A diverse range of microorganisms in soil play critical functions, such as nutrient acquisition, organic matter cycling, soil and plant health maintenance, soil restoration and ecosystem primary production, and are thus considered as beneficial organisms [1]. By increasing crop yield, quality, and shielding the plants from various biotic and abiotic challenges, these microbes have shown several positive effects on the farming system. In addition, environmental protection, an increasing concern across the globe, is the main advantage these creatures provide [2]. Moreover, the rising demand and growing awareness of high-quality food has stimulated the development of sustainable and environmentally friendly agricultural production practices. These methods could be achieved by reducing the usage of chemicals and encouraging the application of beneficial microorganisms [3]. Apart from contributing to sustainability, beneficial microorganisms are also economically efficient [4]. These microorganisms can be symbiotic or non-symbiotic bacteria, actinomycetes, and mycorrhizal and endophytic fungi [5]. Throughout the study of various microorganisms, an emphasis is placed on studying endophytic fungi, as they can yield tangible beneficial effects within the host plant, such as improve plant growth, quality, and enhance host resistance to abiotic and biotic stresses, thereby confirming their significance in the agricultural sector ^[6][7][6,7].

Serendipita indica, formerly known as Piriformospora indica, belonging to the order Sebacinales (Basidiomycota), is one of the beneficial endophytic fungi known to possess numerous advantages and has been studied extensively for decades. S. indica, obtained by Ajeet Verma from the roots of Prosopis juliflora and Zizyphus nummularia in the Thar desert in Northwest India, is characterised by the formation of pear-shaped spores known as chlamydospores [8]. These spores are produced by thin walled, hyaline, and septate hyphae [9]. S. indica is distinguished by the unique trait that it can be cultivated without any plant material on a variety of artificial media, such as aspergillus medium, modified aspergillus medium, potato dextrose agar (PDA) or broth (PDB), malt extract, modified Melin-Norkrans (MMN) medium [10], and jaggery containing medium [11]. This property differentiates it from arbuscular mycorrhizal fungi (AMF), with which it shares biological similarities (Figure 1).

This endophytic fungus can colonise a wide range of plant species and develop symbiotic associations with them ^[12][13][12,13]. Mainly, colonisation takes place in the root zone and begins with the germination of chlamydospores, followed by the formation of a hyphal network on and inside the root. Hyphae form branches and continue to grow by penetrating the subepidermal layers of roots, and eventually, cover the rhizodermal and cortical cells (Figure 2). Mature root segments show intra- and intercellular colonisation patterns, while conductive tissues are free from colonisation. This fungus colonises different root zones, with maximum colonisation in the zone of cell differentiation. In addition, the fungus penetrates root hair cells, forming hyphae from germinating spores ^[14][15][14,15] (Figure 3). This process begins with the biotrophic phase, followed by the cell death phase, to build the symbiotic relationship with the host rhizosphere ^[16][17][16,17].

Moreover, a successful symbiosis between S. indica and the host plant depends on administering the right quantity of inoculum, which can be assessed using a qPCR test [18]. According to Rokni et al. [19], optimising the S. indica inoculum concentration had an impact on the host plant’s development. Plant growth was enhanced by a 1–3% w/w concentration; however, higher doses did not show any positive benefits.

The production of calcium and lectin protein kinase induced during symbiotic association are suggested to be the early signalling factors in S. indica–host plant colonisation ^[20][21][20,21]. Additionally, the synthesis of the plant hormone ethylene is considered crucial for the interaction between plant hosts and fungi [22].

S. indica has been reported to provide numerous advantages to host plants upon colonisation of roots. These advantages include enhanced plant growth, nutrient uptake, and antioxidant activities; increased photosynthetic pigments, crop quality and yields; as well as the ability to mitigate various biotic and abiotic stresses ^{[3][23][24][25][26][27][28]}[3,23,24,25,26,27,28]. Although mycelia and spores from fungi are typically considered the beneficial form of inoculum, some studies have demonstrated that cell wall extracts and culture filtrates from S. indica have a positive effect on plant growth ^[29][30][29,30].

Furthermore, to commercialise S. indica, biofertiliser formulation, known as ‘rootonic’, has been developed by culturing this fungus in a bioreactor ^[12][31][32][12,31,32] and mixing with carrier magnesium sulphite [33]. This endophytic fungus is proving to be an efficient source of bio-inoculant for the agricultural sector, with the least environmental hazards and improved agricultural sustainability.

2. S. indica as a Growth Promoter

S. indica has been found to be a prime beneficial microorganism that improved the growth and development of various plant species under normal and stress conditions ^[3][28][3,28] (Figure 4). This endophytic fungus improved the germination rate in various plant species, such as chilli, cabbage, cucumber, eggplant, maize, okra, spinach, rice, and tobacco ^{[11][34][35][36]}[11,34,35,36]. Improved seed germination under intense cold conditions in beetroot, carrot, cauliflower, onion, and radish has also been reported [17]. The application of S. indica has been reported to promote seedling growth and development by enhancing root and shoot growth, biomass, photosynthetic pigment production, and seedling vigour. ^[36][37][38][36,37,38]. Sheramati et al. [39] reported that S. indica improved the growth of Arabidopsis and tobacco seedlings by enhancing the nitrogen accumulation and expression of genes governing nitrogen reductase and the starch degrading enzyme glucan-water dikinase in their roots. These enzymes are responsible for nitrogen and starch metabolism in seedlings, which is essential for their growth and development. This fungus is reported to promote the plant vegetative development in terms of height, leaf number, shoot and root growth, fresh weight, dry weight, as well as photosynthetic pigment and phytohormone synthesis ^{[3][22][25][34][40][41]}[3,22,25,34,40,41]. Stimulation of nutrient uptake and their efficiency is also reported due to S. indica colonisation leading to improved plant growth, quality, and yield ^[25][42][25,42]. Improvement in other plant physiological attributes, such as inflorescence development, duration of flowering, flower size and number of flowers, have been reported with the application of the endophytic fungus S. indica ^{[25][30][34][43]}[25,30,34,43]. Therefore, S. indica promotes plant growth and development by inducing earliness of host plant reproduction [44]. Furthermore, S. indica improved crop yields by elevating chlorophyll content, flower setting, grain yield, and pod number and size in crops, such as black pepper [43], fennel [45], rice ^[44][46][44,46], rapeseed [25], sweet potato [47], sunflower [48], and tomato ^[49][50][49,50]. In addition to improving the growth and production of crops, S. indica has improved the quality of the products. This impact may be attributable to increased root growth and root hair formation in plants treated with S. indica, which results in improved mineral nutrition acquisition and water uptake ^[9][45][51][9,45,51]. Singhal et al. [24] further proposed that nutrient uptake promoted by this endophytic fungus could be attributable to fungal hyphae penetrating deeper into the soil than root hair and thus absorbing more nutrients from the soil. Assimilation of macronutrients, such as nitrogen, phosphorus, potassium, sulphur, and magnesium is improved in crops, such as Poncirus trifoliata, Triticum aestivum, Brassica napus, Oryza sativa, Panicum miliaceum, Arabidopsis thaliana, etc. ^{[7][25][46][52][53][54]}[7,25,46,52,53,54]. S. indica improves the availability of nitrogen by transforming the unavailable form into nitrate, which is taken up by plants [9]. Moreover, it stimulates the expression of the gene governing NADH-dependent nitrate reductase, which is responsible for nitrate assimilation in crops ^[9][55][56][9,55,56]. S. indica is able to convert organic phosphorus into plant-available form, thus improving the phosphorus uptake by plants under normal as well as under phosphorus-deficient conditions ^[17][57][58][17,57,58]. It also improves the acid phosphatase and alkaline phosphatase activities, leading to an improvement in phosphorus uptake in rice and rapeseed ^[58][59][58,59]. In addition to promoting soil phosphatase activities, S. indica upregulated the expression of the phosphate transporter genes (PT3, PT5, and PT6) in Poncirus trifoliata and thereby enhanced the phosphorus absorption by the plant [7]. S. indica colonisation improved the potassium concentration in tomatoes, resulting in higher levels of lycopene and ascorbic acid and therefore improving the quality of tomatoes [60]. Several studies have reported a significant increase in absorption of other macronutrients, such as calcium, magnesium, and sulphur ^{[7][25][61][62][63]}[7,25,61,62,63]. An increase in the absorption of magnesium and sulphur is suggested due to the enhanced expression of transporters PiMgT1 and PiSulT, respectively, in plants treated with S. indica ^[62][63][62,63]. Moreover, the application of S. indica improved the acquisition of iron and zinc in rapeseed [25] and zinc in lettuce and Brassica napus ^[64][65][64,65]. Furthermore, inoculation with S. indica increased iron content in sugarcane plants by enhancing iron transportation ^[33][66][33,66]. Therefore, the application of S. indica has proven to enhance nutrient uptake in plants, and thus is considered as an emerging plant growth promoter. Table 1 summarises the effects of S. indica on the growth and quality of different host plants.