 2022). Cellulases are crucial in the selective processing of lignocellulosic biological materials (Payne et al., 2015;Jayasekara and Ratnayake, 2019). Lipopeptides are an important class of secondary metabolites produced by bacterial endophytes and consist of cyclic or linear peptides that are connected to lipophilic molecules. Antibiotic efficacy against numerous diseases places these lipopeptides among the most potent available compounds (Narayanan and Glick, 2022), (al Ayed et al., 2022). Endophytes producing lipopeptides were reported in the medicinal plant Cordia dichotoma L., which is native to the Jammu region. These endophytes belonged to the genera Acidomonas, Alcaligenes, Bacillus, Pseudomonas, Peaenibacillus, Ralstonia, Streptococcus, Micrococcus, and Staphylococcus. Many of the lipopeptideproducing endophytes demonstrated antibacterial activity against a wide variety of bacteria, including Salmonella typhi, Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis, Staphylococcus aureus, Klebsiella pneumoniae (Sharma and Mallubhotla, 2022). Aspergillus sp. A9, Aspergillus sp. A36, Penicillium sp. P5, and Penicillium sp. P15, an endophytic fungus isolated from M. guianensis, was found to be an excellent producer of hydrolase enzymes. The lipase and protease produced by Penicillium P15 and Penicillium sp. P5 were able to break down the S. aureus biofilm (Matias et al., 2021). Hydrolytic enzymes like peptidase, amylase, xylanase, and carboxylase are produced by endophytes, which lyse the rigid peptidoglycan or Interaction between fungal endophyte and plant expediates Phyto-stimulation conferring the stress response adopted from (Verma et al., 2022). Choudhary et al. 10.3389/fpls.2023.1193573 Frontiers in Plant Science frontiersin.org Amylase, cellulase, protease (Ayob and Simarani, 2016) E. longifolia Preussia minima, Alternaria sp. Amylase (Zaferanloo et al., 2014) A. altissima F. proliferatum Amylase (Oukala et al., 2021).

Role of endophytes in bio-nanotechnology
Nanotechnology and nanoparticles (NPs) have gained huge attention in the last decade due to their unique and remarkable properties like high surface area to volume ratio and high surface energies . Due to these features, NPs are widely used in medicine, research, drug delivery, electronics and environmental clean-up (Modi et al., 2022). When it comes to drug delivery and medicine biocompatible and non-toxic nanomaterials are the first preference. So, biocompatible NPs could be easily synthesized by using fungal and bacterial endophytes. From the various pieces of literature, it has been revealed that numerous investigators have used both prokaryotic and eukaryotic endophytes for the synthesis of both metal NPs and metal oxide NPs (Ahmad and Kalra, 2020;Modi et al., 2022). All these methods mainly involve bottom-up approaches which involve exposure of metallic ions to the desired endophytes under desired conditions. Investigators have proven that the positively charged metal ions come closer to the negatively charged endophytic surfaces by electrostatic attraction. Further, these ions then get transported to the internal structure of the endophytes via ions channels where the metal ions get reduced to their zero-valent atomic states., which further then get aggregated to form NPs , (Dhara et al., 2023). Figure 5 is showing basic steps involved in the green synthesis of NPs by using endophytic microorganisms.
Till date investigators have synthesized gold, silver, and copper metal NPs from the endophytes, which have been used in all the domains of science. In addition to this, investigators have reported the synthesis of zinc sulfide, copper oxide, cobalt oxide, nickel oxide, etc from endophytes isolated from terrestrial and marine regions. Fadiji and their colleagues showed the role of various NPs synthesized from the bacterial and fungal endophytes in sustainable agriculture by enhancing plant growth and improving disease resistance (Fadiji et al., 2022). Bogas and their team have shown that these endophytes could act as biofactories for nanoparticles. NPs synthesized from such endophytes have immense potential in healthcare applications (Bogas et al., 2022). In addition, Rathore and his colleagues have placed an emphasis on bacterial endophytes, discussing the recent biomedical scope of these organisms, as well as their synthesis, associated challenges, and importance in bio-nanotechnology (Rathore et al., 2022). Mishra and their groups have also emphasized the green synthesis of NPs by using fungal endophytes which have easy scaleup, downstream processing and eco-friendly nature (Misra et al., 2021). The majority of these NPs synthesized from the endophytes have a role as an antimicrobial agent or as an anti-cancer agent. The antimicrobial activity of endophyte-mediated synthesis NPs is shown in Figure 6 while Figure 7 is showing anticancer activity of the NPs synthesized from endophytes. Table 3 is showing a summarized form of various nanoparticle syntheses from endophytes, along with their applications. Schematic representation of the general steps for green synthesis of metal-based NPs using endophytic microorganisms isolated from tropical plant adopted from Bogas et al., (Bogas et al., 2022).

Conclusion
Endophytes are bacterial and fungal species beneficial to plants by fulfilling their requirements for growth and protection. Recent applications of endophytes in the agriculture sector not only accelerate plant growth by providing tolerance against various stresses but also reduces the use of numerous agrochemicals like chemical fertilizers and pesticides and this would make agriculture more sustainable and productive. In addition, the exploration of the insecticidal, antimicrobial, and pest-control activities of endophytes will make them good friends of farmers. Endophytes produce several bioactive compounds with huge industrial and medicinal applications as well as can be involved in the bio-transformation of hazardous chemicals like toxins, pollutants and heavy metals. Although Schematic diagram for the steps involved in the antimicrobial activity of NPs synthesized by endophytes adopted from Bogas et al., (Bogas et al., 2022). Schematic diagram representing steps involved in the anti-cancer activity of NPs synthesized by endophytes adopted from Bogas et al., (Bogas et al., 2022). Choudhary et al. 10.3389/fpls.2023.1193573 Frontiers in Plant Science frontiersin.org the bio-transforming activities of endophytes is still in their infancy.
Thus, future efforts should focus on the industrial and medicinal applications of the bio-transforming endophytes and strengthen their eco-friendly and cost-effective approaches in food safety and in the pharma sector. Endophytes exert various therapeutic activities such as anti-cancer, anti-diabetic, anti-inflammatory etc activities by their bioactive compounds. To date, there is no report on commercially available antibiotics derived from endophytes. Intensive research is required which emphasizes the development of new drugs or antibiotics from endophytes and their mechanism of action. The use of endophytic microbes is a relatively new area of study for the environmentally friendly synthesis of nanoparticles, especially when compared to saprophytic microorganisms. Endophyte-derived NPs have potential applications in medicine, including the elimination of multidrug-resistant bacteria, the transport of genetic elements in genetic engineering, and the detection of disease. At the interface of biology and nanotechnology, this area of study has the potential to usher in a plethora of novel nanomaterials. Incorporating metagenomics, metabolomics, and metabolic profiling approaches for elucidating the biosynthetic pathways adopted by endophytes and plants, as well as creating protein-protein interaction maps, and exploring endophytic nanoparticles, will greatly illuminate future applications of endophytes in agriculture, environment, medicine, and industry.

Author contributions
NC, ND, AG, RV and B-H J contributed in conceptualisation, supervision, review editing and writing, and VY, MC, UB, RG and RC reviewed the manuscript, involved in formal analysis, validation and review editing. MA, LE, WA and NC prepared the final draft. MA, LE, WA, RV and B-HJ contributed in visualisation, supervision, review editing and funding acquisition and RV, RG, B-HJ finalized and submitted the manuscript. All authors contributed to the article and approved the submitted version.

Funding
The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University (KKU) for funding this research through the Research Group Program Under the Grant Number: (R.G.P.2/513/44). This work was supported by t h e M i d -C a r e e r R e s e a r c h e r P r o g r a m ( g r a n t n o . 2020R1A2C3004237) through the National Research Foundation of the Republic of Korea. The authors are also thankful to the Department of Biosciences; Mody University of Science and Technology, Lakshmangarh Sikar, Rajasthan (SM/2020-21/008 and SM/2022-23/008) for providing financial support.

Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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