Keywords

3.1 Introduction

Soil microorganisms greatly influence the nature of the soil and its health through beneficial and harmful activities. Microorganisms present in the rhizosphere mediate certain functions, for instance, decomposition, nutrient immobilization, mineralization, nitrogen fixation, and release of nutrients. In addition to these, microorganisms also possess phosphate-solubilizing ability by converting insoluble phosphates to soluble phosphorus (Pradhan and Sukla 2005) in soil and make them readily accessible to plants.

The father of plant pathology de Bary proposed the term endophyte to refer any organism that resides within plant tissue exclusive of any disease manifestations in host plant (Nisa et al. 2015). All types of plants harbor a wide variety of microorganisms, for instance, fungi, bacteria, and cyanobacteria that posses endophytic properties and play an imperative role in plant metabolism and physiology (Hardoim et al. 2015). During symbiotic association established between endophytes and host plants, both the organisms get mutually benefited, the plant provides nutrients to colonizing endophytes, while the endophytes accelerate biosynthetic pathways for metabolite synthesis that have many applications in agronomy. For example, management of plant growth and novel disease-resistant mechanisms against pathogens.

Endophytes can be isolated from exterior or interior part of sterilized plant tissues. Significant biochemical molecules such as Terpenoids, isoflavonoids, flavonoids, and phenolics are released from plant roots. They may attract the fungi from root region to colonize within the plant as an endophyte. However, the endophytes are depicted to be colonized in different plant tissues associated with the different ecosystems. Fungi can be classified into diverse groups derived from their role and survival, such as epiphytic, endophytic, pathogenic, and mycorrhizal fungi (Porras-Alfaro and Bayman 2011). Some endophytic fungi can find their way to either vertical or horizontal root region and penetrate to the deeper regions of plants.

Endophytic fungi colonize the tissues of host plant by particular route of transmission and this can be either vertical or horizontal method. Endophytic fungi transmit from the mother plant to offspring via seeds (true endophytes). A study carried out by Hodgson et al. (2014) in forbe species, common poppy, knapweed, cornflower, sheep’s sorrel, groundsel, and ribwort plantain, and two endophyte species, Cladosporium sphaerospermum and Alternaria alternata primarily investigated the vertical transmission of species. Horizontal transmission occurs by airborne spores or through soil. Endophytes colonize forbes via leaves of the host through horizontal transmission.

According to earlier fossil records, evolutionary tendency reveals an association between a diverse group of plants and endophytic fungi. Plant endophyte communications resulted in plant growth promotion, uptake of micronutrients, and synthesis of different types of secondary metabolites and bioactive compounds with potential applications in industry, medicine, and agriculture. Endophytic fungi provide protection to plants against plant pathogens, reduce biotic and abiotic stresses, and for the reason that these organisms are considered as eco-friendly bioresources. Endophytic fungi may enhance plant growth by solubilization of potassium, phosphorus and zinc, produces phytohormones, viz., cytokines, gibberellic acids, indole acetic acids, hydrolytic enzymes and Fe-chelating compounds, ammonia and hydrogen cyanide (Rai et al. 2014). Different classes of fungi, for instance, Mucoromycota, Basidiomycota, Oomycota, and Ascomycota were depicted as plant growth promoters and protect the plants under anomalous and abiotic stress conditions. Natural products produced by endophytes were previously reported with potential anti-bacterial, anti-fungal, and anti-protozoal properties. Nonetheless, the secondary metabolites produced by endophytes are proved to have remarkable anti-cancer, anti-microbial, insecticidal properties, pharmaceutical sciences, and in other biotechnological applications (Kusari et al. 2011; Aly et al. 2010; Uzma et al. 2018; Mishra et al. 2017).

3.2 Diversity of Phosphate-Solubilizing Microorganisms

A great extent of microbial species exhibit phosphate-solubilizing ability, these include archaebacteria, bacteria, actinomycetes, and fungi. These microbes reside in the plant tissues exclusive of causing any harmful effects to the host. Generally, they prevail in tissues of the host plant with the symbiotic association. These microbes were previously isolated from different types of plants, including Triticum (Yadav et al. 2018a; Verma et al. 2015, 2016a, b), Oryza sativa (Piromyou et al. 2015), Zea mays, Capsicum annuum L., Saccharum officinarum (Montanez et al. 2012; Thanh and Diep 2014), mustard, citrus (Kasotia and Choudhary 2014), Solanum tuberosum (Rado et al. 2015; Manter et al. 2010;), Glycine max (Mingma et al. 2014), Pisum sativum (Narula et al. 2013; Tariq et al. 2014), Phaseolus vulgaris (Suyal et al. 2015), Helianthus (Forchetti et al. 2010; Ambrosini et al. 2012), and Cicer arietinum (Saini et al. 2015). Fungal endophytes pertaining to diverse genera including Acremonium, Aspergillus, Paecilomyces, Cryptococcus, Fusarium, Curvularia, Rhodotorula, Cladosporium, Alternaria, Phaeomoniella, Chaetomium, Colletotrichum, Berkleasmium, Rhizoctonia, Geomyces, Leptospora, Phyllosticta, Microdochium, Neotyphodium, Ophiognomonia, Glomus, Penicillium, Rhizopus, Trichoderma, Xylaria, and Wallemia have been isolated from various host plants (Suman et al. 2016; Verma et al. 2017; Yadav et al. 2018a, b). Recently, a nematode fungus Arthrobotrys oligospora was identified to solubilize rock phosphate Togo, Tilemi rock phosphate, Kodjari phosphate rock. The fungi solubilized all three types of rock phosphates. Given these, Duponnogs and group (2006) demonstrated the phosphate solubilization ability in vivo conditions.

3.3 Biotechnological Applications of Natural Products from Endophytic Fungi

Biotechnology has opened up numerous avenues for exploitation of endophytic microorganisms in medicine, agriculture, and industry from diverse ecological niches and their applications in agriculture are aptly essential for plant growth, plant protection, and yield (Yadav et al. 2018a; Rana et al. 2019c). Because of their ability to promote plant growth and adapt under extreme abiotic stresses, the endophytic microorganisms have, in fact, captured the attention of the scientific community (Soni et al. 2018; Yadav et al. 2019b, c, d). With the exceptional capacity to produce secondary metabolites, the endophytic fungi may perhaps unearth novel applications in pharmaceuticals, industrial processes, and horticulture (Joseph and Priya 2011). Endophytic bacterial and fungal organisms possess extensive applications as bio-control agents, bio-inoculants, and bio-fortification of micronutrients (Yadav 2019; Yadav et al. 2019a; Yadav and Yadav 2019).

The excessive usage of chemical phosphorus (P) fertilizers to increase agricultural yield sequentially to meet the requirements of escalating global food demand potentially causes soil and water pollution, eutrophication, depletion of soil fertility, and deposition of toxic heavy metals such as arsenic (As), lead (Pb), and selenium (Se) in the soil. Abundant soil microbes together with bacteria, fungi, actinomycetes, and algae are efficient in solubilizing insoluble soil phosphate to convert into the soluble P and making it available to plants. Strikingly, these microbes promote the growth and yield of a wide variety of crops. Thus, it is essential to inoculate phosphate-solubilizing microorganisms (PSM) via the seeds, to the crop and soil, which is a potential strategy to improve the crop yield. Despite their immense significance in the improvement of soil fertility, phosphorus-solubilizing microbes are yet to replace conventional chemical fertilizers in commercial agriculture. Extensive studies are obligatory to comprehend recent approaches in a diversity of phosphate-solubilizing endophytes and their colonizing ability and application to enhance agronomic yield.

3.4 Endophytic Fungi as P-Solubilizers and Growth Promoters

Phosphate-solubilizing fungi enhance plant growth by different mechanisms and those are (Fig. 3.1): provide nutrients to plant by solubilization process, produce biological control substances, i.e., antibiotics and siderophores, provide protection against the plant pathogens, and stimulate growth hormones production (auxins, gibberellins, and cytokines). In agriculture, phosphate-solubilizing fungi play a significant role as bio-inoculants for improvement of plant growth (Khan et al. 2010; Kour et al. 2019b, c; Rana et al. 2019a, b). The competent phosphate-solubilizing fungi were tested under in vitro conditions and selected for large-scale production and eventually distributed to farmers. In addition to that, suitable carrier selection is also paramount for the development of fungal inoculants such as peat, farmyard manure, soil, cow dung, and cafe powder which are being used as suitable carriers. However, a perfect carrier is designed to possess some unique qualities like good absorption ability, sufficient level of moisture pH, aeration stability, pH buffering capacity, and porousness. In addition to these characteristics, the carrier must be eco-friendly, non-hazardous to microbes, plants, animal, and humans. In addition to this, it should be easy to handle, mix, sterilizable, and store. Keeping in view of cost–benefit ratio, the carriers ought to be cheaper and easily available. The carrier enhances the persistence of phosphorus-solubilizing activity; fungal spores after mixing with a carrier can be stored for about 3 months at 30 ± 2 °C. Plethora of examples of commercially available inoculants, i.e., Penicillium radicum and Penicillium bilaiae were demonstrated in large-scale production and shown to possess “P” absorption ability. Various stages involved in bulk scale production and application of P-solubilizing fungi are clearly depicted (Fig. 3.2).

Fig. 3.1
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Mechanisms of plant growth acceleration by endophytic P-solubilizing fungi

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Fig. 3.2
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Production and application of phosphate-solubilizing endophytic fungal inoculants

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Various studies indicating the effect of a single culture and/or mixed culture of phosphate-solubilizing fungus on different plant growth parameters observed the effect of mixed inoculation of P-solubilizing fungal strains (two strains of A. awamori and four of P. citrinum) on growth and seed production of chickpea in pot experiment. Notably, all the isolates were shown synergistic effect and resulted in noteworthy stimulation of root and shoot lengths of legume, height of the plant, seed weight, and number compared to the un-inoculated control.

Phosphorus is a vital nutrient and a part of structural compounds and mediates catalytic reactions in plant metabolism. Phosphorus plays a major role in capturing solar energy and is converted into useful plant compound. Phosphorus is a key component of DNA and RNA. Two phosphate-solubilizing fungi, i.e., Penicillium oxallicum P4 and Aspergillus niger P85 were isolated by Yin et al. (2015) from calcium-rich soils of China. A remarkable increase in plant fresh weight was observed in strain p24 when rock phosphate was supplemented externally. A study carried out on Aspergillus aculeatus P93 has also shown a significant increase in the availability of soluble phosphorus of maize grown in non-amended soil (Yin et al. 2017).

In a recent study, phosphate-solubilizing microorganisms depicted a synergistic effect on plant growth parameters and leaf chlorophyll content (Senthil kumar et al. 2018). In addition to solubilizing phosphates, some may produce potential bio-control agents against plant pathogens. PSM can produce anti-fungal compounds such as flavonoids and phenolics, siderophores, antibiotics and hydrolytic compounds. All of which inhibits growth of plant pathogens.

3.5 Phosphate Solubilization Mechanism by Endophytic Fungi

Based on the availability of type of phosphates (organic or inorganic), endophytic fungi employ suitable mechanism for solubilization of phosphates and endophytic fungi are capable to synthesize organic acids, proteins, OHˉ ions, Ca+2 exopolysaccharides, CO, siderophores, and enzymes, those may play a significant role in phosphate solubilization (Fig. 3.3).

Fig. 3.3
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Production of metabolites by P-solubilizing fungi

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3.5.1 Inorganic P-Solubilization

Various theories explained the key mechanism underlying the inorganic phosphate solubilization. The principal mechanism is dissolving compounds such as hydroxyl ions, siderophores, organic acids, carbon monoxides, proteins, extracellular enzymes, and exopolysaccharides (Sharma et al. 2013). Secretion of organic acids by endophytic fungi into soil seems to be a most important mechanism for inorganic phosphate solubilization primarily organic acids such as glycolic, maleic, formic, lactic acid, gluconic acid, oxalic, tartaric propionic, and succinic acids. The quantity of organic acid varies with the endophytic fungal strain and also the type of organic acid produced is greatly determined by source of insoluble phosphorus. According to Mendes et al. (2013), Aspergillus niger FS1 primarily secrete oxalic acid in higher quantity in treatments with FePO4 and AlPO4, whereas gluconic acid was produced in meager quantity in AlPO4. In contrast to this, Penicillium canescens FS23 produced citric and gluconic acids after treatment with Ca3 (PO4)2, AlPO4, and rock phosphate. Organic matter present in soil is an excellent source of organic phosphorus. The total quantity of organic phosphorous present in soil is as high as 30–50% of total phosphorus. Organic phosphorus in the soil is principally in the form of inositol phosphate (or) soil phytates. Organic phosphorus can be mineralized with the involvement of enzymes such as phosphatases, phytases.

Many studies explained about p-solubilization based on an organic acid concept. Several genes are involved in production of organic acids. Among all organic acids, gluconic acid is more essential in P-solubilization produced by endophytic fungi. The genetic basis of P-solubilization was studied by Kusari et al. (2012). He observed upregulation of pyrroloquinoline quinine and glucose dehydrogenase genes in solubilization of phosphorous. PQQ-dependent glucose dehydrogenase present on cytoplasmic membrane stimulates oxidation of glucose to gluconic acid. Because of production of gluconic acid, the pH of the soil further decreases which make the following ions \({\text{HO}}^{ - 2}_{4}\) and \({\text{HPO}}^{ - 3}_{4}\) (soluble forms of phosphorus) more available. An array of genes is involved in the production of organic acids.

3.5.2 Organic P-Solubilization

Mineralization of organic phosphate carried out by involvement of various enzymes, i.e., phytases, phosphonatases, and phosphatases. Phosphatases dephosphorylation or hydrolyze and phosphoanhydride and organic phospho-ester bonds of organic matter. Among all phosphatases, predominant types of enzymes are phosphomonoesterases (Nannipieri et al. 2011). Based on pH optima they are classified as acid and alkaline phosphatase (Behera et al. 2014). Several genes encoding for alkaline and acid phosphatases with broad substrate specificity were cloned and characterized. However, a considerable amount of phosphatases was secreted by plant roots, and it has been reported that microbial phosphates possess a stronger affinity for substrate when compared to derived by plant phosphatases.

A large quantity of phosphorus is found in fruits and seeds for the reason that it is important for the development of seeds. Phytin is a significant form of “P” in seeds. Phytin is naturally degraded by phytases. This is the main source of inositol phosphate and constitutes for more than fifty percent of organic phosphorus in the soil. Phytases act upon phytate and make available free form of phosphorus. Noteworthy that the phosphonatases and carbon-phosphorus lyases hydrolyze carbon-phosphorus bond of organophosphates and release free phosphate (Rodriguez et al. 2006). Because of scarcity, organo-phosphatases do not add much to the soluble form of phosphate in the soil solution. A plethora of studies revealed that organic acids released by P-solubilizing fungi are much superior to bacteria; therefore, endophytic fungi exhibit greater P-solubilization activity. Motsara et al. (1995) revealed that solubilization of rock phosphate was much higher under in vitro conditions by Rhizoctonia solani, Penicillium, Fusarium oxysporum, and Aspergillus niger.

3.6 Development of Phosphate-Solubilizing Endophytic Fungal Inoculants

Phosphates-solubilizing endophytic fungal inoculants are used as major bio-fertilizers. These bio-inoculants are more eco-friendly when compared to chemical fertilizers. Fungi are depicted as predominant P-solubilizing inoculants when compared to other bacterial inoculants; hence, these strains hold much importance in agriculture. Nonetheless, several fungal bio-fertilizers were already developed by IARI as bio-inoculants which comprises Aspergillus awamori, A. niger, and P. digitatum, P.bilaii strains which were commercialized by Novozymes Biologicals Limited (Canada). Interestingly, the strain P.radicum was recently developed by Bio-Care Technology (Australia) by Gupta and Rodriguez Couto (2018). Similarly, in India, P-bio-fertilizers were produced by Ambika Biotech and Agro Services (Madhya Pradesh) (Pal et al. 2015).

For production of P-solubilizing fungal inoculums, huge amount of endophytic fungal strains are required. There are broadly three phases in development of bio-fertilizers. In the first phase, there is a selection and screening of potential phosphate solubilizers followed by a selection of fungal inoculants. The screening process can be carried but by the cultivation of fungi in modified Pikovskaya’s medium. In second phase, proper endophytic fungal bio-fertilizers can be developed and the third phase includes checking the quality and persistence of P-solubilizing microorganisms and distribution to farmers (Khan et al. 2010; Kumar et al. 2017). Notably, few potent microorganisms are selected, screened, and cultivated in large scale for production of bio-fertilizers under optimized conditions in a suitable fermentation broth. For cultivation of fungi, lower pH (acidic condition) is more suitable at the same time inhibits the bacterial contamination (Nelofer et al. 2016). Once an adequate amount of growth is obtained, the biomass can be extracted and mixed with suitable pre-sterilized carrier material, purified, packed under aseptic conditions, and stored under appropriate conditions before commercialization. At every stage of bio-fertilizer production, it is mandatory to assess the level of contamination as well as for the amount of desired microorganisms.

Definite problems may also be associated with the commercial-scale synthesis of bio-fertilizers, among those sometimes microorganisms unable to survive under in vivo conditions. This may be due to the fact that the bio-inoculants are either difficult to survive under unfavorable environmental conditions or outcompeted by presented microflora (Walia et al. 2017). One of the important strategies to surmount this problem is amalgamation of bio-fertilizers with suitable carriers.

Carriers being used in production of bio-fertilizers should possess definite characteristics like it should be easily mixed with microbes to enhance the sustainability and survival of microorganisms by maintaining optimum pH, an adequate level of moisture and aeration, etc. Thus, the carrier material should possess an excellent moisture absorption ability and pH buffering capacity, nonetheless, it should also be non-toxic to microorganisms, and eco-friendly to plants, animals, and humans. In addition to there, it is easy to sterilize, and easy to mix, handle, and store. In view of the cost–benefit ratio, the carrier should be effortlessly available and cheap. Nowadays, different types of carriers are being used for the production of bio-fertilizers. Smith (1995) has classified the carriers into different categories, first category comprises different types of soils, peat and coal, waste cake powder farmyard manure, plant debris and second group comprises barnyard compost, soya bean oil, shelled nut oil, barnyard compost. The last group consists of inert materials like perlite, rock phosphate, and calcium sulfate vermiculite. These carriers have also been used in combinations. Wang et al. (2015) reported the utilization of different carriers for developing a bio-fertilizer of A. niger and reported a mixture of wheat husk and perlite to enhance the availability of “P” content.

Among soil microorganisms, AM fungi have been found to be a noteworthy component of soil–plant systems (Schreiner and Bethlenfalvay 2003). An AM fungus plays a major role in nutrient and water uptake by plants and provides other benefits to host, such as tolerance under adverse environmental conditions and disease resistance (Pal et al. 2014). Due to their obligatory symbiosis, it is highly difficult to produce AM-based bio-fertilizers in in vitro conditions.

Mass scale production AMF is highly difficult because of its specific nutritional requirements (Pal et al. 2015). The different strategies were reported by Berruti et al. (2016) for the utilization of AMF as bio-fertilizers. According to first strategy AMF-harboring rhizosphere, soil can be used as bio-inoculants; however, this method may not be reliable and may perhaps result in colonization of weeds and pathogens. In other strategies, AM spores that were isolated from root region can be used for this AM fungal organism inoculated on a host trap plant in an inert medium. The trap plant is highly susceptible for Arbuscular mycorrhizal fungal growth, and therefore it is used for production of AM fungi for bulk scale. This is frequently used inoculums for inculcation to crop plants in large scale. This consists of a set of some kind of AM spores present in soil inoculants.

In this method, trap plant should be highly amicable for inoculation of desired AM fungi and also should be ideal for large-scale production of propagates. In addition to these, the trap plant should show intense root development within a short period of time and resist to harsh environmental conditions, suitable for synthesis of fungal propagules (Sadhana 2014). An important observation came from a study of Selvakumar et al. (2016) that maize could be the suitable host trap plant when compared to the Sudan grass for the propagation of Claroideoglomus etunicatum. Other trap plants including Chloris gayana, Sorghum vulgare, Zea mays, Sorghum bicolor var. sudanense, and Ipomea batatas are most common trap plants used for mass scale culturing of Arbuscular mycorrhizal fungi (Sadhana 2014).

By wet sieving and decantation, the AM fungi is regularly isolated from soil (Singh et al. 2010) followed by microscopic observation of AM fungi. Mass multiplication is carried out by collecting a large number of spores by pot culture method. Host trap plant and AM fungi were cultured in natural solid medium containing clay, peat, sand, perlite soil, and different types of composted plant debris. Tamil Nadu Agricultural University designed a method, and according to this, a trench lined with polythene sheet is being used as plant growth pot or tub. Fifty kg of vermiculite and 5 kg of sterilized soil are filled in trench up to 20 cm height. To this 1 kg of AM spores inoculated 2–5 cm below the surface of vermiculite. Sterilized seeds of trap plant are sown in a trench along with an appropriate dosage of nitrogen source urea and superphosphate. After the period of 60 days roots of trap, plants are cut, spores, a mixture of vermiculite, hyphal fragments, and infected root pieces obtained were used as AM fungal inoculants. Without using soil also some researchers developed hydroponics and aeroponics for the cultivation of AM fungi in the presence of trap plants. The major advantage of these methods is that there is a feasibility to produce pure and clean AM spores (Ijdo et al. 2011).

3.7 Application of Phosphate-Solubilizing Endophytic Fungal Bio-Inoculants

Treatments of seed surface with suitable bio-inoculants are the most common choice of inoculation prior to seeding and reported to be the popularly used method (Walia et al. 2013b). However, there are few techniques that are widely used for the inoculation of endophytic microorganisms, viz., soil application, seed treatment, and foliar spraying. In seed treatment process, carrier-coated fungal inoculums are immersed with seeds in a liquid culture medium. In this method, a fungus adhered firmly to the seed surface. Conversely, there are some constraints in this method. Amount of viable fungi adhered on to the seed surface may not be adequate. The plant species are under cultivation at commercial scale by vegetative propagation, and the endophytic P-solubilizing bio-inoculants are usually applied to plant parts before planting in the field (Panhwar et al. 2013; Kour et al. 2019a; Kumar et al. 2019; Yadav et al. 2019e). The shoots developed from such plants are deemed to be more amenable for bacterization by endophytic microorganisms. Application of endophytes to the soil is another method of bio-inoculants application (supplementation of soil with endophytic bio-inoculants). These methods have many advantages which include the following:

  • A high number of P-solubilizing fungi may disseminate per unit area.

  • Less number of interactions may occur between bio-inoculants and chemically treated seeds.

  • This method is more rapid in comparison with seed inoculation technique.

  • These bio-inoculants are more tolerant to dry and desiccated conditions.

In view of above aspects, phosphate-solubilizing endophytic fungal inoculants can be applied by two approaches.

  1. 1.

    Single culture of phosphate-solubilizing fungi can be used as inoculants as single culture approach (SCA).

  2. 2.

    Two cultures can be used as bio-inoculants are called mixed culture approach (MCA).

3.8 Effect of Phosphate-Solubilizing Fungal Bio-Inoculants

Soil is a natural habit for wide variety of microbial communities. The interactions occurring between microbial communities basically impact a physico-chemical property of the soil, and soil fungi performs several imperative roles in the maintenance of soil biochemistry directly or indirectly. Direct endophytic fungi accelerate plant growth promotion through the production of phytohormones, mineralization of salts and ions (Guleria et al. 2014) and in the indirect mechanism; it plays an important role in bio-control agents against phytopathogenic microbe (Walia et al. 2013a). Mehta et al. (2011) studied the growth enhancement of groundnut in association with endophytic fungi. They studied two fungi, i.e., Aspergillus niger and Penicillium notatum supplement of soils tri-calcium phosphate (TCP) under pot culture conditions and reported that there is a remarkable improvement in dry weight and height of the plant. When a mixed culture of fungal strains was employed as inoculants, a substantial improvement was observed in plant height as 81% and plant dry weight as 105% compared to controls (Prasanna et al. 2011; Mehta et al. 2011).

It is pertinent that a number of plants and weight of seeds enhanced remarkably with single or multiple inoculations of fungal strains. Other studies carried out by Priyadharsini and Muthukumar (2017) on pigeon pea revealed that when inoculated with the fungi Curvularia geniculata has shown a significant impact on growth parameters. C. geniculata inoculated seedlings of pigeon pea were taller (26.53%) and showed increased shoot and root dry weight (16.67–33.33%) as compared to un-inoculated control seedlings. In addition to P-solubilization, endophytic fungi also play an imperative role in phytohormone production that can remarkably enhance plant growth. Thus, the exploitation of phosphate-solubilizing fungi is considered eco-friendly, profitable, and sustainable approach for enhancement of crop yield.

3.9 Application of RDNA Technology in Developing Phosphate-Solubilizing Endophytic Fungi

Interaction of endophytic fungi with host plant is relatively an intricate process. Set of genes are involved in such interaction including nitrogen, phosphorus, and other nutrient exchanges between endophytic fungi and host plant tissues were studied; nonetheless, widespread research is desirable to better comprehend the genetic aspects of such interactions. Comprehensive and enhanced knowledge is essential for the involvement of genes and their regulation to undertake genetic manifestation of fungi, which consecutively can be employed for better phosphorus uptake and improved plant growth. The molecular approaches signify a vital role in understanding the genetic aspects of host fungal interactions. Among several molecular approaches, the cloning and gene sequencing methods are most promising and consent to determine which techniques are time-consuming. Nucleic acid hybridizations and probing techniques are required to possess sufficient knowledge of microbial community. However, other molecular methods such as amplified ribosomal DNA restriction analysis or ribosomal intergenic spacer analysis can be employed for the endophytic fungal colonizations.

3.10 Future Applications of Endophytic Fungal Phosphate Solubilizers

In recent times, usage of phosphate fertilizers is highly expensive that cannot be afforded by farmers, particularly in developing countries. Scientists thus have a great responsibility toward society to find some innovative ways from biological sources to make available “P” to crop plants, by an economically efficient alternative for chemical fertilizers. Most of the soils are deficient in available phosphorus to plants and chemical fertilizers are expensive. Due to this interest has been developed in the application of rhizosphere microbes and endophytic fungi with phosphate-solubilizing capacity as bio-inoculants to solubilize phosphate from poorly available sources in soil. Although the potentiality increased for developing such inoculants, their vast applications remain incomplete by intricacy in an understanding of microbial inoculants. These endophytic fungi not only enhance the phosphate availability to the plants but also provide protection to the plants against plant pathogens and stimulate plant growth. The major challenge associated with endophytic fungi is a commercial-scale application, in fact, managing microbial communities to favor plant colonization by beneficial endophytic microorganisms. The contribution of endophytic fungal research may have environmental and economic impacts. Molecular-level research in this aspect is necessary for a better understanding of host endophytic interaction.