Trichoderma: A Game Changer in the Modern Era of Plant Disease Management

Abstract

Trichoderma has been found to have effectiveness against a vast range of plant diseases and can be a good alternative biocontrol strategy in the modern era of plant disease management. It has been found effective against soil borne pathogens and nematodes. Trichoderma has been isolated from variable soils and has multifaceted application other than disease management. Trichoderma enhances plant growth and development by boosting the plant’s capacity to absorb nutrients, increasing systemic resistance to pest and/or pathogen attacks in the future, increasing tolerance to abiotic stresses (such as salinity, drought, and low temperatures). For instance, the stress on organic management in the modern cropping system, Trichoderma is a promising soil enhancer and can have handful applicability for diseases particularly those of soil borne ones. Its competitive mechanism and antagonistic approaches to compete with other pathogens makes it a good fit for future crop management strategies.

1. Introduction

Plant disease management is one of the important strategies for a way forward towards sustainable agriculture and food security. Presently, crop output is being increased by applying herbicides, fungicides, insecticides, fertilizers, nematicides, and soil amendments [1]. Although using pesticides has some advantages, worries about pesticide residue in food are constantly growing. There is growing evidence that pesticide residue in food can have negative effects on both human health and the environment [2]. Most of these pesticides cause chronic toxicities such as genotoxicity, causing disruption in hormonal functions particularly endocrine, kidney damage, reproductive toxicity, alterations in metabolism, liver and bladder toxicity, gastrointestinal problems, etc. in addition to being potential human carcinogens, mutagens, and acetylcholinesterase inhibitors [3, 4, 5]. Taking account to a Joint UNEP and WHO research, 3 million people worldwide are poisoned by pesticides each year, killing about 200,000 people worldwide [6]. Even while high-income countries utilize large quantities of pesticides, the vast majority (95%) of pesticide poisoning incidents take place in developing countries as a result of ignorance, abuse, inappropriate management, etc. [7].

As a result, in recent years, consumers have grown increasingly concerned about the detrimental effects these synthetic fungicides have on both human well-being and the environment. Therefore, research into alternate methods of crop protection has been mandated and has gathered considerable interest from scientists all around the world. The use of biological controls by means of advantageous microorganisms has become increasingly important among these alternatives. Over millions of years of evolution, microorganisms have evolved the ability to detoxify heavy metal ions and pesticides and have become resistant to intoxicants. As a result, they have contributed to the long-term environmental benefits of restoring degraded environments to their natural state [8]. Several biological control agents (BCAs), including Streptomyces, Trichoderma, Clonostachys, Bacillus, Pantoea, Pseudomonas, Burkholderia, and specific yeasts, have been tested. However, the comparative effectiveness of the biocontrol agents against the chemical pesticides particularly in disease control is yet to be achieved. Therefore, the need arises for an alternative to these hazardous chemicals which could be safer to environment as well as to the humans. Although, biological control has shown a promise and effectiveness owing to their safe use and the various interactive mechanisms with plants and pathogens. However, the considerable effectiveness of the biocontrol agents with respect to the chemical pesticides particularly in disease control is yet to be achieved. There has been a debate on these biocontrol agents in terms of economics and technological perspectives worldwide leading to certain political scenarios in many countries where few protective plans to reduce the pesticide use even by up to 50% have been implemented for sustainable agriculture [9, 10, 11].

Among the various biocontrol agents like bacteria, fungus and others, the use of Trichoderma based formulations has grown in popularity due to its special mechanisms to compete with the various pathogens and also its growth enhancing characters. It has been thought that this particular kind of fungi is particularly advantageous for many phases of life owing to its distant sensing properties, and is quick to target and stop the spread of plant diseases, boosts plant development, and has several other features. The most significant genus of filamentous fungi in biological control techniques for managing phytopathogens is Trichoderma [12, 13]. Rhizospheric soils have largely been used to separate soil microorganisms, particularly the species of Trichoderma [14] which are considered as the probable contenders for the biocontrol aspect of plant disease management because of their critical role in inhibiting the activity of up to 80% of some economically significant plant pathogens [15] and nematodes (Table 1) [13]. They also play a significant role in the production of a variety of metabolites [28].

Species/strain	Biocontrol option	Reference
T. longibrachiatum T6	Pepper damping off	[16]
	Cereal cyst nematodes	[17]
T. harzianum	Pepper and Potato Phytophthora blight	[18, 19, 20, 21]
	Phytopthora cactorum	[22]
T. asperellum	Apple canker	[14]
	Pythium and Fusarium; corn sheath blight	[23, 24]
T. harzianum	Rhizoctonia solani; Pythium aphanidermatum	[25]
T. viride	Fusarium oxysporum f. sp. Adzuki; Pythium arrhenomanes	[26]
T. virens	Pythium ultimum; Pythophthora	[27]

Table 1.

Trichoderma species as efficient biocontrol agents against various plant diseases.

Studies across the world have revealed that Trichoderma species possess an outstanding ability to solubilize minerals in the soil, create hormones that promote plant growth, encourage water usage efficiency, stimulate defenses in the host, and antagonize pathogens, all of which together lead to a notable improvement in crop health [29]. It may create a variety of secondary chemicals and easily activates other fungi, creating important enzymes including chitinase, proteases, and β-1,3-glucanase that cause plant defense, systemic resistance, and fierce fight against plant diseases. In order to minimize the toxicity released by plant pathogens, it participates in a crucial detoxification process. In order to promote sustainable agriculture, it is crucial to emphasize Trichoderma’s significance in the treatment of plant diseases [30]. The majority of Trichoderma species are soil-dwelling, globally distributed molds. They take in a range of organic materials and produce a number of enzymes and secondary metabolites that could be advantageous. A few species are aggressive and dangerous to any industry that cultivates specialized edibles, including mushrooms. Positively, a number of species have been exploited as biological control agents because of their traits that are hazardous to plant infections. Furthermore, several Trichoderma species are used to promote plant development [31]. Trichoderma can adapt to diverse environmental circumstances and recognize the host, and the existence of a host is important for the successful colonization of the rhizosphere, plants, and soil [32]. Trichoderma is able to respond to the host by progressively releasing pathogenesis-related proteins (PR-Proteins), such as proteases, chitinases, and glucanases [31].

2. Morphological identification of Trichoderma

A unique sweet or coconut aroma is given off by several species. When examined under a microscope, conidiophores are either borne by the sparse aerial hyphae or are heavily branching, loosely or compactly tufted, frequently formed in various concentric rings, and so on. It is presumable that the typical Trichoderma conidiophore will have a pyramidal shape with paired branches. Phialides can have an enlarged center and can be cylindric or almost subglobose in shape. Chlamydospores could be produced by any species. Even while chlamydospores can be found inside hyphal cells, they are often unicellular subglobose and end short hyphae. Most strains of Trichoderma do not have a sexual stage and solely generate asexual spores. They can be identified by the growth of fleshy stromata that are light or dark brown, yellow, or orange in color. The ascomycete genus Hypocrea contains species that are the teleomorph stage of the Trichoderma fungus [33]. The development of fleshy stromata in hues of light or dark brown, yellow, or orange is what distinguishes them. Ascospores have two cells and tend to be green in color [34]. There are numerous species of Trichoderma which have been found to have application in the crop disease and pest management. Examples of those species have been documented in a wide range of environments, like that of soil from woodlands, vegetable gardens, rotting wood, cultured mushroom compost, grains of cereal, and coastal environments. Include Trichoderma harzianum, T. asperellum, T. virens, T. aggressivum, T. longibrachiatum, T. reesei, T. citrinoviride, T. ghanense, T. hamatum, T. pseudokoningii, T. polysporum, T. tomentosum, T. atroviride and T. gamsii etc. [35, 36, 37].

3. Mechanism of Trichoderma

As biocontrol agents against plant fungal infections, Trichoderma has been reported to have numerous strains. In order to combat the disease or act as a growth stimulant may be attributed to its enormously efficient mechanisms like antibiosis, parasitism, promoting host-plant resistance, and competition. Trichoderma virens and Trichoderma harzianum are active rhizosphere colonizers that produce heat-stable metabolites like ethyl acetate [38] as well as antibiotics like gliotoxin, viridin, and some enzymes that break down cell walls [39, 40]. These chemicals could contribute to the prevention of disease or the stimulation of plant growth [41]. When used as biocontrol agents, the species is particularly efficient against soilborne root diseases like root rot (Rhizoctonia solani), cereal take-all (Gaeumannomyces graminis), gray mold (Botrytis spp.), wilts (Sclerotinia sclerotiorum, Verticillium dahliae) and damping off (Pythium spp.). Conidiophores and chlamydospores are being produced on a large scale, and specialized delivery methods for these spores are being actively developed. Because of their mycoparasitic features along with their capacity to enhance plant health as well as defense, fungi from the genus Trichoderma can be typical biocontrol alternatives in agriculture. Consequently, it is a better plant symbiont at protecting the host from phytopathogens. Trichoderma works by secreting secondary metabolites and effector molecules that facilitate the advantageous interaction of Trichoderma with plants and confer tolerance to biotic and abiotic stimuli. Here, we go over the most recent developments in our knowledge of how this opportunist plant symbiont operates as a means of biocontrol and a plant growth booster [30]. The Trichoderma with its modes of action encompasses a range of procedures notably the synthesis of antibiotics, mycoparasites, competition, and systemic resistance. It also has the capability to encourage plant defense. These are the several modalities of action mentioned below [42].

3.1 Mycoparasitism

Mycoparasitism refers to the direct attack of one fungus on another, also known as direct antagonism [43]. This idea was first proposed in the work of Weindling [44], who revealed how Trichoderma virens hyphae could possibly parasitize Rhizoctonia solani hyphae to suppress citrus seedling disease. Other studies have also demonstrated Trichoderma species’ mycoparasitism of Pythium ultimum and Sclerotium rolfsii [45]. According to Dix and Webster, mycoparasitism is a three-step, complex process that entails chemotrophic development and identification, coiling and interacting hyphae, and the production of certain lytic enzymes [43]. In order to produce infectious structures and secrete enzymes, Trichoderma engages in a mycoparasitic interaction with other filamentous fungi that appears to be automated by host signals [46]. The chemotropic progress of the mycoparasite’s hyphae towards a target is the first noticeable contact between Trichoderma and its host in vitro and is probably a reaction to diffusible signals like oligochitins [47, 48]. Mycoparasite hyphae often coil around or attach to the host by forming hook-like structures when they reach it.

It was speculated decades back that mycelium permeates the host by gradually degrading its cell wall [49]. Following that, it became apparent that Trichoderma does, in fact, emit a sophisticated set of CWDEs (cell wall degrading enzymes) that break down cell walls and membranes [50, 51, 52]. The production of CWDEs, including chitinases, proteases, and β-1,3-glucanases, is what enables Trichoderma spp. to function as mycoparasitic fungi [31, 49, 53]. These results eventually contributed to the identification of the very first genes that contribute to mycoparasitism, whose expression is transcriptionally responsive to the existence of specific microbial host [50, 51]. The vast majority of mycoparasitism-related genes that have been revealed so far encode CWDEs, which are believed to operate in collaboration with secondary metabolites and are speculated to play a role in Trichoderma’s antagonistic ability [54]. In reality, the genus Trichoderma most likely descended from a distant relative that consumed either fungus or arthropods, according to a recent phylogenomic investigation of Hypocrean fungi (which included nine Trichoderma spp.). According to the research, entomoparasitic fungi and the monophyletic family Hypocreaceae had a last common ancestor [55].

It is interesting to take into account that signals leading to the synthesis of cell wall-degrading enzymes, the signal these enzymes produce, and the regulation of their expression in Trichoderma mycoparasitism exhibit significant similarities to insect pathogenicity in species of Metarhizium, possibly the most thoroughly noticed group of entomopathogenic fungi [50, 51, 56, 57]. Additionally, similar to Trichoderma, some Metarhizium species have recently been found to be rhizosphere competent and to form advantageous relationships with plants [58, 59, 60].

Furthermore, at least three Trichoderma genomes (T. atroviride, T. reesei, and T. virens) have had their genomes sequenced, each of which contains genes linked to the synthesis of secondary metabolites. The majority of NRPS are found in T. virens (28), subsequently followed by 19 alongside 16 in its two most close rivals T. atroviride and Fusarium graminearum, respectively. It is important to point out that the genetic material of T. reesei genome lacks half the number of secondary metabolite clusters of genes observed in T. virens and T. atroviride, which have been determined to be productive mycoparasites [61]. Genome-wide gene expression investigations were made possible by the development of genomic technology and consequently, 66 genes that were overexpressed during the start of mycoparasitism were identified by an EST-based investigation of gene expression during the interaction of T. atroviride with Botrytis cinerea/R. solani [62]. The majority of genes were associated with the transmission of signals, amino acid metabolic processes, post-translational processing, and fat catabolism.

Comparative transcriptome analysis revealed that before coming into contact with the host hyphae, the Trichoderma species viz., T. virens, T. reesei and T. atroviride responded in dramatically different ways. The gliotoxin-producing genes were expressed by T. virens, whereas the secondary metabolite-producing genes, β-glucanases, different proteases, and minute secreted cysteine-rich proteins were produced by T. atroviride. It’s important to note that T. reesei boosted the expression of genes related to cellulose breakdown [63]. As a result, various Trichoderma species and even isolates employ a variety of methods, which might constitute an important aspect in the synthesis of biofungicides.

3.2 Plant interactions

Trichoderma accelerates plant development and growth on an array of crops when used in the field as well as axenic environments [64, 65]. In this regard, to help with nutrition and water uptake, the fungal mycelium secretes substances that encourage root growth [66]. Trichoderma species can also alter plant defense mechanisms by engaging with their various signaling pathways. It is widely acknowledged that manipulating the SA route makes it feasible for there to be a link between the fungus T. harzianum and the plant Arabidopsis thaliana because SA limits the volume of root tissue that T. harzianum can colonize [67]. However, fungi like T. atroviride have the ability to delicately alter both the SA and JA pathways to enhance resistance [68]. Secreted proteins are crucial in enabling connection between Trichoderma and plants, as has been demonstrated through interactions between plant-pathogens and mycorrhizae. The Trichoderma genomes contain hundreds of possible effector proteins, according to recent research [18, 32, 69, 70]. In a few cases, evidence has been established that confirms the involvement of effectors in the establishment of the Trichoderma-root interaction. In addition to effectors, T. virens secretes chemicals associated to secondary metabolism that aid the fungus in scavenging reactive oxygen species and hydrolyzing the plant cell wall, according to protein profiles derived from maize root tissue in interaction with the organism [70]. Using the projected secretome, researchers demonstrated differential expression of thioredoxin (tatrx2) from T. atroviride-derived genes with predicted activities pertaining to different families throughout the Trichoderma-plant interaction. Thioredoxins serve as essential effectors due to their ability to counteract the apoptotic events brought on by stress-activated signaling through mitogen-activated protein kinases (MAPK) cascades [71, 72]. The positive interaction between T. atroviride and Arabidopsis which leads in plant resistance may be mediated by tatrx2, which could play a part. It was also found that the CFEM (Common in Fungal Extracellular Membrane) proteins, which have a significant role in appressorium development in Magnaporthe oryzae, are expressed variably by the T. atroviride tacfem1 gene [73]. The genes encoding members of the LysM repeats family, tvlysm1 and talysm1 from T. virens and T. atroviride, respectively, showed differential expression as a result of the Trichoderma-Arabidopsis interaction. When LysM proteins bind chitin oligomers generated by the fungus, they prevent the plant from recognizing them and promote colonization [74], so that they might strengthen Trichoderma’s propensity to associate with plants. Finally, the T. virens gene tvhydii1—encoding a class II hydrophobin—plays an instrumental part in the colonization of plants [18].

The synthesis of phytohormones is an intriguing idea that might help to explain how We surveyed the T. atroviride genome for this review to recognize genes that may be related to the synthesis and signaling of substances like SA or JA, as well as other phytohormones. The SA, JA, Auxins, CKs, GAs, Et, and ABA pathway protein sequences in A. thaliana that are involved in biosynthesis and signaling were used as the seed in a BLASTP analysis against the T. atroviride genome database (v.2.0). The sequences with the highest similarity scores for each protein were selected using BLASTP against the genomes of Marchantia polymorpha and Physcomitrella patens in addition to this method.

3.3 Priming of plant defense responses

The molecular basis of the induced systemic resistance (ISR) brought on by T. hamatum in A. thaliana has been explored through microarrays. It was determined utilizing mutants impaired in multiple defense-related pathways that treatment with T. hamatum expedited the stimulation of the defensive response towards B. cinerea, which was perceived as an ISR-boost [75]. Similar to this, Morán-Diez et al. [76] examined A. thaliana gene expression alterations in response to T. harzianum exposure and discovered changes in genes relevant to abiotic as well as biotic stress responses, involving a number of signal transduction pathways regulated by plant hormones. Their findings are consistent with the theory that the presence of T. harzianum in A. thaliana resulted in the downregulation of genes associated to SA and JA and the upregulation of numerous genes involved in abiotic stress responses. They therefore proposed the theory that plant defenses mediated by JA and SA are less early in the encounter, allowing for root colonization. As a result, the host plant did not perceive Trichoderma as a threat, yet continued exposure to Trichoderma might activate both local and systemic defense responses. The function of some effector proteins has been thoroughly studied since they may be responsible for the activation of the plant defense response. While its T. atroviride ortholog (Epl1) produces systemic resistance to disease in cotton along with maize towards Colletotrichum spp., the T. virens hydrophobin-like protein Sm1 confers resistance in tomato plants from the diseasesA. solani and B. cinerea. The level of expression of genes related to the JA defense mechanism or those that code for peroxidase and α-dioxygenase activity are correlated with this protection [77, 78]. Sm2 and Epl2, their paralogs, appear to be crucial for the Trichoderma-maize interaction in addition to Sm1 and Epl1, improving resistance and limiting the degree of leaf lesions driven by Cochliobolus heterostrophus [79]. Likewise, the T. longibrachiatum hydrophobin HYTLO1 triggers amplification of genes engaged in the SA and JA driven defense pathways, boosting protection against B. cinerea in tomato and pepper plant cultivars [80], TasSwo from T. asperellum also does the same for cucumber plants [81]. Other Trichoderma compounds that are not protein-based have the power to activate plant defense mechanisms. The tomato marker genes for both JA/Et and SA mediated defense pathways were expressed more frequently as a result of the secondary metabolite harzianolide from T. harzianum [82]. Thus, it is suggested that effector proteins and secondary metabolites are important for the development of the Trichoderma-plant symbiosis as well as for initiating plant defensive responses by promoting the production of SA and/or JA. However, it is conceivable that Trichoderma could also directly convey these signaling molecules in addition to the release of protein effectors.

3.3.1 Salicylic acid

One of the most crucial defenses against biotrophic fungus is facilitated by salicylic acid (SA), a phenolic molecule that is accountable for a variety of physiological actions in plants. It also serves as a signal molecule during plant defense. The isochorismate pathway (IC) and the phenylpropanoid pathway are the two processes through which SA is produced in plants. Both pathways start with chorismate as a precursor, which is the end product of the shikimate pathway [83]. Whereas PAL is an important enzyme that catalyzes the preliminary step in the phenylpropanoid route, converting phenylalanine into cinnamic acid, IC synthase (ICS or SID2) is an important enzyme which catalyzes the initial part of the IC pathway, transforming chorismate into isochorismate [83].

3.4 Plant growth stimulation

Numerous studies showing that Trichoderma spp. increase nutrient solubility and root capacity when applied to soil, seeds, or plant surfaces support the effectiveness of these organisms as biofertilizers. Trichoderma has a positive impact on plants, which can be attributed to the fungus’ ability to modify root architecture and/or its synthesis of siderophores and organic acids, which boost nutrient availability [84]. Recent research has revealed that pathogenic fungus-plant interactions are significantly influenced by phytohormones produced by fungi. However, there is little evidence on their involvement in advantageous interactions between fungus and plant.

3.4.1 Auxins

Auxins are a series of indole-derived substances that, among other biological functions, control root initiation, cell division, and elongation in plants. Auxin-producing fungi, that mainly affect the germination of spores and elongation of cells, have been illustrated to encompass certain phytopathogenic fungi, notably species of Fusarium, Rhizoctonia, and Colletotrichum gloeosporioides [85, 86]. Auxins appear to have a physiological function in Phycomyces blakesleeanus as evidenced by the fact that exogenous delivery of auxins stimulated the growth of the fungus and the sporangiophore developing zone [87]. Tryptophan (Trp)-independent and Trp-dependent pathways make up the majority of the very complicated biosynthesis of auxins in plants [88]. In order to synthesize IAA, four Trp-dependent routes have been proposed: indole-3 pyruvic acid (IPA), indole-3 acetaldoxime (IAOx) pathway, indole-3 acetamide (IAM), and the YUCCA (YUC) pathways [88, 89]. Through the application of the IAM pathway in species of Fusarium and C. gloeosporoides and the IPA pathway in Ustilago and Rhizoctonia species, the fungi have been shown to produce auxin [86, 90]. Auxin production is perceived to guide plant root development since T. virens is capable of producing at least two auxin-related substances, commonly indole-3-acetic acid (IAA) and indole-3-acetaldehyde (IAAld), by means of a Trp-dependent mechanism [91]. The speculated connection among the auxins generated by the fungus and the regulation of the architecture of roots, however, is still up for question [92, 93], because volatile organic compounds produced by various Trichoderma strains also stimulate changes in root architecture [93]. In fact, there are evidences which suggest that the volatile 6-pentyl-2H-pyran-2-one (6-PP) generated by T. atroviride alters the architecture of the roots of Arabidopsis, resulting in an increase in plant biomass [66].

3.4.2 Cytokinins

Plant hormones known as cytokinins play functions related to nutrition balance, stress tolerance, root and shoot division, and cell differentiation [94]. CKs play a central role in the development of defensive mechanisms and plant-microbe interactions. Exogenous administration of CKs to A. thaliana renders the infectious agent Hyaloperonospora arabidopsidis resilient to them. Because plants do not activate their defenses in response to cytokinin treatment in the absence of a pathogen challenge [95], cytokinins may also serve as priming agents. Multiple investigations on the priming of advantageous fungi reveal that different Trichoderma species can activate plant defenses [96, 97]. However, CKs encourage hyphal branching in ectomycorrhizal mycelia [98]; and aid M. oryzae in tolerating oxidative stress [10, 99]. Isopentenyl transferase (IPT) or tRNA-IPT, which uses the substrate dimethylallyl-diphosphate (DAMPP) in the first phase of cytokinin biosynthesis in plants, produces N6-(2-isopentenyl) adenosine-5′-triphosphate and -diphosphate ribonucleotides (iPRTP and iPRDP, respectively). Both products are afterwards transformed into trans-zeatin ribonucleotides (tZRTP/tZRDP) by cytochrome P450 monooxygenases (CYP735A1 and CYP735A2). The LONELY GUY (LOG) family of enzymes is used to create the active CK forms in the end [100]. The probable IPT and LOG genes have been described in M. oryzae [99] and Claviceps purpurea [101], and these fungi may produce CKs. At least three receptors, including histidine kinase 2 (AHK2), histidine kinase 3 (AHK3), and histidine kinase 4 (AHK4/CRE1/WOL), are implicated in plant CKs signaling in A. thaliana.

3.4.3 Gibberellins

Gibberellins are the plant hormones that promote growth by breaking down the DELLA proteins that inhibit it [102]. Since its discovery in Gibberella fujikuroi, a fungus that causes stupid seedling disease and is where the term “gibberellins” originated, in the 1920s, they have been well-known among fungi. Although many fungi, including Neurospora crassa, produce these hormones, the biological function of GA in fungi is still not entirely understood [86]. Ent-kaurene is formed by the following processes: (i) biosynthesis from geranyl geranyl diphosphate (GGDP); (ii) cytochrome P450 monoxygenase converts ent-kaurene to GA12; and (iii) cytoplasmic synthesis of GA19 and GA20. There are three stages in the creation of GAs in plants. Salazar-Cerezo and colleagues [103] recently analyzed the parallels and convergence in the GAs biosynthesis pathway between plants, fungi, and bacteria [103]. In contrast to plants, fungi have clusters of genes that are involved in the biosynthesis of GAs, including three cytochrome P450 monoxygenases (P450-1, P450-2, and P450-3), one desaturase (2-oxoglutarate-dependent dioxygenase, or DES), one ent-copalyl diphosphate synthase, and one GGDP synthase gene (ggs2). GAs are less researched in plant-microbe interactions, but their potential significance could make for a fascinating subject for study. The JA signaling repressor JAZ1 is known to be released by the breaking down of DELLA proteins via GAs signaling, which increases SA signaling and increases biotroph resistance while lowering the expression of genes that respond to JA [104, 105]. Salazar-Cerezo and colleagues claim that while GAs as secondary metabolites are not necessary for fungal development or growth, they may have been helpful for survival in their ecological niche because they are primarily produced when the environment is hostile to the fungi [103]. By raising the concentration of GAs in the plant, Trichoderma could promote plant growth and increase plant biomass.

3.4.4 Ethylene

A gaseous phytohormone called ethylene is involved in the germination of seeds, fruit ripening, and senescence of plants [106]. By influencing simultaneously the SA and JA pathways, ethylene also contributes to plant immunity [105]. Sadenosyl-L-Methionine (S-AdoMet) and 1-aminocyclopropane-1-carboxylic acid (ACC) serve as the precursors for the manufacture of ethylene in plants. The key enzymes that catalyze this route are S-AdoMet synthetase (SAM), ACC synthase (ACS), and ACC oxidase (ACO) [107]. Numerous fungi, particularly Penicillium digitatum, B. cinerea, as well as F. oxysporum, have been reported to generate ethylene [85, 86]. Et has an impact on hyphal development and spore germination in fungus. It is known that B. cinerea utilizes methionine as a precursor of α-keto-methylthiobutyric acid (KIMBA), which in turn produces ethylene [108]. Six genes associated with Ethylene biosynthesis have been identified in the T. atroviride genome that were previously unknown to exist in B. cinerea, T. asperellum, P. patens, or Pseudomonas sp. [109]. The genes encoding an S-adenosylmethionine synthetase, a 1-aminocyclopropane-1-carboxylate synthase, and a member of the Iron/ascorbate family of oxidoreductases are thought to be involved in Et biosynthesis [107]. This fungus may have a functional ACC synthase pathway that produces Ethylene. Ethylene affects the physiology of the plant directly and also has a connection to nutritional stress. The amount of ethylene increases when the plant is subjected to nutritional limitation, which improves auxin sensitivity and ethylene sensing and increases stress tolerance [110]. Enhanced biotic and abiotic stress resistance is one advantage of the plant’s interaction with Trichoderma [65]. Given that T. atroviride genome contains genes for Et biosynthesis, it is possible that the fungus’s production of this chemical contributes to the plant’s ability to withstand a variety of stresses.

3.4.5 Abscisic acid

A plant hormones called abscisic acid is a compound that modulates the dormancy of seeds and development, stomatal aperture, and enables plants withstand abiotic conditions including drought and excessive salinity [111]. ABA can have either an advantageous or detrimental effect on defense in plant-pathogen interactions [112]. For instance, ABA has a favorable effect when interacting with Alternaria brassicicola and a negative effect when engaging with Pseudomonas syringae [112]. For instance, ABA has a favorable effect when interacting with Alternaria brassicicola and a negative effect when engaging with P. syringae. Although the molecular specifics are still poorly known, according to Flors and collaborators [113]; ABA can also impact SA-JA cross talk, controlling basal defenses. Numerous fungi have been shown to produce ABA [86]. As it has been confirmed to be active in the M. oryzae-rice [114] interaction and that accumulates during the early phases of the Ustilago maydis-maize interaction, the primary function speculated for this fungus phytohormone is as a virulence factor stimulating plant infection [115] the primary function speculated for this fungus phytohormone is as a virulence factor stimulating plant infection. B. cinerea has a biosynthetic route from isopentenyl diphosphate that has been described [116], It involves the genes bcaba1, bcaba2, bcaba3, and bcaba4 [117], which are homologs to the Arabidopsis ataba1, ataba2, ataba3, and ataba4 genes that catalyze the final steps of ABA production [118]. T. virens and T. atroviride are capable of modulating ABA in plants, as shown by Contreras-Cornejo and colleagues’ [91] discovery that strains of both species may affect stomatal aperture of A. thaliana and leaf transpiration by activating an ABA receptor. Trichoderma might be able to assist plants in reducing the negative impacts of environmental stress through yet another pathway like this. Additionally, in order to modulate stomatal aperture in Arabidopsis and increase the plant’s resistance to water stress, ABA precursor from Trichoderma may be helpful.

4. Conclusion

Although increasing crop productivity, the indiscriminate use of chemical pesticides has created environment related problems. The microbial application having role in green technologies may offer alternative to chemical pesticides. Trichoderma evolved as a versatile microorganism to address environmental issues. The researchers have realized the usefulness of Trichoderma species but general and marginal farmers have remained unaware about its effectiveness. An awareness among farmers along with the identification of indigenous isolates of Trichoderma species is the need of an hour.

Conflict of interest

The authors declare no conflict of interest.