Studies throughout the decades have shown that endophytes are central to plant protection against pathogens (Barkodia et al., 2018). During the direct interaction between endophytes and pathogens, endophytes produce antibiotics to suppress the pathogens and protect the plant from the damage caused by pathogens (Fadiji and Babalola, 2020). Endophytes produce certain secondary metabolites that have antimicrobial properties like antifungal, antibacterial, etc. (Palanichamy et al., 2018). These antimicrobials have a strong inhibitory action on plant pathogens. A single class of endophytes is capable of producing a variety of bioactive agents (antibiotics) such as aromatic compounds, terpenoids, polypeptides, and terpenoids (Balla et al., 2021). Antimicrobials synthesized by some endophytic fungi are listed in Table 1.

Lytic enzyme secretions play a significant role in plant protection from pathogens, particularly when it comes to endophytic microorganisms. Most microbes produce lytic enzymes that aid in the hydrolysis of polymers (Tripathi et al., 2008). Endophytic fungi create a symbiotic relationship with plants when they colonize the internal tissues of plants; this interaction is advantageous to both the plant and the fungus. As pathogens attempt to invade the plant, the fungal endophytes parasitize pathogenic hyphae in various ways, including twisting, coiling, perforation of the pathogenic hyphae, and secretion of cell wall-degrading enzymes leading to lysis and inhibition of pathogens (Waghunde et al., 2017). By degrading the cell walls of pathogens, the lytic enzymes limit the ability of the pathogens to invade and propagate within the plant, effectively enhancing the plant’s natural defense mechanisms. This mechanism not only provides immediate protection but also primes the plant’s immune system for induced systemic resistance (ISR), leading to long-lasting and robust defense responses against potential future pathogen attacks (Fadiji and Babalola, 2020; Trivedi et al., 2020). In lieu of chemical pesticides, the use of lytic enzymes produced by endophytic fungi offers a sustainable and environmentally benign method of plant protection, encouraging healthier and more resilient crops. Despite their involvement in mycoparasitism, these lytic enzymes also contribute to cell wall reformation and recycling during active fungal growth, as well as during aging and autolysis (Kumari and Srividhya, 2020). Endophytic fungi have a variety of enzymes such as hemicellulases,1,3-glucanases, chitinases and cellulases, that break down different types of materials (Haran et al., 1996). It has been reported that in Trichoderma species, a number of enzymes are directly involved in the breakdown of the cell wall of pathogenic fungi for the utilization of pathogenic fragments (Rajesh et al., 2016). Mycolytic enzymes like 1,3-glucanase or chitinase produced by Trichoderma asperellum have the potential to degrade the cell walls of phytopathogens. A respective increase in enzymatic activity and transcripts of 1,3, glucanase, and chitinase enzymes in cultures induced by pathogens has been observed in banana wilt affected plantations (Win et al., 2021). Biocontrol of root-knot nematodes by endophytic fungi showed elevated activity of genes encoding enzymes like glucanase and chitinase and downregulation of genes encoding antioxidant enzymes during the stimulation of the immune system of plants in response to pathogens (Molinari and Leonetti, 2019). Although enzymes lack the ability to act as antagonizing agents independently, but they augment the antagonistic activity of other agents and pathways when merged together. Similarly, the pectinase enzyme has also been reported to help reduce pathogenesis in plants (Fadiji and Babalola, 2020).

Phytohormones of endophytic fungi play a crucial role in plant protection, enhancing defense responses and resistance to pathogens through complex interactions with the plant (Fadiji and Babalola, 2020). The context of plant protection, the secretion of phytohormones by endophytic fungi trigger a sequence of defense mechanisms in the host plant. In response to environmental cues, endophytic fungi that have formed a symbiotic relationship with plants release phytohormones such as auxins, cytokinins, gibberellins, indole acetic acid, and jasmonic acid that act as potent signaling molecules (Baron and Rigobelo, 2022). The two phytohormones, Jasmonic acid (JA) and Salicylic acid (SA) are important defense signaling molecules that regulate the defense responses of plants against disease-causing microbes (Shi et al., 2020). Systemic acquired resistance (SAR) and biotrophic pathogen defense are both impacted by Salicylic acid, which in turn is regulated by the activation of Pathogenesis-related (PR) genes. Localized programmed cell death, a fallout of the hypersensitive response caused by the two phytohormones, jasmonic acid and salicylic acid inhibits the spread of pathogens and protects against stress (Lahlali et al., 2022). These proteins and compounds act as an immediate response to pathogen attack, hindering the progress of invading pathogens and limiting their growth (Rashad et al., 2020). When pattern recognition receptors (PRRs) recognize the pathogens, Jasmonic acid (JA) and Salicylic acid (SA) have the potential to enhance the activity of the enzymes in the phenylpropane pathway of plants resulting in the production of phenolic compounds (such as flavonoids, lignin, coumarins, and tannins), triggering pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI), a robust defense response in plants (Franco-Orozco et al., 2017). Jasmonic acid and ethylene (ET), mainly produced during induced systemic resistance (ISR), are considered important for defense against necrotrophic diseases and beneficial for interactions between plants and pathogens (Li et al., 2019). The phenomenon of JA/ET-dependent systemic resistance has been seen in various plant-associated microorganisms, including Trichoderma asperellum, Penicillium sp., and the endophyte Serendipita indica. But in other pathosystems, S. indica produced resistance without relying on the JA/ET route, while T. asperellum coupled with plants triggered resistance in an SA-dependent manner. This suggests that the roles of phytohormones and their potential interactions are complicated, and the use of a microorganism on the plant is expected to alter the entire hormone profile rather than solely altering the levels of individual hormones. Plant defense signaling pathways have been identified and compounds act as an immediate response to pathogen attack, hindering the progress of invading pathogens and limiting their growth (Rashad et al., 2020). When pattern recognition receptors (PRRs) recognize the pathogens, jasmonic acid (JA) and salicylic acid (SA) have the potential to enhance the activity of the enzymes in the phenylpropane pathway of plants, resulting in the production of phenolic compounds (such as flavonoids, lignin, coumarins, and tannins), triggering pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI), a robust defense response in plants (Bhattacharya et al., 2010). Jasmonic acid and ethylene (ET), mainly produced during induced systemic resistance (ISR), are considered important for defense against necrotrophic diseases and beneficial for interactions between plants and pathogens (Ghozlan et al., 2020). Plant defense signaling pathways have been found to involve ethylene (ET) and abscisic acid (ABA), while auxin, gibberellic acid (GA), cytokinin (CK), brassinosteroids, and peptide hormones may also be involved (Li et al., 2019). Furthermore, ethylene and jasmonic acid also initiate the synthesis of secondary metabolites, including volatile organic compounds (VOCs) and phytoalexins. VOCs can serve as signaling molecules to attract beneficial microorganisms, such as predatory insects or microbes that feed on plant pests, to further enhance plant protection. On the other hand, phytoalexins are antimicrobial compounds that are directly involved in the inhibition of pathogen growth and proliferation within the tissues of plants (Latz et al., 2018). A study conductedal on the functional role of Trichoderma atroviride fungi in controlling the pathogenic activity of Fusarium verticillioides in maize showed the involvement of this endophytic fungi in the synthesis of phytohormones such as salicylic acid, abscisic acid (ABA) and jasmonic acid (Agostini et al., 2019). Similarly, Ren and Dai also found that the interaction between Gilmaniella spp. and endophytic fungi Atractylodes lancea led to increased production of jasmonic acid along -with the production of various volatile antimicrobial compounds such as eudesmol, atractylone, atractylodin and hinesol (Ren and Dai, 2012). Together, ethylene, Jasmonic acid, and salicylic acid create a hormonal response network that is coherent and keeps the plant’s defense mechanisms in place like production of compounds like ethylene and jasmonic acid in response to agents like Penicillium spp., Serendipita indica and Trichoderma asperellum to activate systemic resistance is crucial in avoiding host-pathogen inhabitation or activation of salicylic acid dependent systemic resistance pathway in plants inhabited by Trichoderma asperellum (Latz et al., 2018). These hormonal responses generated between the endophytic fungi and phytopathogens are extremely complicated and involves cross communication and multiple events between plants and the host endophytic fungi (Adeleke et al., 2022; Figure 2).

Moreover, phytohormones influence the establishment of induced systemic resistance (ISR) in the plant. Induced Systemic Resistance entails getting the plant’s immune system ready to react more quickly to future pathogen attacks. This priming effect allows the plant to mount a quicker and more robust defense, effectively protecting it from potential pathogen threats (Baron and Rigobelo, 2022). Despite significant advances in research on signal transmission in induced resistance, there is still a lacuna in allocating roles to each hormone in signal transduction, particularly in complex systems. As a result, there is a need to strengthen a plant’s defense systems and use it as a biomarker to detect induced resistance (Latz et al., 2018). Overall, the secretion of phytohormones by endophytic fungi in plant protection is a finely tuned mechanism that involves a network of signaling pathways and defense responses. By modulating the plant’s hormonal balance, endophytic fungi contribute to a heightened state of preparedness against pathogen attacks, leading to improved plant health, resilience, and reduced reliance on chemical pesticides. Harnessing the potential of plant phytohormones from endophytic fungi offers a sustainable and eco-friendly approach to plant protection, benefiting both agricultural productivity and environmental health (Waghunde et al., 2017; Figure 3).

For the growth and development of plants, phosphorus is a crucial nutrient. However, in most soils, phosphorus is present in insoluble forms like phosphate rocks or mineral complexes. This renders it less available to plants, limiting their growth and overall health (Johan et al., 2021). Endophytic fungi have evolved the ability to solubilize phosphorus from these insoluble forms into soluble forms that the plant can readily absorb. This is achieved through the secretion of organic acids, which are powerful chelators capable of breaking down complex phosphate compounds. One of the primary organic acids produced by endophytic fungi is gluconic acid. This acid is particularly effective in dissolving phosphorus compounds in the soil. When the endophytes release gluconic acid into the plant’s rhizosphere, it reacts with the insoluble phosphorus, converting it into soluble phosphate ions. These soluble phosphate ions become more available for uptake by the plant’s root system (Etesami et al., 2021). Phosphate solubilization by endophytic fungi is a crucial process that aids in protecting plants from pathogens. By facilitating phosphorus uptake, endophytic fungi contribute to improved plant health and vigor (Baron and Rigobelo, 2022) and healthy plants are better equipped to defend themselves against pathogens. When a plant is well-nourished with an ample supply of phosphorus, it can allocate more energy and resources toward its defense mechanisms. The mechanisms employed by endophytic fungi for phosphate solubilization can be categorized into acidification, enzyme activity, and ion exchange (Sharma et al., 2013). Phosphate solubilization by acidification involves the secretion of organic acids like citric acid, oxalic acid, malic acid, gluconic acid etc. into the rhizosphere. These organic acids act as chelating agents for the binding of metal ions present in the soil to facilitate the release of bound phosphate, thus making them available for plant uptake. Fungal strains such as Trichoderma viride, Trichoderma harzianum, Trichoderma vixens, and Trichoderma longibrachiatum have been reported for phosphate solubilization by acidification (Elhaissoufi et al., 2022). Many endophytic fungi such as Penicillium and Aspergillus have been shown to produce various enzymes like phosphatases and phytases which are capable of hydrolyzing the organic phosphate compounds present in soil and converting insoluble phosphate into soluble inorganic phosphate (Chaudhary et al., 2022). Other than these two mechanisms, endophytic fungi can also exchange metal cations present in the rhizosphere with phosphates, thus effectively releasing soluble phosphates into the surrounding soil (El Hassan, 2017). Endophytic fungi provide an additional source of phosphorus to the plants by solubilizing phosphate, thus enhancing their nutrient uptake and promoting overall growth and health. This increased nutrient availability makes the plants more resistant to pathogenic attacks in several ways such, as enhanced plant growth, and induced systemic resistance like phosphate-solubilizing endophytic fungi can trigger systemic resistance in plants. They can stimulate the plant’s defense mechanisms and produce various secondary metabolites that act as natural biopesticides, thereby protecting the plant from pathogenic attacks. Overall, phosphate solubilization by endophytic fungi is a multifaceted mechanism that contributes significantly to plant protection against pathogens. These beneficial fungi play a vital role in promoting plant health and crop productivity while reducing the need for chemical fertilizers and pesticides, making them essential components of sustainable agriculture practices.

For all eukaryotes and almost all prokaryotes, iron is a crucial nutrient, since it is necessary for metabolic function. Availability of iron in two different oxidation states, i.e., ferric (Fe³⁺) and ferrous (Fe²⁺), helps in serving iron both as a cofactor and a catalyst in basic metabolic processes. Despite being one of the metals with the highest abundance on Earth, iron has a low bioavailability due to the fact that iron forms a highly insoluble compound ferric hydroxides in the presence of oxygen (Expert, 1999). Overall, a sufficient supply of iron is necessary for survival. Various species have created controlled systems to maintain homeostasis between sufficient uptake of iron and preventing iron toxicity to avoid cell damage. In plants, animals, and bacteria, iron with high-affinity forms complex with glycoproteins such as lactoferrin or transferrin. Iron is stored intracellularly in ferritin (Arosio and Levi, 2002). Before recent times, little was known about the role of iron in interactions between fungi and their hosts as well as the general regulatory mechanisms that control iron homeostasis. Reductive iron assimilation and Siderophore-mediated iron uptake are the two major systems used by fungi for iron uptake (Oide et al., 2006). Microbes produce siderophores, which are low molecular weight, essentially ferric-specific ligands, as scavenging agents to counteract low iron stress (Sid = Iron, Phores = Bearers; Korat et al., 2001). Except for Lactobacilli, Candida albicans, Cryptococcus neoformans, Saccharomyces cerevisiae all other aerobic and facultative anaerobic microorganisms are known to produce siderophores, which function as iron chelates (Loper and Buyer, 1991). Siderophores are thought to be the result of Fe²⁺ being simultaneously oxidized to Fe³⁺ and precipitated as ferric hydroxide as an evolutionary reaction to the presence of O2 in the atmosphere (Winkelmann and Drechsel, 2008). Variety of siderophores produced by fungi have been classified mainly into five major classes (a) fusigens, (b) coprogens, (c) ferrichromes, (d) rhodotorulic acid, and (e) rhizoferrin (Chincholkar et al., 2000). Some of the fungal siderophores are enlisted in Table 3.

The extraordinarily high affinity of siderophores for ferric ions is the most notable characteristic of Siderophores (Askwith et al., 1996). When competing for nutrients in the soil, the ability to exploit siderophores produced by different microbes is a significant selection advantage. The most significant biotechnological importance of siderophores is in the plant’s rhizosphere, where they nourish the plant with iron, act as a first line of defense against parasites that invade the roots, and aid in the removal of hazardous metals from contaminated soil (Matzanke et al., 1987). Studying the opportunistic human pathogen Aspergillus fumigatus led to the initial discovery of the function of fungal siderophores in illness (Sindhu et al., 1997). In vitro removal of iron from transferrin by siderophores was linked to the survival of pathogen in human serum (Hissen et al., 2004). Another example of role of fungal siderophores can be seen in Candida albicans, a non-siderophore producing yeast. Candida albicans requires Ferrichrome-type siderophores along with Arn1p/Sit1p, a siderophore transporter, for invasion and penetration of its host epithelia (Heymann et al., 2002). The discovery that the metabolic byproduct of the biosynthetic pathway involving the NPS6 gene, encoding a non-ribosomal peptide synthase from Cochliobolus heterostrophus, a phytopathogenic fungus, recognized the role of Siderophores in fungal virulence towards plants (Oide et al., 2007). Many studies on NPS6 gene demonstrated its role in virulence to maize against H₂O₂ hypersensitivity and to be broadly conserved among the filamentous ascomycetes (Lee et al., 2005). Beyond virulence, fungal siderophores also serve to maintain the mutualistic symbiotic relationships between grass and endophytes. The grass symbiont Epichloe festucae uses the NRPS gene sidN to make a new extracellular siderophore that looks like fusarinine-type siderophores. These helpful fungi are never free-living; instead, they are restricted to the intercellular spaces (apoplast) of leaf sheaths and blades, where they do not spread disease. On the other hand, Mycorrhizal fungi frequently create advantageous symbiosis with the roots of terrestrial plant communities, which benefit plant nutrition, including the uptake of micronutrients (Johnson, 2008). Also, these fungi have been found to produce hydroxamate siderophores, and it is believed that siderophore-mediated iron uptake is crucial for the acquisition of iron by the host plant (Haselwandter et al., 2006). Thus, the production of siderophores is a potent strategy employed by various fungal endophytes to grab iron from the environment. Both extracellular and intracellular siderophores play an essential role in numerous fungal-host interactions.

Plant hormones take an active role in symbiotic, defensive, and developmental processes. Ethylene (ET), a gaseous plant hormone that is easily absorbed by plant tissues and has effects even in extremely low quantities, is at the center of these activities (Van de Poel et al., 2015). Some studies have explored the impact of ethylene on the cell division and the finding indicates that ethylene can exert contrasting effects on the cell cycle depending on the specific tissue as well as internal and external stimuli. During the development of the apical hook, ethylene drives cell division in the subepidermal layers, most likely working in conjunction with auxins (Van de Poel et al., 2015). In addition to controlling several aspects of plant growth and development, Ethylene also engages in microbial defense and symbiotic programs which affect overall microbial assembly (Desbrosses and Stougaard, 2011). Moreover, ACC (1-Aminocyclopropane-1-carboxylate) a major precursor of ethylene is a non-proteinogenic α-amino acid that has been reported to play an important role in regulating numerous plant developmental and defense responses. The production of ethylene precursor is mainly regulated at all the major levels (transcription, post-transcription, translation, and post-translation; Lee et al., 2017). The molecule is produced from S’ adenosyl methionine (SAM) in a reaction catalyzed by an enzyme ACC-synthase, releasing MTA (5-methylthioadenosine). Through a series of biochemical reactions, the released MTA is reconverted to methionine to renew the stack of available methionine (Bennett, 2003). Being localized in the cytosol, ACS is one of the members of the Pyridoxal phosphate (PLP) dependent enzymes that uses vitamin B6 as a co-factor for its enzymatic activity (Boller et al., 1979). MACC (malonyl-ACC; Amrhein et al., 1981), GACC (γ-glutamyl-ACC; Martin et al., 1995) and JA-ACC (Jasmonyl-ACC; Staswick and Tiryaki, 2004) are three different conjugates of ACC namely suggesting a complex overall biochemical regulation of ACC pool with eventual effects on the production of ethylene and other developmental and physiological processes. Apart from these conjugates produced in different mechanisms, another distinctive way to metabolize ACC (1-Aminocyclopropane-1-carboxylate) is the deamination of ACC. Initially, ACC deaminase was first reported in bacteria. Many reports suggested that some plant growth-promoting bacteria have the potential to process the plant-based ACC using the ACC deaminase enzyme into ammonia and α-ketoglutarate (Honma and Smmomura, 1978). Being multimeric with an approximate subunit molecular mass of 35-42 kDa (Ullah et al., 2019), ACC deaminase is a sulfhydryl enzyme that requires pyridoxal 5’-phosphate (PLP) for its enzymatic activity (Kushwaha et al., 2020). The cleavage of ACC to ammonia and α-ketoglutarate catalyzed by the enzyme ACC deaminase was discovered in 1978 (Ullah et al., 2019). Following X-ray crystallographic studies, the enzyme ACC deaminase folds into two domains, each of which has an open twisted α/β structure resembling the β-subunit of the tryptophan synthase (Husen et al., 2008). This PLP-dependent enzyme has a low affinity for ACC with a 1.5-15 mM reported Km value (Hontzeas et al., 2004). Many studies have shown that the root exudates contain an explicit amount of ACC that might attract ACC deaminase harboring microorganisms and set up Rhizospheric interaction (Glick et al., 2007). Interaction with the root environment is a must to access the plant-based ACC by the plant growth-promoting microbes containing ACC deaminase enzyme (Payment et al., 2011). Apart from bacterial species, fungi are also exploited for their ACC deaminase activity. Even though the functional principles of the bacterial and fungal ACC deaminase are substantially the same, their structures differ due to sequence variations. Comparison of AcdS gene sequences of various fungal and bacterial strains have shown that approximately 70-90% sequence similarity was shown by the fungal strains with that of bacterial AcdS gene rather than with the fungal gene sequence. Schizosaccharomyces pombe 972 h, Clavispora lusitaniae ATCC 42720, Cyberlindnera saturnus are some of the major examples of fungal strains with higher sequence similarity with bacterial AcdS rather than fungal. It has been hypothesized that horizontal gene transfer is responsible for the transfer of genes from Proteobacteria to the above-mentioned fungal strains (Nascimento et al., 2014) Furthermore, the isolated fungal ACC Deaminase genes similar to Proteobacteria mainly belong to classes Ascomycota and Basidiomycota (Gravel et al., 2007).

In the rhizosphere and phyllosphere of plants, competitive exclusion, a frequent phenomenon, controls the population of a wide range of species (Sikora et al., 2007). In some circumstances, the biological control of root diseases and nematodes is also a result of competition for nutrients and space. Therefore, when several inoculants are required for efficient pest management, the “first-come, first-serve” aspect of colonization and the idea of survival of the fittest are the major factors that could affect the efficacy of biological control (Backman and Sikora, 2008). Because nutrients in soils are typically in scarce supply and difficult to acquire, competition for resources such as oxygen, nutrients, etc. is the active demand between soil-inhabiting microorganisms including pathogens and non-pathogens, and is a key mechanism for the management of diseases that are carried by soil (Stoytcheva, 2011). Due to competition for scarce nutrients and the fact that starvation is a significant and frequent cause of microbiological death, fungal phytopathogens may be biologically controlled (Eisendle et al., 2004). If the requirements both in terms of nutrients, oxygen, space of endophytic fungi, and the disease-causing pathogens are similar, then only competition will occur among them. Species of Fusarium and Pythium, soil-borne pathogens that infect plants through mycelial contact are more prone to competition than those that invade through infection threads (Van Dijk and Nelson, 2000).

Several research have examined how plants respond to disease and parasite invasions for more than 20 years, utilizing several categories. Systemic acquired resistance (SAR) and induced systemic resistance (ISR) are the two types of resistance that researchers are most interested in. ISR is regulated by ethylene or jasmonic acid, which is produced by some non-pathogenic rhizobacteria and not connected to the accumulation of pathogenesis-related (PR) proteins. Salicylic acid mediates SAR, which is a result of pathogen infections and is linked to the production of PR proteins (Tripathi et al., 2008). Invading cells are directly lysed by these PR proteins’ many enzymes, like chitinases and 1, 3-glucanases, which also strengthen cell wall borders and increase resistance to cell death and infection (Gao et al., 2010). ISR generated by endophytes has also been associated with increased expression of genes involved in disease. The ISR process does not directly kill the virus or restrict it. Instead, it strengthens the plants’ natural physical or chemical barriers (Van Loon et al., 1998). ISR and SAR frequently have an antagonistic action that controls the signaling at the cellular level. When SA and JA have an antagonistic effect on biotrophic or necro pathogens, and vice versa, there is upstream and downstream signaling between them (Syed Ab Rahman et al., 2018). For instance, Blumeria graminis f. spp. Tritici, which causes powdery mildew in wheat, is controlled by Bacillus subtilis by inducing disease resistance via the SA-dependent signaling pathway (Xia et al., 2022). The non-expression of the pathogenesis-related genes 1 (NPR1 and NPR3/4), which eventually trigger an antagonistic response via SAR through priming and reveal resistance against secondary infections, is crucially regulated by the pathogenesis-related gene 1 (PR1; Ding et al., 2009). By preventing Botrytis cinerea’s spore germination and mycelium growth through the ISR method of resistance, Burkholderia species (BE17 and BE24) shield grapevine from the grey mold disease (Lahlali et al., 2022). Trichoderma spp. AA2 and Pseudomonas fluorescens PFS are the most powerful antagonists of Ralstonia spp., which causes bacterial wilt in tomatoes by generating ISR in the plant.

Secondary metabolites are bioactive substances that have an important function in ecological interactions, competition, and defensive signaling (Liu et al., 2018). The production of secondary metabolites via a metabolic exchange, which exhibits a complicated regulatory response, is necessary for the formation of microbial contact. These interactions may be competitive, parasitic, mutualistic, hostile, or mutualistic. The most recent advances in imaging mass spectrometry (IMS) technology have been employed to investigate the diverse roles played by mold metabolites in microbial interactions. The growth of phytopathogens is regulated by the antibacterial and antifungal activities of secondary metabolites. While under biotic stress, plants can create secondary metabolites on their own or in collaboration with other endophytes to manage stress and mount defenses (Ludwig-Müller, 2015). To safeguard plants and enhance crop quality, endophytic secondary metabolites are employed as a biocontrol agent. Whereas microorganisms create homogeneous, high-quality metabolites with the highest efficacy in terms of their biocontrol potential, plants produce bioactive molecules that are inadequate and variable in quality (Lugtenberg et al., 2016). An example of plant secondary metabolite stimulation by endophytic fungi is the production of loline (an alkaloid) in the leaves of fescue grasses by endophytic fungus Neotyphodium spp. and Epichloë spp., which protects the leaves from herbivores. The interaction between endophytic fungus Neotyphodium coenophialum and its host plant is another example of plant secondary metabolite stimulation. Neotyphodium coenophialum protects its host plants from aphids the key carriers of viruses, e.g., protection of Festuca arundinacea against Rhopalosiphum padi also inhibit the spread of viruses (Alam et al., 2021).

Hyperparasites are another means by which endophytes protect their host ecologically. In this approach, recognized pathogens or their zoospores are immediately attacked by endophytes (Tripathi et al., 2008) By twisting and piercing the hyphae of the pathogens and creating lyase, which dissolves the pathogen’s cell wall, endophytic fungi trap the pathogens. This sort of interaction among fungi is frequently seen. Hyper parasitism in bacteria has only occasionally been documented. A predatory bacterium called Bdellovibrio bacteriovorus has the peculiar ability to exploit the cytoplasm of other Gram-negative bacteria as food (McNeely et al., 2017). Over 30 fungal species, including Cladosporium uredinicola against Puccinia violae and Alternaria alternata against Puccinia striiformis f. spp. tritici, reported by Zheng et al. to exhibit hyper parasitism against rust pathogens (Jia et al., 2016). The reduction of plant pathogens through microbial predation is another technique. The majority of endophytes show their predatory traits in nutrient-poor environments. For example, fungal endophytes isolated from Taxillus chinensis produce cell wall-degrading enzymes that promote the dissolution and relaxation of the cell wall between the host and Taxillus chinensis (Wu et al., 2021).

Endophytes are described as microorganisms (bacteria or fungal) that are found living in tissue, plant organs, and seeds of almost all vascular plants (Caruso et al., 2022). The interaction between the endophyte and the host plant during their association has proven to be beneficial rather than harmful for both interacting organisms. These benefits are always based on the interaction between endophytes and the host plants (Kamana et al., 2016). Many novel bioactive compounds like antibiotics, antimycotics, antineoplastics, etc. are endophytic products (Shukla et al., 2014). The agro-industry relies heavily on the use of agrochemicals for controlling phytopathogens. This excessive use of chemicals in the agricultural industry has resulted in the development of resistant phytopathogens. The endophytes play an important in maintaining the health of the host plant (Wu et al., 2021). These endophytes can be exploited as a biocontrol agent in the agro-industry. Biocontrol agents (BCA) are described as living organisms or their products that can fight against plant diseases or pests via direct antagonistic action (Xia et al., 2022). Many endophytes are being exploited as BCA as they are effective in controlling plant diseases and can help attain sustainable agriculture. The endophytic fungi that act as biocontrol agents against different phytopathogens are summarized in Table 4.

Endophytic fungi play a pivotal role in safeguarding plants from the destructive foraging of leaf cutting ants. These mutualistic fungal endophytes, which commonly inhabit plant tissues, establish intricate associations with their host plants. One of their primary mechanisms of protection lies in the production of secondary metabolites, such as alkaloids and mycotoxins, which serve as potent deterrents against herbivores, including leaf-cutting ants (White and Torres, 2010). By secreting these chemical compounds, endophytic fungi effectively shield their host plants, rendering them unpalatable or toxic to the ants. This protective alliance not only benefits the plants by reducing herbivore-induced damage but also highlights the fascinating interplay between plants and their endophytic partners in the intricate web of ecological relationships.