Research Progress on the Application of Trichoderma in Plant Abiotic Stress

1. Introduction

About 9 % of the global land is suitable for crop cultivation, with more than half of annual crops damaged by abiotic stresses. Such losses are likely to increase further with the intensification of global climate change [1]. Abiotic stresses include salt stress, extreme temperature, drought, waterlogging, heavy metal pollution, and nutrition stress, among which soil salinization, extreme temperature, and drought are the main adverse environmental conditions affecting crop growth [2]. Different responses caused by stress will seriously affect various life activities in plants, such as enzyme activity regulation, osmotic stress signal transduction, maintenance of cell membrane integrity. Ultimately, these disturbances impair plant growth and development, leading to reductions in crop yield and quality [3]. Salt stress not only disturbs the turgor pressure and chlorophyll content of plants, but also inhibits root growth, thereby reducing plant biomass [4,5]. Drought stress leads to the increase of plant osmotic factors and inhibit photosynthesis [6]. It is of great significance to enhance the tolerance of plants to abiotic stress. Microbial inoculants represent one effective solution to address soil ecological issues such as alleviating abiotic stresses, remediating soil and water pollution, and preventing plant diseases [7]. The application of microbial inoculants in plant cultivation enhances crop resilience to risks and disasters while promoting synergistic growth. Additionally, this approach helps repair the soil environmental damage caused by abiotic stress, so as to improve the agricultural quality and yield in a sustainable manner [8].

Trichoderma belongs to the Deuteromycetes subphylum and represents a common fungal genus in nature. Fungi exhibit broad adaptability to growth environments, engaging in interactions with higher plants, cyanobacteria, algae, insects, helminth, and other fungi, including intergrowth, symbiosis, antagonism, and parasitism [9]. Trichoderma indirectly promotes plant growth and enhances plant vitality via multiple pathways: by enhancing plant stress resistance, promoting the active absorption of nutrients by plants, participating in the bioremediation of contaminated rhizosphere soil, and providing plants with various secondary metabolites, enzymes, and defense-related proteins [10]. Previous studies found that Trichoderma was used as a direct biological control agent (BCA) in crop protection, and its indirect effects stimulated plant immunity, promoted plant growth, and improved the defense capacity against abiotic stresses [11]. When parasitizing or coming into contact with pathogenic bacteria, Trichoderma produces antibiotics and hydrolytic enzymes such as chitinase, β-1, 3-glucanase, lipase, and protease, which limit their growth and activity by degrading the cell walls of phytopathogenic fungi [12]. Trichoderma provides energy by metabolizing carbohydrates such as cellulose and chitin, and reduces soil pH by synthesizing and releasing organic acids such as gluconic acid [13]. Its secondary metabolites such as xylanase, peptide, terpene and pyranone promote plant growth and nutrient absorption, induce systemic resistance and control pathogens in the rhizosphere environment [14]. Therefore, as a beneficial microorganism to plants, Trichoderma contributes to the development of ecologically sustainable agriculture and provides theoretical support for the use of Trichoderma to alleviate the performance of crops under abiotic stress.

2. Trichoderma Species

Trichoderma is widely distributed in soil and plant rhizosphere, and produces various bioactive substances such as cell wall degrading enzymes, secondary metabolites, volatile organic compounds, and reactive oxygen species (ROS) in soil and plant rhizosphere, which plays an important role in crop protection [9]. Common biocontrol Trichoderma species include T.harzianum, T.viride, T.pseudokoningii, T.asperellum, T.longibrachiatum, T.polysporum, and T.brevicompactum [15]. Trichoderma forms symbionts with plant roots, stimulating root development and plant growth, activating seed germination, changing plant metabolic functions, increasing nutrient absorption and utilization, increasing crop yield, improving abiotic stress tolerance, and antagonizing pathogens [16]. Trichoderma rapidly colonizes and modifies the root structure through the interaction with plant roots, thus significantly affecting the metabolic function of plants. This interaction leads to changes in hormone levels, soluble sugars, phenolic compounds, and amino acid content within the plant, and affects photosynthetic rate, transpiration, and water content [17]. In addition, Trichoderma species exhibit high resistance to phytoalexins, phenols, aglycones, terpenoids, and flavonoids released by plants under abiotic stress. This enables Trichoderma to effectively exert its biocontrol and growth-promoting effects in the rhizosphere environment of plants [18].

3. Mechanisms of Trichoderma

There are three application mechanisms of Trichoderma in agricultural production: the first is to colonize in the soil, on plant roots, and in the rhizosphere, occupying physical space and preventing the propagation of pathogens; the second is to control plant pathogens through different mechanisms such as parasitism, antibiotic production, and induction of systemic resistance; the third is to stimulate root development and plant growth through growth-promoting effects [9]. The application of Trichoderma in the plant rhizosphere promotes the morphological characteristics of plants, such as root bud length, biomass, height, leaf number, tillers, branches, fruit, and so on [19]. When Trichoderma was inoculated, the seedling biomass, root structure index, soil nutrients, and soil enzyme activities of Pinus sylvestris var. mongolica annual seedlings were significantly increased [20]. Inoculation of Trichoderma on sugarcane improved the germination rate, tiller number, and yield of sugarcane [21]. When Trichoderma was inoculated on apples, the numbers of soil bacteria and fungi increased, and the relative contents of alkenes, ethers, and citrullines in root exudates increased [22]. More mechanisms and improvement effects of Trichoderma on alleviating plant abiotic stress are shown in Table 1 and Figure 1. Therefore, Trichoderma is widely used in agriculture as a biocontrol agents, biological agents, and soil remediation agents.

4. Application of Trichoderma in Crop Resistance to Abiotic Stress

4.1. Salt Stress

Soil salinization is a global issue, affecting approximately 9×108 hm2 of soil worldwide and resulting in annual agricultural losses of $27.3 billion.Salt stress causes ion toxicity when crops are subjected to it. High concentrations of sodium ions hinder the normal absorption of water and nutrients by plants, disrupting the ROS balance and ultimately leading to plant cell damage [43]. Therefore, salt stress affects seed germination and crop growth, resulting in reduced crop yield and quality [38].By inducing physiological protection mechanisms against oxidative damage within plants, Trichoderma further enhanced the vitality of tomato seedlings and improved the growth performance of tomatoes under osmotic stress, temperature stress, and salt stress conditions [39]. The application of Trichoderma on cucumber increased the root fresh weight by 92.55% and 84.86%, and the root dry weight by 75.71% and 53.31%, respectively, which promoted the growth of cucumber roots under high salt stress and reduced the adverse effects of salt stress on cucumber [40]. Under salt stress, a higher ethylene concentration inhibits root growth and affects the overall development of plants. The ethylene level in plants is regulated by the key enzyme 1-aminocyclopropane-1-carboxylic acid (ACC)-deaminase [41]. Trichoderma enhances salt tolerance of plants by producing IAA and increasing ACC-deaminase activity. IAA stimulates the synthesis of exogenous auxin, thereby promoting ethylene synthesis. However, the activity of ACC synthase (ACCD) in Trichoderma is increased, which decreases the availability of 1-aminocyclopropane-1-carboxylic acid (ACC) required for ethylene biosynthesis. Inoculation with Trichoderma can reduce the ethylene concentration in plants and alleviate the inhibitory effect of salt stress on root growth [42]. Liu et al. found that Trichoderma inoculated in a high salt environment effectively colonized the roots of cowpeas, enhanced photosynthesis by increasing the chlorophyll content, increased the soluble protein content, the activities of POD and CAT in seedlings, and decreased the MDA content (by 1.33% and 10.17%, respectively), reducing the ROS accumulation and salt stress damage. In addition, Trichoderma enhanced the salt tolerance of cowpeas by reducing Na+ accumulation, increasing K+ uptake in cowpeas, restoring the K+/Na+ balance, and alleviating ion toxicity [44].

Apart from ethylene, other plant hormones play a crucial role in regulating the growth and development of plants under salt stress. For example, salicylic acid (SA) activates the immune system of plants, enabling it to effectively resist various stressors, including salt stress. After inoculation with Trichoderma, the biomass, chlorophyll, and carotenoid contents of wheat seedlings increased significantly. Meanwhile, the activities of SOD, POD, and CAT increased by an average of 47.68%, 23.68%, and 38.65%, respectively. This result was closely related to the increase in endogenous SA content (the SA concentration increased by an average of 55.87% by Trichoderma treatment). SA enhanced the salt tolerance of wheat seedlings by activating phenylalanine ammonia-lyase (PAL) and the antioxidant enzyme system (such as SOD, POD, CAT), thus protecting cells and tissues from oxidative damage [45]. Trichoderma also interacted with Arabidopsis thaliana, mainly reflected in the growth and development of the plants. The number of lateral roots of the plants increased, and more auxin accumulated at the root tips. After inoculation with Trichoderma, the levels of ABA, L-proline, and ascorbic acid in Arabidopsis thaliana seedlings increased. This enhanced the ability of root exudates to remove Na+, thereby increasing the IAA level and the antioxidant and osmoprotective states of plants under salt stress. In addition, the auxin signal produced by Trichoderma improved the root branching of Arabidopsis thaliana, thus having a significant impact on plant biomass [46]. Under salt stress conditions, SOS3 perceived the Ca signal and activated SOS2, which then phosphorylated SOS1. This signaling pathway played a central role in plant response to salt stress and was one of the important molecular mechanisms for plants to adapt to a high salt environment [47]. Zhang et al. found that the transcription level of SOS1 was significantly higher than that of other groups after Trichoderma treatment [48]. In the high salinity environment, this gene plays a crucial role in regulating Na+ transport and effectively reduces the damage effect caused by Na+. In addition, the SOS1 gene also cooperates with the Na+/H+ exchanger in plants. When plants are under high salinity conditions, the two cooperate to regulate the cytoplasmic Na+ concentration and maintain intracellular ion balance, thereby enhancing the salt tolerance of plants.

4.2. Drought Stress

Drought is one of the key factors restricting global crop productivity, with an impact ranging from 9% to 10%. Under drought stress, a variety of physiological and biochemical functions of plants are disturbed, which leads to limited plant growth and decreased yield [49]. Trichoderma helps plants enhance their tolerance to drought stress by promoting antioxidant enzyme activity, the accumulation of secondary metabolites and the up-regulation of plant hormone levels. Consequently, it facilitates the growth of crops in drought-stressed soils, effectively improving the drought-resistance capabilities and yield levels of crops [50]. Inoculating Trichoderma under drought stress improves the root biomass, total biomass, nutrient content, and antioxidant enzyme activity of host plants [51]. The length of hypocotyls and roots increased after inoculation of Trichoderma in Brassica napus L., and the yield increased by 16.7% [52]. T.asperellum root dipping treatment improved the tobacco phenotype, increased the relative water content of leaves, enhanced the content of photosynthetic pigments, alleviated the accumulation of membrane lipid peroxidation products, increased the content of osmotic adjustment substances, and increased protective enzyme activity, thereby enhancing the defense ability of tobacco itself [53]. Inoculation of Trichoderma in Pinus massoniana seedlings enhanced the absorption of N, P, and K, effectively reducing the damage to cell membranes and needle tissue structure under drought stress. Meanwhile, it increased the activity of antioxidant enzymes and the content of osmoregulatory substances. In addition, Trichoderma increased the nutrient content of rhizosphere soil and the activities of invertase, phosphatase and urease. It also regulated plant physiological response and soil microbial community, simultaneously increased the rhizosphere soil contact area, secreted extracellular enzymes to activate soil nutrients, and effectively promoted the growth of Pinus massoniana seedlings under drought stress [54].

When plants are affected by drought stress, it leads to a decrease in leaf water potential, a reduction in turgor pressure, stomatal closure, and inhibition of cell growth and expansion [55]. The AQGP gene of T.harzianum represents a key role in improving plant tolerance to environmental stress by biotechnology. As reported by Vieira et al., the overexpression of AQGP of T.harzianum in tobacco improved water use efficiency and drought tolerance of tobacco. Under drought stress, AQGP transgenic lines showed higher fresh stem and root weight, suggesting that plants regulated their water content by altering their ability to retain water in stems and roots [56]. Bashyal et al. found that 1436 genes (57%) were upregulated and 1070 genes (42%) were downregulated in rice plants under drought stress treated with Trichoderma. Trichoderma delayed the adverse effects of drought stress on rice by upregulating the expression of photosynthetic genes, phenylpropanoid pathway genes, glutathione pathway related genes, and stress-enhanced osmotin [57]. Under drought stress, the contents of endogenous auxin, gibberellin, and cytokinin in plants usually decreased, while the contents of abscisic acid and ethylene tended to increase [58]. Trichoderma promoted plant growth and development by enhancing the ability of plants to absorb nutrients and water, and regulating plant hormones such as ethylene, ABA, CK, IAA, GA, and zeatin [59].

4.3. Extreme Temperature Stress

Global climate change causes the frequent occurrence of extreme temperatures, which seriously threatens crop production. When the temperature deviates from the optimal physiological range of plants, both heat stress caused by high temperatures and cold stress caused by low temperatures can cause irreversible damage to plant growth, development, and yield [60]. High temperature stress has become one of the key factors affecting plant growth and development [61]. It leads to the excessive production of ROS, lipid peroxidation, photoinhibition, protein denaturation and degradation, as well as RNA damage. These factors jointly trigger an imbalance in cellular homeostasis [62]. Cao et al. [63] discovered that inoculating Lanzhou lily with Trichoderma promoted the activities of the antioxidant enzyme system (SOD, CAT, and POD), increased the levels of heat-resistant related plant hormones (ABA, SA, and JA), raised the relative water content, elevated the chlorophyll content, and enhanced the net photosynthetic rate, thus reducing cell damage. Trichoderma effectively improved the heat tolerance of lily by promoting ABA synthesis and signal transduction. In addition, Trichoderma highly induced the DEGs of the HSF-HSP pathway. After long-term heat stress, the heat protection genes LzDREB2B, LzHsfA2a, LzMBF1c, LzHsp90 and LzHsp70 involved in the HSF-HSP signaling pathway were up-regulated. In tomato, Trichoderma enhanced heat resistance by regulating ROS metabolism, manifested as an increase in the activities of SOD, POD, PPO, and PAL, as well as an increase in proline and protein contents. Meanwhile, it maintained a relatively high PAL level to strengthen the plant’s defense response to high temperatures [64].

Chilling injury induces plant cold stress, which in turn affects plant growth. It causes a slowdown in plant growth, dehydration, leaf withering, and a reduction in the accumulation of dry matter in vegetative organs such as roots, stems, and leaves. Ultimately, this leads to a decline in plant yield or quality, and in severe cases, can even result in plant death [65]. The colonization of Trichoderma in tomato roots increased the photosynthesis and growth rate of plants, reduced the rate of membrane lipid peroxidation and electrolyte leakage, and increased the water content and proline accumulation in leaves. Meanwhile, the expression of transcription factors NAC1 and dehydrin TAS14 increased sharply, while P5CS expression declined with prolonged cold stress, thus alleviating the adverse effects of cold stress on tomato plants [34]. In addition, some volatile organic compounds released by Trichoderma species induced the expression of cold-related gene AtERD14. This indicated the ability of Trichoderma to mitigate the effects of cold stress on plants [66]. Cold stress can damage the enzyme activity, protein synthesis and cell membrane function of plants [67]. Afrouz et al. found that inoculation of maize seedlings with Trichoderma under cold conditions increased leaf length, root length, root volume and root dry weight, and significantly increased the contents of chlorophyll, carotenoids, and soluble proteins, as well as the activity of catalase. This indicated that exogenous application of Trichoderma enhanced the ability of maize to resist cold stress by optimizing the photosynthetic system, enhancing osmotic adjustment and antioxidant capacity [29].

4.4. Heavy Metal Stress

Industrial activities, sludge wastewater irrigation, and excessive use of chemical fertilizers have led to the accumulation and excess of heavy metals in soil, which not only causes damage to the soil ecosystem, but also threatens crops [68]. Heavy metal pollution destroys crop morphology, physiology and metabolic functions, including reducing ion homeostasis, interfering with photosynthesis, and reducing chlorophyll and carotenoid content [69]. Trichoderma alleviates the toxic effects of heavy metals on plants through multiple mechanisms, including valence state modification, intracellular/extracellular metal localization, metal adsorption and accumulation, and enhanced plant growth and nutrient uptake [70]. When Trichoderma alleviated heavy metal copper stress, the survival rate of onions increased under different copper concentrations. After inoculation with Trichoderma, the biomass in each treatment was mostly higher than that of non-inoculated ones. Moreover, physiological indicators such as the correlation coefficient of copper content in tissues, chlorophyll, MDA, and Pro also changed accordingly. The overall growth condition of onions was improved to a certain extent, reducing the negative impacts of copper stress [71]. Trichoderma contains various bioactive substances, such as polysaccharides and proteins. These substances bind to surrounding heavy metal ions through electrostatic adsorption, complexation, chelation, ion exchange, and covalent adsorption [9]. Polysaccharides play a crucial role in enhancing the tolerance of Trichoderma to lead (Pb2+). By increasing the proportion of uronic acid in the polysaccharide and the proportion of the main chain, the contact area between the polysaccharide and Pb2+ and the proportion of carboxyl groups can be expanded, thereby providing more binding sites for Pb2+, which helps reduce the amount of free Pb2+ and consequently mitigates heavy metal toxicity [72]. High concentrations of cadmium cause serious damage to plants, accompanied by symptoms such as yellowing, chlorosis, stem necrosis, stunted growth, and wilting poisoning. In sunflower plants inoculated with Trichoderma, the fresh weight, dry weight, plant height, and leaf area increased. Through quantitative analysis of glucose, fructose and starch, the results showed that the metabolic process of related carbohydrates was promoted, antioxidant capacity was significantly enhanced, and absorption of nutrients such as K, Ca, Mg, Fe, nitrate and P was also increased [73]. Trichoderma application modified soil structure by increasing porosity and surface area, thereby enhancing the immobilization capacity of heavy metals, and optimizing the function and synthesis of secondary metabolites such as total proteins and phenols [74]. By inoculating Trichoderma, the adverse effects of cadmium and lead toxicity on Vigna radiata (L.) were reduced, and its growth and development under metal stress were promoted [75]. The inoculation of Trichoderma in mine tailings soil contaminated by various heavy metals promoted the dissolution of heavy metals in mine tailings and removed a large amount of lead and other heavy metals from liquid media containing single and multiple metals [76]. Inoculation with Trichoderma enhanced the zinc absorption capacity of plants, effectively reduced the translocation of heavy metals to the aboveground parts, and reduced the level of oxidative stress, thereby promoting the healthy growth of hairy vetch and eliminating the toxic hazards of heavy metals to hairy vetch [77].

4.5. Waterlogging Stress

Under waterlogging stress, the stomata of plant leaves close, chlorophyll degrades, and the photosynthetic rate consequently declines. Meanwhile, waterlogging stress inhibits root respiration and induces the excessive accumulation of ROS such as H₂O₂ and O^2-. These changes not only adversely affect the vegetative and reproductive growth of plants but further trigger cell death, ultimately leading to plant senescence [78]. T.harzianum enhanced the waterlogging tolerance of tomato seedlings by maintaining antioxidant status, regulating glucose metabolism and regulating the expression of key genes [79]. Niu et al. [80] also demonstrated that waterlogging priming induced stress memory by regulating stomatal conductance, maintaining ROS homeostasis, adjusting osmoregulatory substances, and regulating the expression of key genes mediated by H₂O₂. This alleviated the damage to tomato photosynthesis when waterlogging recurred. Trichoderma secreted plant hormones and secondary metabolites, enhanced the antioxidant system of plants, and affected physiological functions through polyamine production and regulation of gene expression to reduce the adverse effects of waterlogging stress [81,82,83,84,85,86,87,88,89,90].

5. Outlook

Trichoderma alleviates abiotic stress in plants through multiple mechanisms. Physiologically, it mitigates the adverse effects of abiotic stress by regulating stomatal aperture, increasing the relative water content of plants, and enhancing physiological parameters such as leaf number, root surface area, and root diameter. Biochemically, it activates the plant oxidative defense mechanisms, enhancing the ability to effectively remove ROS, reducing oxidative damage, and thus improving the plant resistance to abiotic stress. It also increases the contents of substances such as proline and sucrose, helping plants to lower the osmotic potential and maintain cellular water balance. Additionally, it produces or regulates the release of various plant hormones in plants, such as IAA and GA, and influences the composition of the rhizosphere microbial community, prompting the flora to produce cytokinins and antioxidants [91,92,93]. At the molecular level, it regulates the expression of plant-related genes and transmits signals through signal molecules such as transcription factors, providing support for plants to resist abiotic stress [94,95,96].

Although Trichoderma has a positive effect on coping with various abiotic stresses such as salt stress, drought stress, and extreme temperature stress, there are still some problems in its application. On the one hand, it is difficult to screen and isolate Trichoderma from extreme environments. On the other hand, the mechanism of the interaction between Trichoderma and plants is not yet fully understood. Moreover, the actual effect of field inoculation is unstable and restricted by various factors. For example, in experimental culture dishes, the colony growth of Trichoderma strains was inhibited by several fungi such as Aspergillus niger, Colletotrichum gloeosporioides, Alternaria alternata, and Fusarium. Also, the soil competitiveness of Trichoderma was tested in a corn field trial in Yakima, Washington State, USA. The results showed that the competitiveness of Trichoderma in the soil was limited, and it could not be detected a few weeks after planting [82].

In the future, it is necessary to further study the molecular mechanisms of interaction between Trichoderma and plants. Technologies such as multi-omics joint analysis should be used to explore the colonization, assembly, and growth patterns of Trichoderma in complex environments, and comprehensively analyze the mechanisms of Trichoderma in response to abiotic stress [97,98,99]. This is of great significance for understanding the molecular mechanisms of Trichoderma-plant interaction in promoting plant growth and alleviating plant stress. It is also necessary to optimize the preparation and application techniques of Trichoderma agents, explore Trichoderma strains with multiple functions, and reduce the limitations of single strain inoculation. When verifying the functions of strains, more experiments in natural environments should be carried out to improve the stability of the application effect of microbial agents in different environments. In addition, the synergistic effects of Trichoderma with other biological agents need to be explored. Trichoderma has the potential to be used in combination with low-concentration chemical pesticides, which improve the control effect on plant diseases without affecting the concept of sustainable and green development of agriculture. Trichoderma can also be combined with other biological agents to form biological agents beneficial to plant growth and disease resistance, which is widely applied in organic agricultural production. In the future, it is necessary to further explore the synergistic effect of Trichoderma and other biotic or abiotic factors to improve the mitigation effect on crop abiotic stress.

As an important soil microorganism, Trichoderma has great potential in alleviating abiotic stress in plants. Through in-depth research and technological innovation, Trichoderma is expected to become an important tool for sustainable agricultural development. It can improve soil nutrient availability, remediate agrochemical contaminated soil, promote crop growth, increase yield and quality, while reducing the use of chemical pesticides and minimizing the impact on the environment. With the deepening of the research on Trichoderma, it is believed that Trichoderma will play an increasingly important role in future agricultural production.