Trichoderma spp.: Approach for Bio-Control Agent

Abstract

The novel technologies in all areas of agriculture have improved agricultural production, but some modern practices cause environmental pollution and human hazards. The recent challenge faced by advanced farming has been to achieve higher yields. Thus, there is an immediate need to find eco-friendly solutions. Among the various types of species being used as biocontrol agents, fungi of the genus Trichodermaare a very large group of microorganisms widely used as biocontrol agents against different kinds of plant pathogens. Trichoderma spp. are asexual, free-living organisms that are abundantly present in all types of agricultural soils. Recent studies have shown that Trichoderma can not only prevent diseases but also promote plant growth, improve nutrient utilization efficiency, enhance plant resistance, and improve the agrochemical pollution environment. Trichoderma spp. behaves as a low-cost, effective, and eco-friendly biocontrol agent for different crop species. This chapter provides information on Trichoderma as a biocontrol agent, its biocontrol activity, and plant disease management programs.

1. Introduction

Plant diseases caused by viruses, bacteria, fungi, or other macroorganisms have a significant impact on the reduction in food production, growth, and development in agriculture, which results in serious economic losses every year. The first option that farmers frequently use to control plant diseases is chemical pesticides. The primary benefit of these pesticides is their immediate use and “solution” to the issue. Chemical pesticides used over an extended period of time can contaminate soil and water, affecting both humans and animals, while occasionally leaving toxic residues that can encourage the growth of particular resistant species. In addition, pesticides have negative impacts on soil microbiomes, soil-dwelling species (such as beneficial insects, like pollinators), and the overall health of terrestrial and aquatic ecosystems [1]. In recent decades, Chemical pesticides, which are the most widely used technique for defending plants against fungal diseases, have placed serious pressure on the agricultural environment [2].

In order to avoid the negative effects of chemical pesticides, researchers are searching for alternate solutions to these issues, such as the use of biocontrol agents (BCAs) for disease control, minimizing the damage caused by plant pathogens, and an integrated approach with other chemicals that is environmentally safe. One of these methods is the use of BCAs, which are based on living microorganisms or their metabolites and products of natural origin that reduce the population of plant pathogens [3]. These biological disease management strategies are effective, long-lasting, eco-friendly, affordable, and safe for human health. At present, integrated pest management strategies and avoidance or regulation of pesticides by using Aspergillus, Gliocladium, Trichoderma, Ampelomyces, Candida, and Coniothyrium are some examples of fungal agents, and other bacterial agents include Pseudomas, Bacillus, and Agrobacterinum [4]. Among these BCAs, Trichoderma spp. is one of the most adaptable and has been used for a long time to control plant pathogenic fungus and lessen the usage of pesticides on economically significant crops. T. harzianum and T. virens prevented the pathogenic fungus from the growth of Ganoderma [5].

Fungi that act as biocontrol agents have become useful tools in the modern agricultural system. They have the capacity to ameliorate abiotic stresses like drought, salinity, extremely high or low temperatures, and heavy metal impacts, as well as the harmful effects of plant pathogens. Trichoderma spp., which are beneficial fungi, have received a lot of attention because of their high reproductive capacity, ability to survive in adverse conditions, prolific production of secondary metabolites, and resistance to plant pathogenic fungi [6]. Additionally, they have been used in biotechnological applications and are crucial to agricultural endeavors because of their capacity to reduce biotic (species and vegetable variety and edaphic microbial interactions) and abiotic stresses (type of soil, water potential, temperature, and pH) and improve plant growth and yield [7]. Colonization with Trichoderma and plant root is the initial stage of successful interaction, signal exchange, and elicitor generation that results in symbiotic relationship between them. Produced enzymes and antibiotics move toward fungal pathogens, successfully breaking down their hyphae to allow entry into the host cell. Both enzymes and antibiotics have antifungal properties and work together to combat fungi [8]. Trichoderma are known to be resistant to Alternaria alternate, Botrytis cinerea, Rhizoctonia solani, Sclerotinia sclerotiorum, Pythium spp., and Fusarium spp. [9, 10], as well as nematode [11]. After successful colonization, they cause induced systemic resistance (ISR) in the plant, which results in the signaling of several hormones and eventually promotes development [8]. Abiotic factors like salt, drought, heavy metal buildup, and severe temperatures have an impact on crop yield all around the world. However, recent studies have shown that Trichoderma induces tolerance against abiotic stresses and improves growth in plants [12] like radish, cucumber, pepper, bottle gourd, periwinkle, bitter gourd, chrysanthemum, lettuce, and tomato [13]. Trichoderma-colonized plants produce substances like auxins (indole-3-acetic acid: IAA), ethylene (ET), gibberellins (GA), plant enzymes, antioxidants, phytoalexins, and phenols that offer long-lasting resistance to abiotic stresses and improve the root system’s ability to branch. The main advantage of priming the plant for certain stress responses is that it provides a faster and stronger response if the stress occurs again.

The purpose of this chapter is to describe the role of Trichoderma species in detail as a biocontrol agent and biofertilizer and provide a brief overview of the commercially available products of Trichoderma species for application.

2. An overview of the genus Trichoderma

Trichoderma is a genus of filamentous, facultative, primarily asexual (the teleomorphic forms are Hypocrea), and anaerobic fungus that are widely distributed around the world and typically colonize decaying wood and other types of organic plant matter. Trichoderma is an essential component in the mycobiome of a wide range of soil ecosystems (such as farmland, prairie, forests, salt marshes, and deserts), including temperate and tropical areas, Antarctica, and tundra [2]. Trichoderma genus is a group of saprotrophic fungi that are widely distributed and frequently inhabit woody plants as endophytes [14]. The systematics and taxonomy of these fungi have changed when Persoon first coined the word “Trichoderma” in 1794 [15]. According to the present genus, it is a member of the eukaryota domain, fungi kingdom, ascomycota division, pezizomycotina subdivision, sordariomycetes class, hypocreales order, and hypocreaceae family. Over 370 Trichoderma species have previously been identified in the genus Hypocrea/Trichoderma, such as T. harzianum, T. viride, T. asperellum, T. hamatum, T. atroviride, T. koningii, T. longibrachiatum, and T. aureoviride [16, 17]. Trichoderma species are effective biocontrol agents in soil ecosystems because of their capacity for rapid growth, ability to use a variety of substrates, and resistance to numerous toxic chemicals, including fungicides (such as azoxystrobin, 3,4-dichloroaniline, and trifloxystrobin), herbicides, and other organic pollutants [18, 19].

Trichoderma species have the potential to act as biological plant protection agents. This was initially described in the early 1930s. According to the findings of researcher Weindling [20], the T. lignorum strain uses necrotrophic mycoparasitism to defend citrus seedlings from the pathogen Rhizoctonia solani. Since then, Trichoderma’s biocontrol properties have been thoroughly studied for the treatment of diseases caused by several types of soil phytopathogens [21]. The mechanisms by which Trichoderma decreases the occurrence of plant diseases include competition for nutrients and space, the synthesis of antifungal metabolites, mycoparasitism, production of lytic enzymes that degrade the cell walls of fungal plant pathogens, as well as the induction of plant resistance [2]. According to Di Marco et al. [22], the T. harzianum, T. hamatum, T. longibrachiatum, T. koningii, T. viride, T. polysporum, and T. asperellum have the most effective biocontrol capabilities. Numerous studies have shown that the majority of Trichoderma spp. can create bioactive compounds and have antagonistic effects on plant pathogenic fungi [23]. These bioactive compounds, such as cell wall-degrading enzymes and secondary metabolites, can effectively increase crop resistance, lower plant illnesses, and stimulate growth of plants [24].

Trichoderma has a wide range of applications in agriculture due to its biocontrol, biostimulation, and biofertilization abilities. Trichoderma is recognized as the genus with the highest potential for biocontrol as it has the most isolated antifungal bioactive chemicals [25]. According to Rush et al. [25], Trichoderma species account for more than 60% of the fungal BCAs. For example, approximately 250 Trichoderma-derived biofungicides are used in India, but compared to biological management, Indian farmers still rely more heavily on synthetic chemical fungicides [26].

2.1 Trichoderma biology

The mycoflora of the genus Trichoderma, which are typically global in distribution and exhibit a wide range of genetic variety, are generally found in areas with access to decomposing plant materials, primarily cellulosic materials [27]. Trichoderma species are considered to be an imperfect fungus that belongs to the order Hypocreales of the Ascomycota. They can be easily separated from natural soil, decaying plant organic matter, and wood. Trichoderma multiplies and grows very quickly in different nutrient sources like Malt Agar (MA), Czapek Dox Agar (CDA), and Potato Dextrose Agar (PDA). It also produces conidia/spores of various shades that are characterized by a green color, especially at the center of a growing spot or in concentric ring-like zones on the agar surface (Figure 1), and some species also produce thick-walled chlamydospores. The most significant characteristic of this genus is its ability to parasitize other pathogenic mycoflora, particularly those that are associated with root rot and wilt diseases [29]. Trichoderma species have been described as endophytic fungi, even though they are typically found as opportunistic plant symbionts in all types of soil, including agricultural soil, orchard soil, and forest soil [30].

Figure 1.

Three different isolated strains of Trichoderma spp. [28].

2.2 Morphological characteristics

Trichoderma species are mostly identified based on their morphology, although this method is not precise enough to distinguish between species’ differences in diversity. According to [31], most Trichoderma cultures develop quickly in the ideal range of 25–30°C but not at 35°C, yet some species grow well at 35°C. This plays a key role in differentiating species with similar morphology. T. harzianum can be distinguished from species like T. atroviride and T. aggressivum that have a similar morphology by growing it at 35°C. After 96 hours, T. harzianum develops well and sporulates at 35°C, whereas T. aggressivum and T. atroviride cannot have colonies with a radius higher than 5 mm after 96 hours. Mycelia development and color features can be more easily identified in a rich media like Potato Dextrose Agar (PDA). Trichoderma uses a number of compounds, including carbon and nitrogen sources, for its sporulation. Sporulates of Trichoderma also produce powder masses with branched conidiophore bearing bright green, or occasionally colorless, grayish, or brownish, conidia [32], which is characteristic of genera such as Myrothecium, Clonostachys, and Aspergillus as well as Penicillium, respectively. Most of the time, their surfaces are smooth, but other species, such T. viride, have rough conidia [31]. The conidiophore is poorly defined, but it is mostly branched and contains unicellular conidia and phialides at the tip of the branched hyphal system that are not visible on one-week-old media. Phialides of Trichoderma spp. are often sterile hyphae creeping septate, forming a flat, firm, and tuft conidiophores erect originating from a short branch, whereas conidia have double-layered walls that have an electron-dense rough outer layer (epispore) and a moderately electron-dense inner layer. Conidia color morphology varies from species to species, but it is normally green or can be gray, white, and yellow. Colonies of Trichoderma spp. can develop slowly or swiftly, depending on the species. Their aerial mycelium is frequently restricted, floccose to arachnoid, and reverse colorless to dull yellow. Various isolates have a distinct aroma that is similar to coconut. Conidiation may be variously vibrant, loosely tufted, or produce compact pustules that initially appear white but eventually turn green (rarely brown). The conidiophores of T. harzianum with phialides and phialospores are depicted in Figure 2. Shah et al. [33] claim that the light green conidia of T. viride are globose to subglobose, whereas those of T. harzianum are globose. T. pseudokoningii showed little, light green conidia.

Figure 2.

Phialides and phialospores of (a) Trichoderma harzianum and (b) Trichoderma viride (taken from the Université de Bretagne Occidentale website).

3. Biocontrol mechanisms of Trichoderma spp. against phytopathogens

Plant diseases are caused by the interaction among various components, including the host, pathogens, and environment, that is, the disease triangle. Bioagents are the organisms that prevent disease by interacting with different disease triangles. The interaction of pathogens and bioagents allows for modification of the soil environment to establish favorable conditions for effective biocontrol techniques against plant disease. Biocontrol agents involve a variety of processes in achieving disease control. However, conclusive evidences for the involvement of a specific factor in biological control are identified by the strong correlation between the factor’s appearance and biological control. The study of various strategies for managing phytopathogens and plant diseases is the most significant and fascinating aspect of Trichoderma. Trichoderma uses competition, parasitism, antibiosis, induction of defense responses in host plants, and/or a combination of these strategies to control a variety of organisms.

3.1 Mycoparasitism of Trichoderma

A complex mechanism known as mycoparasitism or hyperparasitism allows an antagonistic fungus (mycoparasite) to parasitize on another fungus (host) and ultimately result in the death of pathogen cells [2]. The majority of the Trichoderma fungus is classified as a necrotrophic mycoparasite. Trichoderma parasitize a variety of mycoparasites, especially soilborne pathogens. Over 75 Trichoderma species have been identified with high mycoparasite potential and have the ability to lyse and attack plant pathogenic fungi like Alternaria alternata, Botrytis cinerea, Rhizoctonia solani, Sclerotinia sclerotiorum, Pythium spp., and Fusarium spp. [10]. Trichoderma necrotrophs have a mycoparasitic effect on fungi, which involves chemotaxis and prey sensing, attachment to the host, and physical attack through vigorous branching and coiling around the hyphae of the host. Moreover, Trichoderma can also create penetration structures that are similar to pathogen appressoria, or appressoria-like structures [34]. The act of mycoparasitism consists of three fundamental phases. This role can be performed in the rhizosphere of plants, an ecosystem where Trichoderma has successfully colonized and where the biological control of potential pathogens is crucial to prevent plant diseases. First, Trichoderma identifies its host (or potential plant pathogenic fungus), in which the formation of oligochitins has been considered as a sensor molecule [35]. Similar to this, it is understood that during the preceding stage, a number of gene-encoding proteases and oligopeptide transporters are expressed prior to interaction with the fungal host. Second, hydrophobin-like proteins might be useful when Trichoderma comes into contact with the plant pathogenic fungus, which results in the production of papillae or appressoria-like structures. The third step takes place when Trichoderma coils around the pathogen hyphae and begins to degrade it by producing cell wall-degrading enzymes (CWDE), such as cellulases and hemicellulases, chitinases, proteases, xylanase, pectinase, lipase, amylase, arabinase, protease, 1,3-glucanases, and lytic enzymes among other secondary metabolites (Table 1 and Figure 3), which are essential for the mycoparasitism/biocontrol process [41]. These enzymes degrade the pathogen cell walls, which are composed of chitin and glucan polysaccharides. Production and regulation of lytic enzymes, which include the chitinolytic enzymes β-N-acetyl glucosaminidase, endochitinase, and chitobiosidase, are essential for promoting cell wall degradation, mycelial autolysis, chitin assimilation, and fungal parasitism and inhibiting spore germination of other plant pathogenic fungi, which are produced by T. harzianum, T. atroviride P. karst, and T. asperellum [42]. For plant protection, Trichoderma produces several types of volatile metabolites, such as 6-n-pentyl-2H-pyran-2-one (6-PAP) [43]. The enzymes β-1,3- and β-1,6-glucanases determine the hyperparasitic capability of Trichoderma in response to Phytophthora sp. and Pythium species. Endo- and exoproteases are proteolytic enzymes of Trichoderma that are responsible for enzyme secretion for the control of Botrytis cinerea, Rhizoctonia solani, and Fusarium culmorum. Ceratin cellulase enzymes, such as exo-β-1,4-glucanases, endo-β-1,4-glucanases, and β-glucosidases, synthesized by antagonists, also play a key role in hyperparasitism. Chemical attack and degradation of the pathogen’s cell wall by hydrolytic enzymes and antifungal compounds produced by Trichoderma is the final stage of the mycoparasitic interaction, ultimately causing host death (Figure 4) [44].

Secondary	Metabolites effect	Reference
Peptaibols	Inhibit the activity of β-1,3 glucan synthase	[36]
Gliotoxin, Gliovirin, Koninginins, Trichothecenes, and 6-Pentyl-Α-Pyrone (6-PAP)	Antifungal	[37]
Trichodermin, Suzukacillin, and Alamethicin	Assist in destroying the cell walls of other pathogenic fungi	[38]
Harzianic acid, Tricholin, Massoilactone, Viridin, Glisoprenins, and Heptelidic acid	Antibiotic	[37]
Jasmonic acid and Terpenes	Repel insects from consuming plant leaves	[39]

Table 1.

Secondary metabolites produced by Trichoderma spp.

Figure 3.

Example of SMs produced by Trichoderma spp. chemical structure of 1 6-pentyl-α-pyrone, 2 koninginin A, 3 viridin, 4 harzianopyridone, 5 harzianic acid, 6 harzianolide, 7 T39butenolide, 8 dehydro-harzianolide, 9 cerinolactone, 10 gliotoxin, 11 coprogen, 12 aspinolide C, 13 trichodiene, 14 harzianum A [40].

Figure 4.

Mycoparasitism by Trichoderma. (A) Interaction between colonies of Trichoderma and a fungal prey. (C) Main factors and structures used by Trichoderma during antagonistic interaction. (B and D) Host invasion. Transmission electron micrographs of T. Atroviride parasitizing Rhizoctonia solani. (B) T. Atroviride (T) penetrates R. solani (R). (D) T. atroviride (T) grows inside R. solani (R) hypha [45].

Numerous Trichoderma genes that code for proteases and oligopeptide transporters have been identified during interactions between different Trichoderma species and the host. Additionally, it has been proposed that pathogenic host sensing is mediated by class IV G-protein-coupled receptors (GPCRs), which operate as sensors for oligopeptides and other chemicals released from phytopathogen cell walls by the activity of protease enzymes [10]. The conserved signaling cascade of G-proteins, which consists of the Gα, Gβ, and Gγ subunits, is used for further signal transduction. Mutants of T. atroviride that lack the Gα subunit are completely incapable of mycoparasitism, have decreased chitinolytic activity, and are unable to produce the antifungal chemical 6-pentyl-α-pyrone [10]. Furthermore, during mycoparasitism in Trichoderma, mitogen-activated protein kinases (MAPKs), particularly pathogenicity MAPK (TmkA and Tmk1), are implicated in signal transduction pathways [46]. The antagonistic interactions between S. rolfsii, R. solani, and Pythium ultimatum and T. virens are reduced when TmkA is removed.

Trichoderma’s mycoparasitism extends beyond the host’s pathogenic hyphae, and these fungi may use the pathogen’s conidia as a source of further biocontrol. The Trichoderma TkZ3A0 strain’s hyphae were able to parasitize phialides with macroconidia of the F. culmorum phytopathogen (isolated from winter wheat with severe fusariosis symptoms) and exhibited clear chemotaxis and adhesion to their structures. This strain has a high degree of similarity to the T. koningiopsis species. Even in the initial phases of the interaction, macroconidia’s shape changed and their ability to germinate was inhibited (mycostasis) [47].

3.2 Competitive role of Trichoderma

Starvation is the most prevalent cause of mortality for all living organisms. Microorganisms typically die from starvation because there are not enough nutrients in their local surroundings, which include soil and plant surfaces. Trichoderma successfully established themselves in the environment, both pathogens and biocontrol agents are competing hard with one another for resources like food and space. Competition is a process that occurs between microorganisms when there is a scarcity of micro- and macronutrients, and the competition for those nutrients results in the natural management of fungal communities and the development of phytopathogens [48]. Competition for micro- and macronutrients, which are carbon, nitrogen, and iron, is essential for the interactions between beneficial and harmful fungi and is associated with biocontrol systems. Trichoderma is usually considered as a strong competitor against soilborne fungal pathogens. It exhibits higher capacity to mobilize and quickly absorb the nutrients needed for the growth of the pathogen fungi, leading to nutritional deficit and preventing the growth and reproduction of the pathogenic fungi [49]. Rapid germination rate of Trichoderma has produced a wider accessible area and a greater supply of nutrients for growth. The most effective utilization of nutrients depends upon the ability of Trichoderma spp. to obtain energy from the metabolism of carbohydrates such as cellulose, chitin, glucan, and glucose, which are frequently present in the mycelial environment [50]. Trichoderma has a strong capacity for environmental adaptation [51]. It can seize nutrients and space close to the plant rhizosphere by its rapid growth and reproduction, consume oxygen in the air, and inhibit the growth of plant pathogenic fungi [52]. Trichoderma can effectively suppress the growth of plant pathogenic fungi since its growth rate is much faster than that of plant pathogenic fungi. After 24 hours in the soil, Trichoderma can quickly attach to plant roots for reproduction, and the hyphae quickly wrap plant roots to create a protective layer that prevents pathogen invasion and removes surrounding pathogens. According to Risoli et al. [53], T. harzianum grew 2.0–4.2 times more quickly than B. cinerea. Trichoderma mycelium competed with Fusarium graminearum by adhering, twining, interpenetration, and other mechanisms, causing Fusarium graminearum’s mycelium to become distorted and subsequently disappear [54]. Through rapid growth and reproduction, Trichoderma can weaken and exclude the gray mold pathogen in the same habitat by capturing water and nutrients, occupying space, using oxygen, and other things [55]. Trichoderma accomplishes this by biosynthesizing and releasing organic acids including gluconic, citric, and fumaric acids to cause a decrease in soil pH. Additionally, these organic acids help to dissolve micronutrients and mineral cations such phosphates, iron, manganese, and magnesium.

Siderophores, which are formed under iron-deficiency stress and have a low molecular weight (less than 10 kDa) chelator molecule with a high affinity for iron (Fe), are considered to be of the greatest significance in relation to the competitive phenomenon in Trichoderma [56]. The Fe ions serve as cofactors for numerous enzymes and are crucial for the healthy growth and development of both microbes and plants. Generally, the three classes of microbial siderophores—hydroxamate, catecholate, and carboxylate—are separated based on the chemical structure and the position of their coordination sites with iron [57]. The majority of the times, fungi produce hydroxamate-type siderophores, like coprogens, ferrichromes, and fusarinines, which have the structural component N5-acyl-N5-hydroxyornithine [58]. Iron is mostly found in soil in the form of Fe³⁺ under neutral pH conditions and in the presence of oxygen. Fe tends to produce insoluble iron oxides in the aerobic environment, making it unavailable to plants. Siderophores have the ability to interact with insoluble iron (Fe³⁺) and subsequently change it into soluble Fe²⁺, which is readily absorbed by plants and microbes (Figure 4).

In the absence of iron supplies from an associated niche, Trichoderma may inhibit the growth and activity of target soil pathogens. The interaction of the pathogens Fusarium acuminatum, Alternaria alternata, and Alternaria infectoria with T. harzianum was studied in vitro, and the results revealed that the studied fungi died from nutrient starvation. Further, the iron competition was suggested as one of the essential components in T. asperellum’s antagonistic relationship with F. oxysporum f. sp. lycopersici and tomato plants’ defense against wilt disease. Trichoderma has been shown to successfully compete with phytopathogens from the genera Colletotrichum sp., Botrytis sp., Verticillium sp., and Phytophthora sp. for simple and complex carbon substrates [59]. Further, the powerful inhibitory impact of Trichoderma on the pathogens B. cinerea, F. graminearum, and Macrophomina phaseolina was mediated by enzymes that create competition for nutrients and space [60].

Therefore, the coordination of many strategies, including the competition for nutrients, which is considered to be among the most essential, is required for the control management of some pathogens (such as B. cinerea) by using Trichoderma [61].

3.3 Induced systemic resistance of Trichoderma

One of the most significant mechanisms of Trichoderma’s biocontrol actions is induced systemic resistance. It is well-known that Trichoderma spp. are able to induce systemic and local resistance in plants against plant pathogens through alterations in their physiological and biochemical processes, as a result of complex interactions between microbial biomolecules and plant receptors [62]. While preventing the pathogenic fungus from proliferating and growing, it can also encourage crops to produce self-defense mechanisms for obtaining local or systemic disease resistance. This causes the synthesis of regulatory plant proteins that may contribute in the defense mechanism by recognizing the microbial biomolecules, as well as the formation of the defense-enhancing chemical salicylic acid throughout the entire plant as a systematically induced defense mechanism [63]. Trichoderma-induced plant disease resistance is achieved by two methods: first, regulate the elicitors or effectors that initiate the plant disease resistance response, and second, the cell wall-degrading enzymes (such as celluloses, chitinases, and glucanases) produced by Trichoderma release oligosaccharides that can induce plant resistance [64]. Elicitors play a key role in the initiation of Trichoderma-induced systemic resistance (TISR) in plants. Currently, there are more than 10 Trichoderma elicitors known to promote plant resistance, including Sm1, QID74 hydrophobic protein, MRSP1, chitin-degrading enzyme, xylanase, cellulase, endopolygalacturonase, sucrase, and antibacterial peptides. These substances primarily come from five Trichoderma species: T. asperellum, T. viride, T. atroviride, T. harzianum, and T. virens[65, 66, 67, 68].

The elicitors are divided into two categories: (1) race-specific elicitors that only activate gene-to-gene defense in particular host cultivars and (2) general elicitors released from pathogenic and nonpathogenic strains that activate non-race-specific defense in both host and nonhost plants. Numerous classes of elicitors have been identified, such as oligosaccharides (glucans, chitins, and oligogalacturonides), proteins and peptides (elicitins and endoxylanase), glycopeptides and glycoproteins (e.g., glycopeptide fragments of invertase), glycolipids (e.g., lipopolysaccharides), and lipophilic substances (e.g. fatty acids). In plants, the activation of signal transduction pathways by elicitors causes physical, biochemical, and molecular changes such as ion flow across the membrane, the production of reactive oxygen species (ROS), the construction of a physical barrier that inhibits the spread of phytopathogens (callose deposition and reinforcement of the plant cell wall), and the synthesis of various defense compounds (such as phytoalexins, volatile organic compounds, enzymes, and phytohormones) [69].

Numerous plant species, including monocotyledonous and dicotyledonous, have an enhanced immune response when nonpathogenic Trichoderma fungi are present. The recognition of conserved domains, such as the pathogen-associated molecular pattern (PAMP) or the microbe-associated molecular pattern (MAMP), is the main component of the plant defense response. Both MAMP-triggered immunity (MTI)/PAMP-triggered immunity (PTI) and effector-triggered immunity (ETI) are induced by these domains in plants [47].

According to Saravanakumar et al. [70], Trichoderma-coated maize seeds significantly increased in peroxidase (POD) and phenylalanine ammonia lyase (PAL) activity, and the plants were resistant to curvularia leaf spot of maize.

3.4 Antibiosis effect of Trichoderma

The biological control mechanism referred to as antibiosis involves the production and excretion of secondary metabolites, including compounds of a different chemical nature with cytotoxic activity, which can reduce or inhibit the growth of phytopathogens by secreting antagonistic substances [71]. Trichoderma species have the ability to create antimicrobial secondary metabolites, such as trichomycin, gelatinomycin, chlorotrichomycin, and antibacterial peptides, which may inhibit the development of several types of pathogens. This inhibiting process is known as antibiosis. Antibiosis is one of the primary mechanisms by which Trichoderma and other biological pest controllers, such as plant growth-promoting bacteria (PGPB), work [72, 73]. More than 180 secondary metabolites of Trichoderma have been classified into different categories of chemical compounds based on their roles in competition, iron-chelating metabolites, inducers of plant resistance, plant growth-promoting metabolites, and antibiotics [40]. These substances can be divided into peptaibols, water-soluble substances, and volatile or nonantibiotic substances. These substances have a variety of roles in the biological control of various pathogens and are also able to function as antibacterial agents, encourage plant growth, and serve as valuable building blocks for the creation of agricultural antibiotics [74]. Some secondary metabolites have the ability to modify the metabolism and growth of plants. Some Trichoderma strains, including T. viride, T. harzianum, and T. koningii, have been shown to be able to produce and secrete 6-PAP, a volatile metabolite that can inhibit the growth of colonies to varying degrees, and some of them can inhibit the growth of colonies by more than 80% [75, 76, 77]. These volatile metabolites are crucial to the biocontrol of a variety of pathogenic species, including Fusarium oxysporum, Botrytis cinerea, and Rhizoctonia solani [78]. For example, T. virens species produce trichodermamides, whereas T. koningii produce Koninginins, both of which have antibacterial and antifungal activity. Additionally, in T. harzianum and T. virens substances like azaphilones, viridins, nitrogen heterocyclic compounds (such harzianopyridone and harzianic acid), and volatile terpenes have been characterized and are used in the biocontrol of pathogenic fungus [79]. Different Trichoderma spp. have also been studied for their ability to produce hydrolytic enzymes and proteases, including exo- and endochitinases, chitinases, xylanases, glucanases, lipases, and endo- and exopeptidases, among others, with antifungal activity [80]. The most prevalent volatile organic compound (VOC) from T. atroviride is 6-pentyl-2H-pyran-2-one (6-PP), which, together with other VOCs produced by the fungus, promotes plant growth and controls sugar transport in Arabidopsis roots [81].

4. Trichoderma as biocontrol agent against soilborne pathogen

The rapid use of chemical fertilizers considerably contributes to the current condition of environmental deterioration through the use of fossil fuels, the production and release of carbon dioxide, and the contamination of water supplies. Globally, environmental deterioration is currently a major concern. The most effective way to prevent environmental deterioration is by using biological agents [82]. Biocontrol is the use of biological populations to control the proliferation of pests. A few of the biocontrol mechanisms by which BCAs can prevent the growth of soilborne pathogens include the capability to grow faster than soilborne pathogens for nutrients and space, the production of numerous potent plant-degrading enzymes like lytic enzymes and proteolytic enzymes, and the production of more than 200 antibiotics that are extremely toxic to all macro- and microorganisms. It is believed that the ability to produce various antibiotics will enhance biological control by preventing a variety of microbial competitors, some of which are likely plant diseases [28].

Antibiotics such as 2,4-diacetylphloroglucinol, produced by Pseudomonas fluorescens F113 against Pythium species, which causes damping-off disease, and gliotoxin, produced by Trichoderma virens against Rhizoctonia solani, which causes plant root rot, have been reported to be involved in the suppression of plant pathogens. Furthermore, BCA enzymes can also hydrolyze chitin, proteins, cellulose, and hemicellulose. As a result, plant pathogens are quickly inhibited. Several types of BCAs can produce enzymes that are beneficial against particular plant diseases [83].

Trichoderma has a significant application value and potential for biological management of plant diseases. The use of Trichoderma to control plant diseases has been studied all around the world. Trichoderma spp. are effective biocontrol agents that are widely used against soilborne diseases. Some of the important gene types that are essential to the function of biocontrol include protease, chitinase, glucanase, tubulins, cell adhesion proteins, and stress-tolerant genes. These genes play a role in the degradation of cell walls, hyphal growth, stress tolerance, and parasitic activity. For example, chitinase breaks down glycosidic linkages, while xylanase breaks down hemicellulose and so forth. Trichoderma fights against other plant pathogenic fungi and promotes the growth of the plant and its roots. For plant pathogenic diseases, it uses a variety of management strategies, such as antibiosis, mycoparasitism, induced host cell resistance, and competition for nutrients and space. The antagonist’s proliferative potential in the soil is a key factor in the efficient biocontrol of soilborne pathogens. Trichoderma species are recognized as prospective biological control agents due to their capacity to either increase resistance or restrict the growth of a number of phytopathogenic fungus. Trichoderma’s induction of systemic resistance is another important method of pathogen resistance. Through mycoparasitism, competition, or the production of antibiotics, plants can defend themselves directly [84].

As a biocontrol agent, T. viride and T. harzianum are known to have various degrees of inhibitory effects on pathogens including Fusarium oxysporum, Rhizoctonia solani, Pythium aphanidermatum, Fusarium culmorum, Gaeumannomyces graminis var. tritici, Sclerotium rolfsii, Phytophthora cactorum, Botrytis cinerea, Pseudocercospora spp., Colletotrichum spp., and Alternaria species [85, 86, 87, 88, 89, 90]. Trichomonas has been widely used in the biological management of tobacco root rot, tomato, potato, rice, wheat, and other plant diseases (Table 2).

Disease	Crop	Pathogen	Biocontrol strain	References
Root rot disease	Soybean (Glycine max (L.) Merr. cv)	Pythium arrhenomanes f. sp. adzuki	T. viride	[91]
	Corn (Zea mays)	Fusarium oxysporum f. sp. adzuki
	Cocoyam (Xanthosoma sagittifolium)	Pythium myriotylum	T. asperellum	[92]
	Pepper plants (Capsicum annuum)	Rhizoctonia solani	T. harzianum	[93]
	Eggplant (Solanum melongena L.)	Macrophomina phaseolina	T. harzianum, T. polysporum, T. viride	[94]
Damping-off	Pepper (Capsicum annuum)	Phytophthora capsici	T. harzianum	[95]
	Cucumber (Cucumis sativus)	Pythium sp.	T. harzianum	[96]
	Cotton (Gossyphtm hirsutum)	Rhizoctonia solani	T. harzianum	[97]
	Cotton (Gossyphtm hirsutum)	Pythium aphanidermatum, Pythium ultimum, Rhizopus oryzae	T. virens	[98]
Wilt	Tomato (Solanum lycopersicum)	Fusarium oxysporum f. sp. Lycopersici (FOL)	T. asperellum	[99]
	Melon (Cucumis melo)	F. oxysporum	T. harzianum T-78	[100]
Fruit rot	Chili (Capsicum annuum)	Alternaria tenuis	T. harzianum	[101]
	Tomato (Solanum lycopersicum)	Rhizoctonia solani	T. viride, T. virens,; T. harzianum	[102]
Brown spot	Tobacco (Nicotiana tabacum)	Alternaria alternata	T. harzianum	[103]
Brown root rot	Peanut (Arachis hypogaea)	Fusarium solani	T. harzianum	[104]
Anthracnose gray mold	Strawberry (Fragaria ananassa)	Colletotrichum acutatum, Botrytis cinerea	T. hamatum, T. atroviride, T. longibrachiatum	[105]
Head blight	Wheat and other small grain cereals (Triticum aestivum)	Fusarium graminearum, Fusarium culmorum	T. gamsii	[106]
Sheath blight	Rice (Oryza sativa)	Rhizoctonia solani	T. harzianum	[107]
Blossom blight	Alfalfa (Medicago sativa)	Sclerotinia sclerotiorum	T. atroviride	[108]
Web blight	Bean (Phaseolus vulgaris)	Sclerotinia sclerotiorum	T. viride	[102]
Collar rot	Tomato (Solanum lycopersicum)	Sclerotium rolfsii	T. virens, T. harzianum	[102]

Table 2.

Various diseases controlled by Trichoderma spp.

5. Trichoderma as biofertilizers

Severe environmental conditions are one of the factors that have decreased crop growth and productivity. Crop productivity would suffer as a result of abiotic stresses caused by climate changes, such as temperature rise, increased carbon dioxide levels, protracted drought, and hurricanes [109]. Additionally, these circumstances have an impact on the proliferation and distribution of plant pathogens and pests, adding to the biotic stresses imposed on crops [110]. Genetically uniform improved crop are frequently vulnerable to invasion caused by non-native pests and pathogens. Irrigation practices in dry environments affect soil nutrients and increase salinization. Salinization has been associated to decreased soil microbial activity and also has an impact on physical properties of soil, such as producing compaction. Compacted soil with low oxygen content affects root growth, which limits nutrient and water intake and reduces the production of plants. As a result, reports indicate that the average production of important crops is facing losses of up to 50%. In order to maintain agricultural production and productivity under various environmental challenges, Trichoderma spp. application as biofertilizer offers a sustainable and environmentally friendly alternative. In addition, they also play an essential role as a biocontrol agent, protecting plants from a variety of disease attacks. Trichoderma spp. interacts with plants to promote root growth, assist in nutrient uptake, and enhance the plant’s resistance to abiotic stresses, all of which increase the production of agricultural products [47]. According to previous studies, Trichoderma was able to colonize roots and affect the transcriptome, proteome, and epigenome of plants, improving their capacity to tolerate external stresses. In the presence and absence of abiotic stress factors, Trichoderma spp. treatment has positive effects on plants (Table 3).

Strain as; biofertilizer	Crops	Application mode	Beneficial outcome
T. azevedoi	Lettuce	Simple exposure	Increases carotenoids and chlorophyll with reduction in the white mold attack to about 78.83%
T. afroharzianum	Tomato	Seed inoculation or treatment	Helps in the secretion of phytohormones like homeostasis, antioxidant activity, phenylpropanoid biosynthesis, and glutathione metabolism
T. harzianum,	Chinese cabbage	Irrigation	Increased yield by 37%; Increased enzyme activity in the soils and providing more inorganic nitrogen and phosphorus content to the soil
T. asperellum,
T. hamatum,
T. atroviride
T. brevicompactum,	Tomato	Seedling drenching	Improved growth and yield due to the production of indole-3 acetic acid
T. gamsii,
T. harzianum,
T. harzianum Rifai,	Tomato	Seed treatment	Improves phosphorus uptake
T. asperellum,
T. brevicompactum,	Tomato	Seed drenching	Improves phosphorus solubilization
T. gamsii,
T. harzianum
T. erinaceum	Rice	Seed treatment	Improves germination, vigor, and yield
T. harzianum T22	Tomato	Seed treatment	Improves soil fertility, level of minerals and antioxidants, nutrient uptake, and yield
T. harzianum T22	Tomato	Soil amendment as compost	Increase in yield to about 12.9%
T. viride	Sugar cane	Powder as fertilizers	Improves nutrient uptake
T. asperellum T34	Cucumber	Seedling drenching	Enhanced nutrient uptake
T. harzianum	All crops	Compost	Enhances residue decomposition resulting in availability of soil nutrients
T. simmonsii	Bell pepper	Seedling drenching	Improves yield to about 67%
T. reesei	Chickpea	Seed treatment	Enhanced mineral uptake
T. harzianum	Mustard	Soil inoculation	Improved nitrogen absorption and increased yield to about 108 and 203%
T. harzianum	Chili	Soil inoculation	Increased yield
T. harzianum	Barley	Seed inoculation	17% increase in yield
T. viridae	Wheat	Soil and seed inoculation	75.8% increase in yield with improved nutrient absorption
T. viridae	Potato	Soil inoculation	Increased yield with an average of 16.25 tubers/plant
T. viridae	Red beet cabbage	Seed inoculation	29% increase in yield
T. harzianum	Onion seedlings	Seedling inoculation	Enhanced growth and yield
T. asperellum	Maize	Soil granules	Improves yield

Table 3.

Trichoderma spp. as biofertilizers function in increasing plant production and growth [111].

5.1 Plant growth enhancement by Trichoderma spp.

Trichoderma frequently coexists with the root ecosystems of the host plants. Therefore, Trichoderma is generally referred to as a genus of symbiotic, opportunistic, and nonpathogenic microorganisms that colonize the roots and regulate plant pathogens. Trichoderma spp. not only inhibit the growth of pathogens but also increase plant growth, serve as a biofertilizer, and stimulate plant defense mechanisms. According to Hyakumachi and Kubota [112], plant growth-promoting fungi (PGPF) are fungi that can play a role in the growth of plants. The main effects of PGPF are frequently seen in the production, quality, and growth of the crop. Recent studies have shown that Trichoderma spp. can make a great PGPF. It has been shown that some Trichoderma strains may penetrate the epidermis and create a robust and persistent colonization of root surfaces. Trichoderma species have positive impacts on plants, such as promoting growth, enhancing seed germination and viability, improving root structure and condition, and increasing photosynthetic efficiency, blooming, and yield quality [113]. Trichoderma has been shown to enhance the growth of lettuce, pepper plants, and tomatoes. The production of phytohormones and phytoregulators is the most significant stimulating factor at almost all stages of plant growth and development by creating a favorable environment and production of a large amount of secondary metabolites [47]. Trichoderma spp. produce secondary metabolites that regulate plant growth, such as koningin A (Trichoderma koningii) and 6-pentyl-alpha-pyrone (T. harzianum). Other significant activities carried out by Trichoderma spp. include increased solubilization of phosphate minerals (such as Fe, Mg, and Mn), reduction in soil pH, and formation of gluconic and citric acids as well as micronutrients.

The main advantage of Trichoderma spp. for plant growth is the development of roots. This theory is confirmed by recent studies [114] that have shown Trichoderma spp. produces or controls plant hormones like auxin, harzianic acid, and harzionalide, which are responsible for promoting root development. According to Yedidia et al. [115], it was found that plants inoculated with T. harzianum on the 28th day developed significantly greater roots. Additionally, this study found that the content of Cu, P, Fe, Zn, Mn, and Na increased in the inoculated root (Figure 5). At the same time, it was found that the concentration of Mn, Zn, and P had increased by 70%, 25%, and 30%, respectively, in the plant’s shoot. The study supports the theory made by Contreras-Cornejo et al. [116] that a high root surface area caused by Trichoderma spp. inoculation permits the root to explore a larger area of soil. This makes it possible for plants to absorb more macronutrients and micronutrients from the soil, which is advantageous for them when fighting for minerals with other organisms or when there is a lack of minerals. The identification of Trichoderma spp. metabolites could be helpful in the application of novel biofertilizers in agriculture as an alternative to synthetic/chemical fertilizers. To increase crop yield, using Trichoderma spp. as a biofertilizer in combination with fungi as inoculants may be beneficial. Additionally, it reduces pollution caused by the overuse of synthetic/chemical fertilizer in the agricultural sector (Figure 6).

Figure 5.

Transformation of the insoluble form of iron (Fe³⁺) into a soluble and easily assimilating form (Fe²⁺) by siderophores produced by Trichoderma fungi [117].

Figure 6.

Trichoderma spp. increase the nutrients uptake by root improvement [118].

5.2 Plant root colonization by Trichoderma Spp.

Many rhizosphere Trichoderma species may colonize the root surfaces of monocotyledonous and dicotyledonous plants, which may have a significant effect on the plant’s metabolism. Trichoderma species recognize the host plant and adhere and penetrate plant roots to colonize it. Trichoderma spp. colonization of plant roots enhances plant defense by causing the production of 1,3-glucanase, peroxidases, phenylalanine, chitinases, and hydroperoxidase, which activated signaling of the plant’s biosynthetic pathways and led to the accumulation of low-molecular weight phytoalexins. Since it colonized such a diverse range of host plants, Trichoderma most likely developed efficient techniques to get over plant defenses. Trichoderma harzianum and plant interacted physically in a symbiotic way where Trichoderma stimulate the increased activity of metabolites such as peroxidase and chitinase, thus protecting the plant from disease in exchange for it providing a nutritional niche for the fungus. Yedidia et al. [119] used an electron microscope to investigate the physical interaction between T. harzianum T-203 and a cucumber plant and discovered that the fungus pierced the root and spread across the epidermis and outer cortex, which prompted the production of more peroxidase and chitinase. Consequently, the interaction seems to be a symbiotic one in which Trichoderma lived in the nutritional niche offered by the plant and the plant was protected from disease.

Various studies have confirmed Trichoderma’s capacity to colonize the root surface in addition to the rhizosphere soil. Previously, the root hair and elongation zone was the main area where Trichoderma colonized the roots. However, recent research has shown that these fungi have colonized the root cap border cells (RBCs) zone. RBCs play a vital role in the interaction between plants and soil microbes. During root extension, the RBCs are discharged into the soil environment from the cap cells’ outer layer. These cells are thought to be a valuable source of nutrients and biologically active substances for microorganisms, along with the related root exudates. Additionally, RBCs have an impact on the structure and content of the microbial rhizosphere population and serve as attractants for microbes, promoting interaction with plants and root colonization. RBCs are also considered to be an important factor in protecting plants from a variety of biotic and abiotic stresses [120]. The ability of Trichoderma to colonize RBCs, as shown for the wheat roots injected with DEMTkZ3A0 strain conidia, suggests a major role of these fungi in the indirect defense mechanism against phytopathogens through the creation of a mantle-like structure.

6. Commercially available Trichoderma-based bioproducts

Trichoderma spp. are considered as one of the most extensively studied fungal agents as microbial biocontrol agents (MBCAs) in agriculture and are sold as biopesticides, biofertilizers, growth promoters, and naturally occurring resistance-inducing agents. Since these compounds are developed to protect plants from adverse environmental conditions, it is important to establish set standards for their production and maintenance because changes in pH, temperature, and other environmental factors may have an impact on the formulation’s activity. Additionally, it should not be phytotoxic, be economical, dissolve quickly in water, be easy and affordable to obtain carrier materials for, and be compatible with agrochemicals. The distribution of the different Trichoderma spp.-based products for important agricultural diseases is listed in Table 4.

Bioproducts name	Species strain	Company, Country
Topshield, Rootshield	T. harzianum T-22	Bioworks, Geneva, N.Y.
T35	T. harzianum	Makhteshim-Agan Chemicals, Israel
Harzian 20, Harzian 10	T. harzianum	Natural Plant Protection, Noguerres, France
F-stop	T. harzianum	Eastman Kodak Co., United States TGT Inc., New York
Supraavit	T. harzianum	Bonegaard and Reitzel, Denmark
Solsain, Hors-solsain, Plantsain	Trichoderma spp.	Prestabiol, Montpellier, France
ANTI-FUNGUS	Trichoderma spp.	Grondontsmettingen De Ceuster, Belgium
Ty	Trichoderma spp.	Mycontrol, Israel
GlioGard and SoilGard	T. virens (Gliocladium virens)	Grace-Sierra Co., Maryland, USA
Bip T	T. viride	Poland
Promot Plus WP Promot PlusDD	Trichoderma spp., T. koningii, T. harzianum	Tan Quy, Vietnam
TRiB1	Trichoderma spp.	National Institute of Plant, Vietnam
TRICÔ-DHCT	Trichoderma spp.	Can Tho University, Vietnam
Vi – DK	Trichoderma spp.	Pesticide Corp., Vietnam
NLU-Tri	Trichoderma virens	Ho Chi Minh University of Agriculture and Forestry, Vietnam
Biobus 1.00WP	Trichoderma viride	Nam Bac, Vietnam
Bio – Humaxin Sen Vàng 6SC, Fulhumaxin 5.15SC	Trichoderma spp.	An Hung Tuong, Vietnam
BioSpark Trichoderma	T. parceramosum, T. pseudokoningii, and Ultraviolet irradiated strain of T. harzianum	BioSpark Corporation, Philippines
Biocure F	T. viride	T. Stanes and Company Limited, European Union, available in India
Trichoderma viride powder	T. viride	Organic Dews, Bio Organic, India
Bio-Shield, Bioveer	T. viride	Ambika Biotech, India
Mycofungicyd, Trichodermin	T. viride 16, T. lignorum	Bizar-agro LTD, Ukraine
ICB Nutrisolo SC e WP	T. viride, T. harzianum, T. koningii and Trichoderma spp.	ICB BIOAGRITEC Ltd., Brazil
Trichosav-34	T. harzianum A-34	Institute for Research in Plant Protection (INISAV), Cuba
Trichosav-55	T. harzianum A-55	Institute for Research in Plant Protection (INISAV), Cuba
Antagon TV	T. viride	Green Tech Agroproducts, Tamil Nadu, India
Trichostar	T. harzianum	Green Tech Agroproducts, Tamil Nadu, India
Gliostar	T. virens	GBPUAT, Pantnagar, India
Bioderma	T. viride, T. harzianum	Biotech International Ltd., India
Bio Fit	T. viride	Ajay Biotech (India) Ltd., India
Ecofit	T. viride	Hoechst Schering Afgro Evo Ltd., India
Trichoguard	T. viride	Anu Biotech Int. Ltd. Faridabad, India
Biocon	T. viride	Tocklai Experimental Station Tea Research Association, Jorhat (Assam), India

Table 4.

Description of commercially available bioactive products of Trichoderma spp. worldwide [111].

7. Conclusion

Currently, the primary strategy for controlling plant diseases is chemical control, which is accomplished by spraying pesticides and fungicides. Although chemical control has a positive effect and is beneficial in boosting agricultural production, it also has negative effects on people and the environment and increases pathogens’ resistance. Scientists and their studies have demonstrated that Trichoderma is nonpathogenic to plants and exhibits various biocontrol features, which makes it one of the most efficient organisms against several types of plant pathogens, and reduces the use of harmful pesticides in the agricultural sector. Trichoderma spp., which are primarily found in soil and the rhizosphere region, are antagonistic to the majority of plant pathogens. A biocontrol program is only established when the biocontrol agent is able to successfully control the relationship between the host plant and pathogen. Trichoderma has a well-established ability to control this interaction. It has also been shown that the fungi improve plant defense mechanisms. Furthermore, Trichoderma provides a variety of control strategies for various plant pathogens, which gives it an edge over other phytopathogen control strategies. Mechanisms that are usually involved are mycoparasitism, antibiotics, competition for nutrients, and stimulation of systemic resistance in plants. Another fascinating feature is that Trichoderma species produce a number of secondary metabolites that significantly affect plant growth or prevent pathogen growth along with promoting localized and systemic resistance as well as stress tolerance in plants. Currently, Trichoderma species are being used in the sustainable disease management system for managing plant diseases. These methods of managing plant diseases may be safer for the agroecosystem, overall environment, the health of the soil, and human health. They would also be the proper move in sustaining agricultural output to satisfy the demands of growing populations. As a result, farmers have the choice of using it to enhance agricultural yields and produce higher quality. Trichoderma spp. can be used for waste/organic material decomposition, contaminated area purification, and decreasing diseases and enhancing plant growth.