Plant Growth Promoting Fungi (PGPF) for Ecologically Sound Agriculture and its Market Trend Evolution

Introduction

There is a growing demand for food to fulfill the requirement of the world with ever increasing human population has driven farmers to significantly escalate crop production. Consequently, there has been a widespread adoption of synthetic fertilizers, pesticides, and herbicides in large quantities.^1-4 However, the continuous use of these chemicals has led to decline in soil quality and lead to various detrimental consequences, including groundwater contamination, reduced microbial diversity, reduced crop yield and quality, development of susceptibility to pathogens etc.⁵ The excessive use of chemical fertilizers became prominent during the Green Revolution, spanning from1940s to the 1970s in Mexico, and subsequently extended to numerous other developing countries.^{1, 3} Though, it significantly boosted the crop production, but it also brought forth negative implications with enduring impacts on various facets of human society, such as soil and water pollution and several health hazards stemming from the harmful chemicals present in these agrochemicals.⁶ To mitigate these concerns, sustainable framing practices that aim to reduce the use of chemical fertilizers have gained popularity. This paradigm shift is driven by the belief that organic crops are healthier and have a lower environmental impact compared to conventionally grown crops.⁷ In this context, fungi can play a crucial role in ecosystems and offer numerous benefits in various fields, including food production, medicine, agriculture, and healthcare advancements.⁸ Plant growth promoting fungi (PGPF) are beneficial fungi (mycorrhizal, rhizospheric, endophytic) contribute to growth and development of plants. Their ability to enhance plant growth makes them suitable for organic based agriculture and presents a new innovative approach with reduced reliance on traditional inorganic fertilizers.⁹ Fungal biofertilizers comprise fungal inoculums either alone or in combination.¹⁰ The PGPF extends to the root system of the plants, aiding in nutrient and water uptake and improving the physical properties of the soil by modifying its structure. For example, fungal hyphae can create macro-aggregate by entangling soil particles with each other.¹¹ Among the well-studied root associated fungi are Arbuscular Mycorrhizal Fungi (AMF), which forms symbiotic relationship with approximately 80% of the land plants species including agricultural crops. The AMF provides numerous benefits to the plants including enhanced uptake of mineral nutrients and water in exchange of carbon source from plants.¹² The association between PGPF and plant roots has been demonstrated to influence plant growth, enhance mineral nutrient absorption, boost biomass production, and improve crop yields.¹³ In addition to increasing plant yields and growth, PGPF suppresses plant pathogens in the rhizosphere by producing plant hormones, hydrolytic enzymes, and mineral solubilization (N, P, and Fe). Other pathways include siderophore synthesis, saprophytic colonization competition, mycoparasitism, and the induction of systemic resistance.¹⁴

Various studies have emphasized the potential benefits of different fungi (Table 1). For example, Phoma sp. when inoculated, increased the fresh biomass and number of cucumbers,¹⁵ while, Aspergillus niger and Aspergillus caespitosus enhanced content of protein, carbohydrate, total phenolic contents, anti-oxidant activity and diosgenin in Trigonella foenum-graecum.¹⁶Aspergillus terreus fungi have exhibited antifungal properties effects Aspergillus fumigatus, a human pathogen.¹⁷ Furthermore, in a study by Sarkar.⁹ reported that Trichoderma harzianum TaK12 and Trichoderma aureoviride TaN16, both are phosphate solubilizing and IAA producing PGPF, enhanced the maximum shoot and root length of rice plants, While, Penicillium olsonii was found to improve the height, dry weight, leaf area, and total chlorophyll content of the rice seedlings.¹⁸ In another study reported the application of a consortium PGPF such as Aspergillus sp., Penicillium sp. and Rhizopus sp. resulted in significantly increased wheat yield. This study demonstrated the strong potential of PGPF as biofertilizer.⁸ Overall, the use of these fungi in sustainable agriculture practices offers promising solutions to reduce the reliance on chemical fertilizers and improve crop productivity. Few examples of PGPF are Penicillium sp., Trichoderma sp., Pythium sp.,¹⁹ Trichoderma harzianum, Trichoderma aureoviride,⁹ Rhizoctonia oryzae, Aspergillus foetidus, Penicillium allahabadense, Daldinia eschscholtzii, and Sarocladium oryzae,²⁰ Acrophialophora jodhpurensis,²¹ and Aspergillus niger.²² 

Table 1: Fungal strains as plant growth promoting fungi tested on different crop plants

PGPF	Test Plant	Effect on the Plant Growth
Penicillium commune	Vigna mungo	Increased growth parameters such as shoot and root length, fresh and dry weight of the shoot and root length.134
Talaromyces sp.	Capsicum annuum	Antagonism against the pathogen (Colletotrichum capsici) and presented significant enhancement in the seed and plant growth parameters.135
Aspergillus flavus	Solanum lycopersicum	Antagonisms against Alternaria phragmospora and increase in the plant growth parameters such plant fresh weight and plant length.136
Penicillium sp.	Pennisetum glaucum, Solanum melongena and Lycopersicon esculentum seedlings	Enhanced seed germination and seedling vigour.137
Aspergillus niger	Triticum aestivum seedlings	Increased the assessed biometric parameters, reduction in the population of pathogenic fungi Gibberella, Fusarium, Monographella, Bipolaris and Volutella.50
Phoma sp.	Cucumis sativus	Increase in the plant biomass and length after 6 and 10 weeks of planting.15
Penicillium sp., Pythium sp., Trichoderma sp.	Pennisetum glaucum	Enhancement of seed germination, vigour and diseases protection against downy mildew of pearl millet.19
Aspergillus niger and Aspergillus caespitosus	Trigonella foenum-graecum seeds	Elevated levels of diosgenin, protein, carbohydrates, and total phenolic content as well as antioxidant activity.66
Trichoderma harzianum and Trichoderma aureoviride	Oryzae sativa	Increased shoot and root length of rice plant.9
Penicillium olsonii	Nicotiana tabacum	Increased amounts of proline, CAT, SOD, and total chlorophyll result in enhanced plant salt tolerance. Furthermore, the treated plants’ roots gathered less Na+ but their leaves acquired more K+.18
Acrophialophora jodhpurensis	Lycopersicon esculentum	Notably higher plant growth metrics and slower disease progression brought on by A. alternata.21
Aspergillus sp., Penicillium sp., Rhizopus sp.	Triticum aestivum	Significant increase in the plant growth and yield.8
Trichoderma viride	Vigna radiata, Vigna mungo and Sesamum indicum	Increase in the fresh and dry weight, seed germination, vigour index and inhibition of Fusarium oxysporum, and Aspergillus niger.68
Aspergillus falvus,  Aspergillus niger,  Penicillium citrinum, Penicillium chrysogenum, and Trichodermakoningiopsis	Triticum aestivum	Systemic resistance to Rhizoctonia solani induced.88
Penicillium chrysogenum	Zea mays	Increased shoot length, dry and fresh biomass, total chlorophyll and proline content.115
Aspergillus niger	Phaseolus vulgaris	A rise in the fresh weight, dry weight, and length of the shoot as well as the root.22
Trichoderma harzianum	Solanum lycopersicum	Decreased F. oxysporum wilt disease and increased chlorophyll content, shoot length, fresh and dry weight of shoot and roots.138
Aspergillus flavus, Aspergillus niger, Mucor circinelloides and Pencillium oxalicum.	Lycopersicon esculentum	Increased growth of healthy and infected plants against F. oxysporum.139
Penicillium menonorum	Cucumis sativus	Cucumber roots and shoots had higher dry biomass, and their concentrations of protein, starch, chlorophyll, and P rose by 16%, 45%, 22%, and 14%, respectively.140
Ampelomyces sp. and Penicillium sp.	Lycopersicon esculentum	Ampelomyces sp. Increase in plant growth under drought condition while Penicillium sp. increased plant growth and root biomass under salinity stress (300 mM).116
Gibberella intermedia	Waito-C Oryzae sativa germinals	Increase in the shoot growth by the production of gibberellins.141
Trichoderma longibrachiatum	Triticum aestivum	Relative expression of SOD, POD, and CAT genes was significantly up-regulated in wheat seedlings under salt stress. Increased in the relative water content (leaves and roots), chlorophyll content, and root activity, accumulation of proline content in leaves, antioxidant enzymes-superoxide dismutase, peroxidase, and catalase.117
Fusarium sp.	Salt-sensitive Oryzae sativa variety IR-64	High assimilation and chlorophyll stability index.119
Bipolaris sp.	Glycine max	Under 200 mM NaCl stress, there was a notable increase in the length, fresh and dry weight, and chlorophyll content of the shoots and roots.69
Stemphylium lycopersici	Zea mays	Increase Ca2+, K+, Mg2+, N, and P contents under salt stress, antioxidant enzyme. Decreased MDA content, Na+ ion content, Cl− ion, Na+/K+, and Na+/Ca2+.142
Paraglomus occultum	Lycopersicon esculentum	Higher root and shoot length, shoot dry weight, yield as well as increased in potassium, calcium, Mgand Fe content.143
Rhizophagus intraradices	Solanum tuberosum	Increase mini-tuber number, mini-tuber weight, shoot dry weight, root dry weight, chlorophyll content, ascorbic acid content, K, Zn and Fe content.144
Rhizophagus irregularis	Lycopersicon esculentum	Substantial improvement in the growth and quality of the crop, total dry weight, survival rate, N content and P content.145
Penicillium pinophilum	Lycopersicon esculentum	Enhanced growth indices and fruit weights, significant reduction in disease incidence caused by Verticillium dahliae.146
Trichoderma longibrachiatum ,T. asperellum, and T. atroviride	Glycine max	Antagonistic activity against Rhizoctonia solani.147
Penicillium oxalicum and Aspergillus brunneoviolaceus	Solanum melongena	Height, leaf size, root length, dry and fresh seedling weights, and early flowering of seedlings all increased.148
Penicillium chrysogenum	Arachis hypogaea	Prevented plant diseases Groundnut bacterial wilt caused by Ralstonia solanacearum.149
Trichoderma koningii	Cynara cardunculus	Plant height, diameter, chlorophyll content, and leaf dry weight all significantly increased.150
Lecanicillium psalliotae	Elettaria cardamomum	Compared to untreated plants, there was a significant increase in the length and width of the shoots and roots, their biomass, the number of secondary roots and leaves, and the chlorophyll content of the leaves.151
Xylaria regalis	Capsicum frutescens	Notable increases in the length of the shoots and roots, their production of dry matter, and their levels of phosphorus, nitrogen, and chlorophyll.152
Trichoderma virens	Zea mays	Systemic resistance induced against the foliar pathogen that causes corn leaf blight, Cochliobolusheterostrophus.153
Trichoderma harzianum	Pistacia vera	Showed the largest percentages of growth inhibition against the pathogens Rhizoctonia solani, Sclerotinia sclerotiorum, and Aspergillus flavus.154
Aspergillus niger and Aspergillus parasiticus	Vigna radiata	Greater growth in comparison to the uninoculated control in terms of fresh weight, dry weight, number of leaves, shoot length, and root length.155

Brief History of Use of Fungi in Agriculture

The practice of Rhizobial inoculation in agriculture has a long history, spanning almost a century.^{23,24,1, 2, 3,4} Microbes including fungi have been utilized for enhancing plant growth and combating both biotic and abiotic stresses. Roberts,²⁵ conducted pioneering research demonstrating the antagonistic interactions among microorganisms in aqueous environment, specifically highlighting the antagonistic relationship between Penicillium glaucum and bacteria. This work marked the inception of the term “Antagonism” as it pertains to the field of the microbiology. In the early 20^th century, Hartley,²⁶ utilized13 antagonistic fungi to control damping-off caused by Pythium debaryanum in forest nurseries.²⁷Trichoderma species were among the first PGPF recognized for their beneficial effects on plants. The recognition of Trichoderma species’ potential as biocontrol agent of plant diseases dates back to the early 1930s. Over the years, numerous reports have been added to the list, documenting their effectiveness in controlling various diseases.^{28,29,30,31,32,33,34} Trichoderma lignorum (viride) initially demonstrated the biocontrol potentialagainst Rhizoctonia solani, and later the list expanded including other fungal pathogens such as Rhizopus, Sclerotium rolfsii, and Pythium, Phytophthora.³⁵ Numerous studies have shown the value of antibiotics in biocontrol activities. For example, Weindling, ³⁶, showed how Trichoderma lignorum could be used as a biocontrol agent to fight citrus seedling disease caused by the pathogen Rhizoctonia solani. Two years later, Weindling,³⁷, reported that a strain of the same fungi produced a substance he called “gliotoxin,” which acted as a “lethal principle” released into the surrounding medium, allowing the biocontrol agent to engage in parasitic activity. However, further investigation revealed that the fungus that produced gliotoxin was not T. lignorum, but rather another fungus called Gliocladium virens,³⁸, which was later reclassified as Trichoderma virens.³⁹ During this foundational research period, many cases of successful biocontrol with PGPF species have been attributed to mechanisms such as mycoparasitism and/or antibiosis.^{40, 41} During the 1960’s, studies on mycorrhizal fungi began and soon various fungal strains were identified for plant growth promoting abilities with first fungal product (Beauveria bassiana) brought into the market named “Boverin” which was produced by the former Soviet Unionin the year 1965.^42,43 Howell and Stipanovic,⁴⁴ discovered and characterized a novel antibiotic, gliovirin, derived from Gliocladium (now Trichoderma) virens (GV-P), which exhibited potent inhibition against Pythium ultimum and certain Phytophthora species. This discovery has led to the commercial cultivation of various Trichoderma species for safeguarding and improving the growth of numerous of crops in the United States.⁴⁵ Penicillium bilaiae has been created as a commercial formulation called Jumpstart® and produced to the market as a wettable powder in a number of countries.⁴⁶ In the recent years, many more fungal strains have been employed in the production of biofertilizers, pesticides, biocontrol agents.

Initiatives to Use PGPF Products in Agriculture System

Various countries have enacted policies to reduce reliance on chemical pesticides, fertilizers, and insecticides by promoting the development and use of biological alternatives. For instance, to make the registration of biopesticides easier, the Environmental Protection Agency (EPA) of the United States founded the Biopesticide and Pollution Prevention Division (BPPD) in 1994.⁴³ Japan’s Ministry of Forestry, Fisheries, and Agriculture (JMAFF) synchronized its system with EPA Regulations in 1996. Similar to this, the European Pesticide Regulation EC No. 1107/2009 in Europe assesses biopesticides in order to promote the use of less hazardous materials by streamlining the registration procedure (2009/128/EC).⁴⁷ This shift along with the development of biopesticides and genetically modified crops, has led to a decline in the synthetic pesticides market over the past few decades.¹⁶ Throughout the 1980s, partnerships such as those between the Nitrogen Fixation in Tropical Agriculture (NifTAL) and the United Nations Educational, Scientific, and Cultural Organization (UNESCO) (e.g., N2 Africa project) have encouraged cooperation through Microbiological Resource Centers (MIRCENs) for the production of biofertilizer in Africa.⁴⁸ In the United States, legislation such as: The Federal Insecticide, Fungicide and Rodenticide Act (FIFRA), the Federal Food, Drug, and Cosmetic Act (FFDCA), and the Food Quality Protection Act (FFQPA) have a major impact on the rate at which biocontrol can enter the market and whether it remains there. To control plant diseases, microbial products need to be registered with the EPA as biopesticides.⁴⁹ On June 5, 2019, The European Commission ratified and implemented the EU Fertilizer Regulation (EC) No 2019/1009, which has replaced old Regulation (EC) No 2003/2003 on July 16, 2022; broadened not only the scope of fertilizer to include mineral fertilizers and inorganic fertilizers, but also covering organic fertilizers, bio-stimulants and fertilizer made from such recycled materials. China, has taken strong steps to mitigate nitrogen fertilizer pollution, such as formulating regional and crop specific fertilization standards, promoting high efficiency fertilizers, adopting new fertilization methods, and substituting with organic manure for synthetic fertilizer. Removing fertilizer subsidies price and improving nitrogen use efficiency are central to China’s fertilizer policy reform and are crucial for reducing N fertilizer use.⁵⁰ In October 2002, the Department of Agriculture; United States introduced the National Organic Program (NOP). This program includes only those organic products that comply with USDA regulations to be labelled as organic within the US.¹⁶ In the European Union’s implemented rules in 2018 paved the way for creating a regulatory framework for bioinoculants and regulating their sale on the internal market with CE mark, additionally steps were taken in 2020 to further refine this framework (https://blog.sathguru.com).

Government of India has taken several initiatives to encourage the adoption of organic farming and use of biofertilizer such as ‘Paramparagat Krishi Vikas Yojana’, ‘Mission Organic Value Chain Development for North Eastern Region’, ‘National Food Security Mission and National Mission on Oilseeds and Oil Palm’.⁵ One significant initiative was the ‘All India Network Project on Soil Biodiversity-biofertilizers (AICRP SBB), initiated by the Indian Council of Agricultural Research (ICAR) in 1976 under the name the ‘All India Coordinated Research Project on Biological Nitrogen Fixation (AICRP BNF)’. This initiative was to promote the use of biofertilizers made from microbial strains (bacteria, fungi, and blue green algae) (aicrp.icar.gov.in), developing more of bacterial biofertilizer than fungalones. The nation produced 1,00,000.69 tons of carrier-based biofertilizers in 2021–2022 compared to 79,436.70 tons in 2019–20. (https://www.thehindusbusinessline.com). Besides this, in 2020, Govt. of India adopted a significant step by making it mandatory for every farmer purchasing urea to also buy biofertilizer. Due to these initiatives and the government’s active promotion of safer and more sustainable farming practices, the Indian biofertilizer market is forecasted to experience a robust compound annual growth rate (CAGR) of 12.5% during 2023-2028 (www.fortunebusinessinsights.com). Besides these, another report has provided an extensive list of Indian manufacturers involved in the production of fungal bioinoculants.¹⁰ In 2006, the Indian government included biofertilizers and organic fertilizers within the regulatory framework of the Fertilizer (Organic, Inorganic or Mixed) (Control) Order (FCO), 1985.⁵ Owing to these green inputs’ substantial advantages over chemical alternatives, a number of government agencies, including the Department of Biotechnology, Ministry of Science and Technology, and the Ministry of Agriculture and Farmers Welfare, Govt. of India have actively supported the research, development, and marketing of these products.⁵¹ Several manufacturers, including MD Biocoals Pvt. Ltd. Lovatde in Haryana, Multiplex Bio-Tech Pvt. Ltd. based in Karnataka, Ambika Biotech &Agro Services in Madhya Pradesh, Biotech International Limited in Delhi, and Shree Biocare India and Shree Biocare Solution Pvt. Ltd. in Gujarat, are actively involved in the production of fungal biofertilizer. In July 2019, ICAR introduced the technology for biofertilizers in a capsule form, named one-gram capsule containing microbial population equivalent to 1 kg pack of powder or a 1 litre bottle (www.fortunebusinessinsights.com). In India, the leading biofertilizer producers are Krishak Bharti Cooperative Ltd (KRIBHCO) and Indian Farmers Fertilizer Cooperative (IFFCO), which are also the prominent players in the chemical fertilizer sector.⁵Although the biofertilizer industry is still in nascent, there is ample opportunity for setting up more biofertilizer-producing facilities in the future. Currently, India already has more than 100 active biofertilizer facilities, and their capacity is rapidly increasing.⁵¹ About 35 commercial companies and 32 integrated pest management (IPM) centers are receiving the necessary guidance and support from the Ministry of Agriculture and Farmers Welfare to manufacture biopesticides.⁵² Many state departments of agriculture and horticulture in several states (Karnataka, Kerala, Uttar Pradesh, Gujarat, Tamil Nadu, and Andhra Pradesh) have set up numerous advanced biocontrol facilities in an effort to speed up the creation of screened prospective biocontrol agents. Microbial pesticide manufacture is also carried out by ICAR institutes and a small number of State Agricultural Universities.⁵³ India now has 410 biopesticide production facilities, of which more than 130 are in the private sector⁵⁴ suggesting a growing interest and potential popularity of biopesticides and other biofertilizers. In contrast to the EU, which has appropriate quality standards and proof requirements for biostimulants, the US lacks a specific framework and fits biostimulants into the existing channels, which has resulted in exaggerated promises and a buyer beware atmosphere. Indian rules demand precise specification of the bio-stimulants, tolerance limits, and testing techniques have been set in order to prevent bogus claims, similar to the EU model. (https://blog.sathguru.com).Table 2 presents the list of fungal PGP products available in the market.

Table 2: List of some commercial PGPF based biocontrol agents available in market

Biopesticides	Trade name	Formulation	Targets
Trichoderma viride	Bioderma	1.0% WP	Soil-borne pathogens.51
Myrothecium verrucaria	DiTera®	Dry flowable	Crop-damaging nematodes, including those that burrow, sting, cyst, and cause root-knots.156
Pochonia chlamydosporia	KlamiC®	Granulate	Root-knot and false root-knot nematodes.157
Beauveria bassiana	Myco-Jaal	2.15 % WP, 10 % SC or 1.0 %, 1.15 %	Coffee berry borer, diamond back moth, grasshoppers, white flies, aphids.51
Paecilomyceslilacinus	Yorker	1.0 %	White fly.51
Verticillium lecanii	Verisoft	1.15 %	White fly, coffee green bug, homopteran pests.51
Trichoderma harzianum	Maru sena 1	4g/kg seed	Fusarium spp., charcoal rot disease.158
Aspergillus versicolor	Maru Sena 2	–	Soil borne pathogens.158
Bacillus firmus	Maru Sena 3	–	Dry root rot.158
Beauveria bassiana	Boverin	–	To control the Colorado potato beetle and the codling moth.42

Note: WP: Wettable powder; SC: Suspension concentrates.

Mechanism of Plant Growth Promotion by PGPF

The PGPF can enhance the plants growth through both direct and indirect methods (Fig. 1). Direct methods involve activity such asPhosphate solubilization (Fig. 2a) and production of indole 3-acetic acid (IAA).On the other hand, indirect methods encompass the production of siderophore (Fig. 2b), induced systemic resistance, and the ability to confer biotic and abiotic stress such as toleranceto heavy metal and salinityetc.

Figure 1: Plant growth promoting fungi (PGPF) enhanced plant’s growth by various direct and indirect methods Click here to view Figure

Phosphate Solubilizing Fungi

After nitrogen, phosphorus is the second most crucial mineral nutrient for plants in terms of quantitative requirement. While, soils contain a considerable amount of phosphorus in both organic and inorganic forms, its availability is limited due to its predominantly insoluble nature.^1,3,4 The effectiveness of applied phosphorus (P) fertilizers is typically limited to around 30% due to its fixation in soils. In acidic soil, P is fixed in the form of iron/aluminium phosphate, while, in neutral to alkaline soils, P fixation occurs in the form of calcium phosphate. These fixation processes hinder the availability of P for plants, leading to reduced efficiency of P fertilizers.⁵⁵ Like plant growth promoting bacteria, which solubilize the phosphate in the soil and make it available of the plant^2,3 certain fungal species also able to helps the plant growth by solubilizing the phosphate in the soil. Interestingly, when compared to bacteria, fungi seem to have more advantages as phosphate solubilizing microbes (PSM), as it can reach to spread up to large area around the plants root and make the nutrient available to the plant. Phosphate-solubilizing fungi (PSF) utilize three primary mechanisms to facilitate phosphate solubilization. These mechanisms encompass (a) the discharge of metabolites, (b) biochemical mineralization, and (c) biological mineralization.⁵⁶ Solubilization of inorganic phosphorus by PSF mainly occurs by the release of organic acids (glycolic acid, oxalic, tartaric, and citric acid, formic acid, gluconic acid, and fumaric acid), whereas, organic phosphorus is solubilized by the release of various enzymes (phosphatases, phytases and phosphonatases).^56-57 Talaromyces yunnanensis, Gongronella hydei, Aspergillus hydei, and Penicillium soli.⁵⁸ The PSF have specialized in solubilizing phosphate by releasing organic acids. These acids serve several functions, including (i) reducing pH levels, (ii) bolstering the chelation of cations, (iii) engaging in competition with P for soil adsorption sites, and or (iv)generating metal complexes alongside insoluble P elements like calcium (Ca), aluminium (Al), and iron (Fe). As a result, this process leads to the liberation of P.⁵⁹

Indole- 3-acetic acid production

Auxin is a plant hormone that exerts significant influence over various processes related to plant tissue formation, including growth, cell division, cell differentiation, and protein synthesis¹and produced as secondary metabolites. Among the types of auxins, indole 3-acetic acid (IAA) plays a vital role in plant growth. Although plants produce a limited amount of endogenous IAA that is not directly utilized and the exogenous IAA derived from fungal isolation can be applied in biological fertilizers to enhance results and provide optimal benefits.⁶⁰ Fungi produce IAA from tryptophan (precursor of IAA) that is present in root exudates and releases it under a symbiotic association.⁶¹ In fungi, IAA synthesis can occur through two distinct pathways Trp-dependent and Trp-independent pathways with majority of studies concentrated on Trp-dependent pathways [(indole-3-acetamide (IAM), indole-3-pyruvic acid (IPyA) and tryptamine (TAM)].⁶² The IAA synthesized by the fungi has the ability to stimulate the formation of lateral roots and the development of root hairs, ultimately leading to improved nutrient uptake by the plants associated with it. This, in turn, results in increased shoot or fruit biomass production.⁶³ Many studies have been undertaken on IAA producing fungi that promote plants growth. For example, IAA producing fungi Sordariomycetidae sp. when tested on Arabidopsis thaliana enhanced lateral roots (3.1-fold).⁶⁴ In another study, Aspergillus awamori effectively established colonization on Zea mays roots, resulting in the promotion of host plant.⁶⁵ Trigonella foenum seeds treated with mixture of PGPFs (Aspergillus niger and Aspergillus caespitosus) exhibited elevated levels of protein, carbohydrate, total phenolic, diosgenin content (342.374 µg ml⁻¹) and antioxidant activity, compared against individual PGPFs or distilled water.⁶⁶ Penicillium olsonii,¹⁸ Acrophialophora jodhpurensis,²¹ Aspergillus sp., Fusarium sp.,⁶⁰ Talaromyces trachyspermus,⁶⁷ Trichoderma viride,⁶⁸ Bipolaris sp.⁶⁹ are few of the PGPF that produce IAA and enhance plants growth and development.

Siderophore Producing Fungi

The fourth most common element in the crust of the earth is iron⁷⁰ and is essential to all living organism’s processes of growth and development.⁷¹ It regulates the biosynthesis of several substances such as nucleic acids, porphyrins, antibiotics, siderophores, aromatic compounds, cytochromes, pigments, vitamins, and toxins.⁷² Iron exists in two states in aqueous solution: Fe2+ and Fe3+. However, plants and microbes cannot use the Fe3+ form of iron because it forms insoluble hydroxides and oxides, which limits the iron’s bioavailability.⁷³ Microbes have devised various strategies for acquiring iron, one of which involves the utilization of siderophore to scavenge iron via specific receptor and transport systems. Siderophores are ferric ion chelators with a high affinity and low molecular weight,^3,74, that are excreted by few plants and aerobic microorganisms. They aid in overcoming iron insolubility by chelating with metal ions and facilitating their uptake inside the cell.⁷⁵ They are capable of forming very stable and soluble complexes with iron.⁷⁶Microbial siderophores are recognized for their ability to bolster plant growth in condition of limited iron availability.⁷⁷ Fungal derived siderophores, characterized as potent iron-chelating compounds with high affinity, exists as linear to cyclic oligomeric secondary metabolites.⁷⁸ Unlike bacteria, fungi produce mostly hydroxamate type of siderophores with the exception of few fungi.⁷⁹ So far, only two non-hydroxamate siderophores that have been identified and thoroughly studied to date. These include rhizoferrin, obtained from Rhizopus microspores and pistillarin, synthesized by the marine fungus Penicillium bilaii.⁸⁰ A Catechol type of siderophore is produced by Rhizopus sp.⁸¹ Many studies have been done to study the production of siderophore producing fungi (Table 1), the most common one being Aspergillus fumigatus and Aspergillus nidulans with findings revealing a shared repertoire of 55 siderophores types.⁷⁴ Fungal hydroxamate siderophores have been categorized into three primary structural families including fusarinines, coprogens and ferrichromes.⁷⁹ Fusarinineis known to be most common among the genera of Aspergillus, Fusarium, Gliocladium, and Paecilomyces.⁸² Another siderophore ASP2397 derived from Acremonium persicinum MF-347833 considered to be a novel antifungal compound akin to ferrichrome.⁸³ The major siderophore in two ectomycorrhizal fungi, Laccaria bicolor and Laccaria laccata have been identified as the ester-containing siderophore linear fusigen. Triacetylfusarinine C, coprogen, and ferricrocin are also present in trace amounts.⁸⁴ Aspergillus niger obtained from the rhizosphere zone of healthy cultivated Viciafabaplants produced a trihydroxymate siderophore, ferrichrome.⁷⁵ It has been reported that Trichoderma harzianum can produce maximum carboxylate and hydroxamate type of siderophore than another Trichoderma sp. like T. asperellum, T. longibrachiatum, and T. viride.⁸⁵ These siderophores have a strong binding strength for Fe (III), but they may also attach to other metals like as Pb (II), Cr (III), Al (III), and actinide ions. This suggests that they may find application in the bioremediation of heavy metal contamination, pharmaceuticals, and the management of industrial waste.⁷⁹

Figure 2: a. Phosphate solubilizing fungi forming halo zone around the colony on Pikovskaya’s agar medium; b. Siderophore production by the fungal isolate on CAS agar medium. Formation of orange halo zone around the fungal colony indicates the siderophore production. Click here to view Figure

PGPF as Biocontrol Agents

Chemicals such as, pesticides, herbicides, fungicides etc. used in the agricultural system has affected environment and human lives to a great extent.  They are known for endocrine disruption, antagonization of natural hormones in the body, immune suppression, reproductive abnormalities, hormone disruption, and cancer.⁸⁶ Biocontrol agents (BCA) being the living organisms are used to fight against insects, phytopathogens, weeds, and pests. Certain fungal strains have proven to be a great substitute to conventional chemical pesticides and insecticides (Table 2). Fungi can act as biocontrol agents (BCA) by different mechanisms such as direct antagonism (hyper parasitism), antibiosis, competition for micronutrients such as iron, mycoparasitism, hydrolytic enzymes, induced resistance, and rhizosphere competence.⁸⁷ The use of fungal strains has many benefits over the commercial harmful agrochemicals such as no development of resistance in the target, eco-friendly, renewable resource. Additionally, they exhibit a relatively rapid reproductive rate, encompassing both sexual and asexual processes, along with a brief generation time. They display specificity towards their targets. Furthermore, when devoid of a host, fungi possess the capability to endure within the surroundings by transitioning their parasitic behavior to saprotrophic nourishment, thereby upholding a state of sustainability.²⁷Trichoderma harzianum was the first fungal strain to be officially available as biocontrol agent in the market when it was listed in United States Environmental Protection Agency (EPA). In another study, the ability of PGPFs like Aspergillus falvus, Aspergillus niger, Penicillium citrinum, Penicillium chrysogenum, and Trichoderma koningiopsis found to stimulate induced systemic resistance (ISR) in Triticum aestivum was compared to that of benzothiadiazole (BTH), a chemical inducer. ⁸⁸ The results demonstrated that treatments with plant growth-promoting fungi (PGPF) resulted in the over-expression of the defensive genes, resulting in fewer disease symptoms when compared to both the BTH and the control group. Presently, the assessment of fruit loss after harvesting caused by phytopathogenic fungi is assessed to constitute over 50% overall agricultural fruit production; whereas, in India the post-harvest loss of fresh fruit and vegetables range from 4.6 to 15.9%.⁸⁷ Many fungal strains have been selected and tested for biocontrol agents in vitro and in the field condition. Entomopathogenic fungus Beauveria bassiana is employed to manage harmful insects, including white flies, thrips, mites, aphids, and their different life stages which cause damage to various crop plants.¹¹

Heavy Metal Tolerant Fungi

In modern agricultural practice, toxicity of heavy metals represents a significant abiotic stress that jeopardizes sustainable agriculture, diminishes crop productivity, and disruptsthe natural soil microbiota.⁸⁹ Soils contaminated by heavy metals as a consequence of mining activities are mostly covered only by sparse herbaceous vegetation with low productivity and species diversity.⁹⁰ Various sources contribute to the presence of metals in the soil, encompassing activities such as fossil fuels combustion, mining and processing of metal-containing ores, disposal of municipal wastes, application of fertilizers and pesticides, utilization of sewage sludge as soil amendments, and the use of batteriesand pigments.⁹¹  It is widely known that heavy metals cannot undergo chemical degradation and must either be physically extracted or immobilized.⁹² Various reclamation strategies are employed to repair the polluted site; but, when the concentration of the contamination is relatively low, they are either ineffective or highly expensive. Under these conditions, a revolutionary method called bioremediation—which restores soil by utilizing live creatures like microbes—is employed.⁹³ Numerous studies asserted that a variety of factors, including the combined influence of soil physical-chemical features and hazardous pollutants such heavy metals, are responsible for changes in the community structure of soil microbiota.^94,95,4 Because nutrients such accessible P, K, and organic matter encourage the proliferation and metabolism of microbes, which effectively reduces the heavy metal pollution, soil physio-chemical characteristics regulate the toxicity of heavy metals.⁹⁶ Mitigation of heavy metal toxicity in plants through ectomycorrhiza has been demonstrated in a number of experiments.⁹⁷Plant species, fungal species, and the kind of linked heavy metal are the main factors that determine how the AMF affects plants growing on contaminated media. However, the AMF increase plant resistance and heavy metal tolerance.⁹⁸ (Table 3). Trichocladium, Ilyonectria, Umbelopsis, Pochonia, and Pseudogymnoascus has been reported as taxa resilient to multiple metals.⁹⁹It has been demonstrated that Diversispora spurcum and Funneliformis mosseae diminish the amounts of zinc (Zn), lead (Pb), and cadmium (Cd) in the shoot of maize plants compared to the roots because they promote heavy metal accumulation in the subterranean portion of plants.¹⁰⁰ In the moderate dose of metal contamination, Aspergillus sp. was found to have a strong tolerance towards Cu, Pb, As, and Zn. In the severe level of heavy metal contamination, however, Aspergillus sp. showed a positive correlation with Ni and Cr. Therefore, it has been demonstrated that microbes associated with plant roots may affect the availability and uptake of heavy metals by plants in the rhizosphere.¹⁰² However; this can only be achieved when the fungus can maintain the growth of its mycelium. Ultimately, this enhanced nutritional provision should result in improved health and growth of trees accompanied with the most resilient isolates.¹⁰³ In addition, a wide range of other extracellular materials are released by soil fungus, such as enzymes and organic acids (such as fumaric acid, citric acid, and gluconic acid) that can change the bioavailability and speciation of heavy metals in soil.¹⁰⁴ Numerous fungi have been demonstrated to be able to thrive in high quantities of hazardous metals thus far.¹⁰⁵ They possess a few heavy metal (HM) tolerance mechanisms, including precipitation, mineral weathering, bio-absorption, volatilization, intracellular metal compartmentalization into fungal cell walls, and metal sequestration or accumulation.¹⁰⁶ Reports revaled that, the fungi Aspergillus flavus and Aspergillus fumigates achieved a removal rate exceeding 70% for Cr (VI), while the fungal isolate Aspergillus fumigatus achieved a 74% removal rate for Cd (II). When arbuscular mycorrhizal fungi were inoculated on Calendula officinalis plants, the uptake of heavy metals (Cd and Pb) was decreased, enhancing the beneficial secondary metabolites in contrast to non-mycorrhizal plants.^107,108; while, another study reported that, isolates of Aspergillus niger and Aspergillus flavus were tolerant to Cr and Pb.¹⁰⁹ Fungal oxidation can precipitate metals as insoluble metal oxalates, reducing metal bioavailability and increasing resistance to harmful metals.¹¹⁰ Fomitopsis cf. Meliae and Ganoderma aff. steyaertanum were observed to have the ability to convert zinc sulfate of Zn, Cu, Cd and Pb into oxalate forms.¹¹¹ 

Table 3: Heavy metal tolerant plant growth promoting fungi

Heavy metal tolerant fungi	Test plants	Elements	Effect on plant growth
Piriformospora indica	Medicago sativa	Cd	Significant increased biomass and nutrients uptake, minimized the Cd concentration in the shoots.159
Aspergillus welwitschiae	Glycine max	Cr-VI, As-V	Higher plant biomass. Increasedenzymatic antioxidants (ascorbic acid oxidase, catalases,1,1-diphenyl-2-picrylhydrazyl and peroxidase activity).93
Rhizophagus irregularis	Agrostis capillaris	Pb, Cd, Zn, Cu	Lower carotenoids/chlorophyll ratio and enhanced chlorophyll concentrations and.160
Glomus mosseae	Phaseolus vulgaris and Triticum aestivum	Zn, Cu, Pb, Cd	Increased root/shoot dry weight, protein content,chlorophyll content and total lipid in wheat plants, antioxidant enzymes.161
Trichoderma virens PDR-28	Zea mays	Cd, As, Zn, Pb, Cu	Increased dry biomass of roots/shoots,increased total soluble sugars, protein, Chlorophyll, and starch.162
Penicillium simplicissimum	Vigna radiata	Cu, Pb	Reduced Cu and Pb toxicity, observed from good root and shoot growth.27
Philalocephala fortinii, Rhizodermea veluwensis, Rhizoscyphus sp.	Clethra barbinervis	Cu, Zn, Pb	Increased K uptake in shoots, decreased amounts of Cu, Ni, Zn, Cd, and Pb in roots, and improved development of C. barbinervis seedlings.164
Claroideoglomusetunicatum	Zea mays	La	Reduced shoot La concentration but but increased root La concentration in maize. Enhancednutrition uptake (K, P, Ca and Mg content) in shoot.165
Rhizophagus intraradices	Oryzae sativa	As	All rice cultivars showed lower ratios of inorganic/organic As concentrations in their grains, and inorganic As in rice were converted into the less hazardous organic form dimethylarsinic acid (DMA).166
Glomus mosseae	Sesbaniarostrata, Sesbaniacannabina, Medicago sativa	Cu, Zn	The development of root nodules was significantly stimulated, N and P uptake was enhanced, and the concentration of metals, including Cu, in the shoots of the three legumes was lowered.168
Pseudomonas fluorescence PGPR-7 and Trichoderma sp.T-4	Cicer arietinum	Cd	Maximum increases in germination %, root dry biomass, vigour index, and chl-a and chl-band carotenoid content were seen with 25 μg Cd/Kg + PGPR-7 + T4 treatment.168
Fusarium sp.CBRF44 and Penicillium sp.CBRF65	Brassica napus	Pb, Cd	Improved the biomass of rape significantly and boosted the effectiveness of Pb and Cd extraction.169
Penicillim oxalicum and Fusarium solani	Triticum aestivum	Cu, Cd	Increased germination,root and shoot length of the plants.170
Mucor sp. MHR-7	Brassica campestris	Cr, Zn, Mn, Cu, Co	Enhanced ability to withstand multi-metal contamination.171
Serendipita indica	Ocimumbasilicum	Pb, Cu	Lowered metal content in the shoot and increased dry weights of the roots and shoots.172
Neotyphodium coenophialum	Lolium arundinaceum	Cd	Increased biomass, tiller count, accumulation of Cd, and movement of Cd from root to shoot.173
Exophiala pisciphila H93	Zea mays	Zn, Pb, Cd	Growth acceleration and decreased heavy metal toxicity.174
Serendipita vermifera sp. P04	Populus sp. clone INRA 717-1B4	Cd, Zn, Pb, Cu	Higher biomass in the shoots along with increased root tips.175
Gaeumannomyces cylindrosporus	Zea mays	Pb	Increased photosynthetic efficiency and improved lead resistance.176

Cd: Cadmium; Cr: Chromium; As: Arsenic; Pb: Lead; Zn: Zinc; Cu: Copper; La: Lanthanum; Mn: Manganese; Co: Cobalt.

Salt Tolerant PGPF

Microbes like fungi possess the capability to accumulate compatible solutes, which helps them counteract osmotic imbalance between their cytoplasm and the external environments. Additionally, they express various Na+ transporters to regulate and minimize cytoplasmic Na+ concentrations.¹¹² In case of AMF, the tolerance to salt stress could be regulated by genes relatedto water-channel proteins (aquaporins), Δ1-pyrroline-carboxylate synthetase (LsP5CS); Na⁺/H⁺ antiporters, ABA (Lsnced) and late embryogenesis abundant protein (LsLea).¹¹³ The halo tolerant fungal strains can stand a very high level of salinity presenting its potential to help the plants grow in such harsh condition. They help the plants to acclimatize to the harsh environment by providing better acquisition of essentials nutrients (phosphorus, nitrogen, potassium etc.), inducing chemical and physiological changes.¹¹⁴For example, less than 150 mM and 300 Mm of salt stress maize plants were inoculated with Penicillium chrysogenum, significant improvements were observed in various growth parameters compared to the group without it under both saline conditions. Specifically, the maize plants treated with PGPF exhibited higher shoot length; fresh and dry biomass; total chlorophyll and proline content.¹¹⁵ Penicillium olsonii cultured from the rhizosphere of tobacco plants enhanced the plant salt tolerance by increasing the levels of total chlorophyll, proline, CAT, and SOD activities. Furthermore, the treated plants exhibited reduced Na+ accumulation in their roots while showing increased K+ levels in their leaves. Penicillium olsonii culture filtrate was also observed to stimulate the expression of five genes associated with salt stress (NtHKT1, NtCAT1, NtNHX1, NtSOD, and NtSOS1).¹⁸ Further, two more endophytic fungi namely Ampelomyces sp. and Penicillium sp. reported to promote the growth of tomato plant under drought and salinity stress (300mM) respectively.¹¹⁶ Similarly, Trichoderma longibrachiatum when inoculated on the wheat plant under salinity stress helped the plant to adjust to the stress conditions and increased the leaf’s relative water content, and the roots along with the chlorophyll content and root activity was observed over the control. In another study, it was reported the enhanced production of antioxidant enzymes-superoxide dismutase, peroxidase, and catalase in the seedlings.¹¹⁷ Certain fungi exhibit the remarkable ability to thrive in highly saline environment (hypersaline), showcasing their halo tolerance. Among these fungi, Wallemia ichthyophaga and Hortaea werneckii stands out as particularly significant, offering substantial potential in the field of biotechnology¹¹⁸, Other halo tolerant fungi are Fusarium sp.¹¹⁹ and Bipolaris sp.⁶⁹

Commercialization of PGPF Products

The market for PGPF based products has witnessed remarkable growth in the recent years observed by the increase in the demands for biopesticides, biofertilizers, and bio-stimulants derived from PGPF.⁵ The biofertilizer market is experiencing significant growth globally with countries such as Argentina, Canada, China, Europe, India and the United States leading the way as these nations have recognized the substantial advantages of biofertilizers and are actively promoting their adoption.⁴⁸ The key drivers behind this growth are the rising awareness of sustainable agriculture practices, strict regulations in the chemical inputs, and the desire for organic and eco-friendly solutions.  Global market of biofertilizers has been over serving significant gained in terms of profit. According to the estimates, the worldwide biofertilizers industry is projected to have a size of USD 2,314.30 million in 2023 and is anticipated to expand to USD 4,096.84 million by 2028, exhibiting a compound annual growth rate (CAGR) of 12.10% throughout the projected time frame of 2023-2028 (mordorintelligence.com). While, according to the report by Fortune Business Insights, the biofertilizer market in India is projected to experience a significant growth. The market is estimated to increase from USD 110.07 million in 2022 to USD 243.61 million by 2029, at a CAGR of 12.02% during the calculated period (www.fortunebusinessinsights.com). With the concern for the environment, most of the developed countries are moving towards the organic farming encouraging the uses of biofertilizers. However, to develop a reliable market products there are many aspects of the formulation to be considerate of. Whether a formulation is straightforward, cost-effective, and easily transportable or not, as it impacts both the duration of the product’s shelf life and the methods of application on crops. Furthermore, deciding whether to manufacture the strains in liquid or solid form is an important consideration as they a significant role in determining the practicality and effectiveness of using the fungal strains as BCAs in agriculture system.¹¹Different types of formulations can be used such as granules, micro-granules, wettable powders, wettable/water-dispersible granules, dusts, biomass suspension in water, oils, and emulsions etc.⁴²The top consumers of fungal biofertilizers are Europe, America and Latin America because of the strict regulation these counties have applied to chemical fertilizers.¹²⁰ Also, the highest producers of bio-fungicides are USA and France.¹²¹ Kiwa Bio-Tech Products Group Corporation, Lallemand Inc., Bayer Crop Science, Rizobacter, and BASF SE are some of the key companies that manufacture biofertilizer in the global market. In 2012, the predominant bio-inoculants utilized were primarily Rhizobium (nitrogen fixing bacteria), accounting for 79% of the worldwide demand. In addition to other bio-inoculants such mycorrhizal products, phosphate-mobilizing bio-inoculants accounted for 15% of the market. N-fixing products now dominate the market, but demand for P-mobilizing products—including mycorrhizal—is predicted to rise. So far, there are approximately twelve manufacturers of mycorrhizal inoculums in the EU, distributed among countries like Germany, the UK, Spain, Czech Republic, France, and Switzerland and along with over 20 others globally with the majority located in the USA.¹²² Moreover, to boost more production of fungal agricultural products, the EU has initiated COST action to formulate a strategy in mycorrhizal technology. This will concentrate on the standardizing production techniques for AM fungal inoculums and establishing regulations to enhance soil quality and agricultural health.¹²³Mycotal (Verticillium lecanii), Biogreen (Metarhizium anisopliae), Trichoderma 2000. (Trichoderma harzianum), Fusaclean (Fusarium oxysporum), Casst (Alternaria cassia), Luboa 2 (Colletotrichum gloeosporioides f. sp. cuscutae), Ketomium® (Chaetomium globosum and C. Cupreum), Promote® (Trichoderma harzianum and T. viride), SoilGard® (Gliocladium virens), AQ10 bio-fungicide (Ampelomyces quisqualis) are some of the PGPF product which has brought into the market.^{121, 86} The share of the biopesticides in the market of Indian pesticides was a mere 4% in 2014. However, there has been a noteworthy growth in their usage, with biopesticides accounting for 9% of the overall pesticides’ consumption in India by 2020, representing a significant increase of 40% between 2014 and 2019. So far, a total of 970 biopesticides has been registered by the central insecticides board registration committee (CIBRC).⁷ This suggests promising potential for fungi to serve as eco-friendly Argo-product on the times ahead.

Future Scope and Limitations

PGPF can play a significant role as biocontrol agents in an integrated agriculture management system.^124,88 They are microbes that aid in the solubilization, biological fixation, and mineralization of significant macro- and micro-elements that are necessary for plant good health. Even while tremendous advances have been achieved in our comprehension of interactions between plants and microbes on many different levels, there are still a number of gaps that must be filled in order to fully utilise the advantageous features. Commercial formulations are employed as biofertilizers and significantly contribute to the sustainability of agriculture, whether they are single-strained or utilised in consortia. In order to boost agricultural output, it is crucial to find novel strains, make existing strains more effective, and thoroughly research plant-microbe interactions.¹²⁵ The goal of future study should be to regulate microbial communities in rhizospheric soil in an integrated manner. The development of biotechnological and molecular methods will increase our comprehension of the cellular mechanisms and signalling pathways behind growth and DP resistance as an outcome of interactions between plants and microbes.¹²⁶ Recent advancements in biotechnological tools and consistent modification may be helpful in creating the PGPF to give crop plants advantageous traits. Regular investigations on the genetic stability and ecological preservation of the genetically modified strain are required.¹²⁷ One of the main problems with inoculants technology, however, is the survival of the microorganisms during storage. This survival is influenced by various factors, such as the growing medium and the physiological state of the bacteria at harvest,¹²⁸  the process of dehydration, rate of drying,¹²⁹ the temperature of storage and water activity of the inoculums.¹³⁰ Under normal storage settings, the inoculants’ shelf life is reduced to three to six months as a result of all these factors. Therefore, research into extending the shelf life of inoculants or developing new carrier inoculants formulations is becoming more and more important.¹³¹ It is essential to create efficient and useful methods for the mass cultivation, shipping, storage, formulation, and use of these fungi. What’s more, work must be put into persuading the growers that PGPF might be a helpful supplement to their current crop management programmes.¹³² Use of biofertilizers, biopesticides, bio fungicides etc. has huge attention in the past few decades. Because of the potential of reducing the use of harmful agrochemicals government of many countries have already adapted the use such bioproducts. However, it is still at infancy stage. Many developed countries have already started to strictly regulate the application of chemical pesticides and fertilizers in an attempt to protect the environment. With harnessing genomic tools for PGPF selection, improvement in the formulation techniques and conducting field trials for evaluating long term effects it identifies future prospects. And also, many schemes have been initiated by the government to help the farmers to provide funds to encourage to opt for organic way of agriculture.⁵ Global markets and different companies have been trying their best to reduce the cost of production so that farmers can opt for sustainable product over the chemical products which are cheaper at the present time.⁴² In spite of the fact that numerous positive effects have been verified by experts around the world, nothing much is known about the significance of PGPF. Chemical pesticides and fertilisers, which have long-term negative impacts on the environment as well as the health of plants and people, are nonetheless purchased by farmers.¹³³ Despite the potential benefits of PGPF several challenges hinder their widespread adoption in mainstream agriculture.  Some of the limitations faced are as follows:

Limitations such as Lower shelf life as the biofertilizers, contamination of the carrier material for the fungal biofertilizer also limits the efficiency of the biofertilizers.

Limited distribution of the biofertilizer among the farmers and in the market

Host specificity is another limitation of the PGPF as biofertilizer. Effect of climatic condition on the PGPPF, for instance one fungus can have certain optimum temperature requirement to be effective to enhance the plants growth.

Other constraints are the inconsistent response on different soils, crop and environmental conditions. There is no universal benchmark for bacterial and fungal viability or performance due to the natural diversity of these species and the flexibility of their roles.¹²²

Also, the fraud companies selling fake products under the names of big companies create distrust among the farmers.⁵

Difficulty in producing large quantities due to high production costs and an expensive data registration process that offers no legal protection.³⁵

Conclusions

In conclusion, this review highlights the substantial contribution of PGPF in advancing sustainable agriculture practices. It underscores their potential to boost soil health, increase nutrient availability, and suppress plant diseases. Furthermore, review discusses the evolving market trends and identified potential challenges and future prospects associated with PGPF based products. Overall, the integration of PGPF into agricultural system holds promises for a more ecologically sound and economically viable approach to farming. The use of synthetic agrochemicals has had a significant and detrimental impact on the environment and human well-being. If this trend continues, a substantial portion of land will suffer from degradation, and there will be an increase in the occurrence of severe diseases affecting mankind. PGPF has proven to be a great substitute to pesticides and chemical fertilizers through many studies. However, still great deal of understanding is required to fully utilize the potential of PGPF in plants growth and development as application of PGPF depends on many factors such as abiotic factors, biotic factors, and formulation and combination of the fungal strains. Since many PGPF are host specific, deeper study on a single PGPF instead of studying only the superficial characters can also help a lot to get the proper manual of applying the particular PGPF into the field.

CRediT authorship contribution statement

Mum Tatung: Formal analysis, writing original draft. Anu Seng Chaupoo: Formal analysis, writing original draft. Chitta Ranjan Deb: Conceptualization, Supervision, editing data and correction of manuscript. 

Acknowledgement

MT is extremely thankful to the Council of Scientific and Industrial Research (CSIR), Govt. of India, New Delhi for financial support as JRF and SRF. Facilities used from the DBT sponsored Institutional Biotech Hub (File No. BT/22/NE/2011), UGC-SAP(DRS-III) and DST-FIST programme are duly acknowledged.

Funding Sources

There was no funding for this review. However, Mum Tatung received fellowship for her doctoral research from the Council of Scientific and Industrial Research (CSIR), Govt. of India, New Delhi as JRF and SRF. Other sources of facilities used for the current work was from the DBT sponsored Institutional Biotech Hub (File No. BT/22/NE/2011), UGC-SAP(DRS-III) and DST-FIST programme.

Conflict of Interest

The author(s) do not have any conflict of interest.

Data Availability Statement

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Ethics Statement

his research did not involve human participants, animal subjects, or any material that requires ethical approval.

Informed Consent Statement

Both the authors agreed to submit the paper to ‘Current Agriculture Research Journal’ for publication with CRD and MT equal share of authorship.

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