Trichoderma is a fungus, while plant growth-promoting microorganisms (PGPM) in soil include fungi (PGPF) and rhizobacteria (PGPR). Generally, PGPR can be either endosymbionts or free-living in the rhizosphere and help in enhancing plant growth and development through myriad mechanisms. The benefits of PGPMs are limited to not only improving growth and development but also imparting abiotic and biotic stress tolerance/resistance capabilities to the plants that they are associated with.
Recent advances in understanding plant-PGPM interactions have provided us with deep insights into the precise molecular mechanisms involved in these beneficial interactions under adverse environmental conditions. These microbes are known to impart biotic stress tolerance to plants by acting as ‘biocontrol agents’. The mechanisms involved in such ‘biocontrol’ processes include the secretion of toxic chemicals such as antibiotics and HCN, which inhibit the growth of pathogenic microorganisms in the vicinity.
Secretion of siderophores by PGPMs is known to limit Fe availability to pathogenic bacteria, thus starving them of an essential nutrient. Moreover, PGPM secretions such as exopolysaccharides, phytohormones (auxins, cytokinins, gibberellic acid, abscisic acid, etc.), the enzyme ACC deaminase (which decreases ethylene accumulation), etc. have been known to help plants in abiotic stress alleviation. These chemicals ultimately modulate the (a) expression of major, stress-responsive genes and their corresponding proteins, (b) important metabolic pathways, and (c) regulation of phytohormone signaling in plants.
Plant growth-promoting fungi (PGPF) are other PGPMs used as biopriming agents. The inoculation of fungi via seed priming promotes colonization, association, and proliferation of PGPF on seeds. The most common and described PGPF is Trichoderma harzianum because of its wide biocontrol against phytopathogens and biostimulant activity on plants even under stress conditions. Plant growth-promoting microbes (PGPMs) are a diverse group of beneficial soil microorganisms that colonize plant roots or the rhizosphere (soil adhering to the roots), contributing positively to plant growth and environmental stress amelioration. These microbes thus constitute a widely diverse soil microbiome that has co-evolved with plants for mutual benefit.
There are broadly two types of PGPMs: (a) plant growth-promoting bacteria/rhizobacteria (PGPB/R) and plant growth-promoting fungi (PGPF). Some soil microbes endocolonize the roots by penetrating the root cells (intracellular or i-PGPMs), whereas others are free-living in the rhizosphere (extracellular or e-PGPMs). i-PGPMs are obligate symbionts, and examples include root nodulation, nitrogen-fixing PGPR (Allorhizobium, Azorhizobium, Rhizobium spp.), and arbuscular vesicular mycorrhizae (AVM). e-PGPMs are free-living and associate with plants through facultative symbiosis. Examples of e-PGPR include Azotobacter, Azospirillum, Bacillus, Burkholderia, Pseudomonas, etc. and those of e-PGPF include several species of the genus Trichoderma. An important attribute of these soil microbes is their ability to help plants mitigate biotic and abiotic stresses. However, before understanding how these PGPMs help in stress tolerance/resistance in plants, it is essential to understand the different types of environmental stresses that plants encounter and how they deal with them.
Proper usage of useful microbes in seeds or soil is necessary for their better effectiveness and function. Various methods like application in soil, inoculation with seeds, foliar spraying, and seed coating are also used. However, increasing concern on microbial viability at seed levels reveals that doing research on potential methods that guarantee survival and colonization of seeds by microorganisms is of great significance. Trichoderma species stimulate and increase plant growth by special mechanisms including biological control of soil diseases by production and activity of enzymes, production of antibiotics, and penetration into the body of pathogenic fungi. Moreover, Trichoderma, as the most common saprophytic fungi in the rhizosphere, acts as a parasitic fungus for other fungi and can help plants overcome abiotic stresses such as drought, salinity, cold, and heat. Accessibility of these fungi to insoluble nutrients in soil leaves some indirect effects on the plants and has a direct effect on them due to their ability to absorb more nutrients. Trichoderma which is able to fight pathogens biologically increases nutrient uptake, enhances plant growth and strength, and makes it resistant against pathogens. Generally, Trichoderma is useful for agriculture specifically acts as biological control agents against a wide range of plant pathogens to improve plant’s growth capacity. Due to the increasing significance of using biological fertilizers to achieve sustainable agriculture, it is attempted in this study to evaluate the potential of Trichoderma in improving plant growth and productivity and its role in tolerating biotic and abiotic stresses.
Minimized losses caused by weak seedlings, poor growth condition, plant pathogens, and chemical costs of disease control can increase production rates. Increasing plant growth can be achieved by feeding artificial hormones or by using microbial potential, especially when they are classified as plant growth-promoting fungi(PGPF). Some plant growth-promoting and antagonistic fungi called Trichoderma are commonly used, since they are able to biologically control a wide range of plant pathogens. Also, they are useful for plant growth under in vivo and in vitro conditions. However, their efficiency largely depends on the physical, chemical, and biological conditions of the soil. Trichoderma species can colonize many plant roots, decompose plant debris, and are significantly involved in biodegradation.
Special mechanisms of Trichoderma make them beneficial fungi which can improve the plant growth, and, on the contrary, inhibit growth in plant pathogenic fungi. Several popular mechanisms useful for plant growth are biological control of soil diseases, production of antibiotics, penetration into the body of pathogenic fungi, detoxification and increased transfer of sugar and amino acids in plant roots, induction of resistance to environmental stresses, increased nutrient uptake by improving the solubility of the element, secretion of growth hormones and hormone-like, and finally, formation of enzyme cellulase which can directly stimulate ethylene synthesis in the plant as a response to the presence of pathogens. Some Trichoderma, like T. harzianum, can build indole acetic acid (IAA) hormone which is also involved in plant growth.
Any increase of plant roots caused by Trichoderma fungi can be related to secretion of indole acetic acid. In addition, the in vitro stimulating effect of root growth by these fungi is reported. Trichoderma fungi are able to accelerate plant growth by raising the level of indole acetic acid. Trichoderma fungi enhance the growth and germination indices in radish. Likewise, seed treatment with Trichoderma may improve seed condition and plant quality in long term.
Trichoderma has also several benefits for agriculture. It is commonly used as a biological control agent against a wide range of plant pathogens and can improve plant growth capacity. Frequently, its stimulating effect on plant growth is observed by modifying soil conditions. Based on reports, increased seedling growth response is caused by the fact that Trichoderma strains mostly depend on the ability of survival and establishment in rhizosphere. Besides, it can be inferred from the findings that Trichoderma’s effects on seedling growth and vigor mainly contribute to the type/species of treated Trichoderma.
Applying Trichoderma strains to soil or using it directly to seedling roots is an economical and effective way to obtain strong tomato seedlings that can be later cultivated on the main fields. Besides its ability to provide an antimicrobial effect, Trichoderma can stimulate the biological activity of the resident microbial antagonist population and, consequently, promote plant growth. The development of tomato root system along with building several organic acids (e.g., gluconic, citric, or fumaric acids) in rhizosphere by Trichoderma (which reduces soil pH) can increase solubility of insoluble compounds, availability of micronutrients, and uptake of plant nutrients. An increase in absorption of plant nutrients and its transfer from the roots to the vegetative parts accompanied by the resulted plant stimulants can also enhance the agronomic traits of tomato seedlings.
The mechanisms (both direct and indirect) used by Trichoderma strains can further be used to affect seed germination and seedling vigor. In an experiment conducted to evaluate the effects of using three strains of Trichoderma including T. harzianum, T. viride, and T. asperellum, the significant effect of these strains on germination, number of true leaves, branch length, root length, and seedling vigor, fresh and dry weights of tomato seedlings was observed compared to control.
Moreover, effectiveness of Trichoderma varies based on the physical, chemical, and biological conditions of the soil. Trichoderma species colonize many plant roots, break down plant debris, and play a significant role in biodegradation. Numerous studies were conducted on the positive and enhancing effects of Trichoderma on growth in various plants. Inoculation of maize seeds with Trichoderma asperellum for 1.5 h increased seedling length and H+ ATPase activity. Treatment of wheat seeds with Trichoderma harzianum for 30 min also increased plant height, chlorophyll content, and the length of root and tillers. Treatment of chickpea seeds with Trichoderma asperellum in potted conditions improved plant growth indices as well.
Treatment with Trichoderma asperellum for 24 h on a variety of eggplant, red pepper, guava, okra, squash, and tomato increased seed germination, plant growth indices, chlorophyll, phenylpropanoid, and lignin activity. On the contrary, chlorophyll depletion is a negative result of stress on plants, yet such reduction significantly interferes with prevention of light inhibitory damage and reduces the light received by the leaves. Trichoderma can also stimulate vegetative growth and simultaneously transfer carbohydrates to vegetative reservoirs to use them for building chlorophylls.
Inoculation of seeds with Bacillus subtilis and Trichoderma harzianum for 24 h increased germination, number of leaves, stem length, root length, plant height, number of spikes, number of florets per spike, and flowering time in Antirrhinum majus. Treatment of soybean seeds with Trichoderma fungi can also increase the rate of cumulative emergence of seedlings. Dual application of beneficial microorganisms using a combination of a bacterium and a fungal strain on onion and carrot seeds significantly affected the number of the recycled microorganisms on rhizosphere and ultimately increased root and plant growth.
Likewise, inoculation of red pepper plant with Trichoderma viride for 3 and 12 h increased germination, root and stem length, biomass, and seedling vigor index, but in no research, Trichoderma fungi did not increase the dry weight of millet seedlings under drought stress compared to prime treatment. Seed inoculation with Trichoderma viride showed a significant increase in root and shoot growth of chickpea. Different species of Trichoderma fungi improved plant growth by producing growth hormones such as gibberellin, auxin, and cytokinin. This increased seedling growth of tomato plants was related to synthesis of growth hormones such as indole acetic acid.
Infections of plant tissues with Trichoderma which induce cell division through cytokinin and formation of special structures represent a bilateral symbiotic relationship with the plant. Meantime, synthesis of cytokinin in the root increases the lateral roots. It can also build chloroplasts with expanded viscosity in leaves so that chlorophyll and photosynthetic enzymes are synthesized more quickly. Under drought stress, Trichoderma growth-stimulating fungi increased root length in cucumber and bitter gourd more significantly than control. These fungi can produce an auxin-like activity by building harzianolide and 6-phenyl alphapyrone. Hence, seemingly, the main reason for increasing root growth in fungal treatments compared to bacterial treatments is an increase in auxin/cytokinin ratio. Similar results were also reported on increased root and shoot length and increased yield made by Trichoderma harzianum. As a result, biological treatments promote and activate plant growth and resistance mechanisms, respectively.
In summary, Plant growth-promoting microbes (PGPMs), represented by Trichoderma, play a crucial role in promoting plant growth and enhancing their resilience to stress. However, the efficacy of PGPMs is profoundly influenced by factors such as soil physical and chemical properties, microbial flora, and application methods. By continuously deepening our understanding of the mutually beneficial plant-PGPMs symbiosis and optimizing their agricultural application strategies, we hope to further tap the potential of these beneficial microorganisms, promote their widespread application in green and sustainable agricultural development, and provide an efficient and environmentally friendly biotechnology approach to achieving high-yield, high-quality, and stress-resistant crops.
The Most Representative: Trichoderma harzianum, Trichoderma viride, Trichoderma Asperellum, Penicillium Bilaiae, Beauveria bassiana, Mycorrhizae.