Piriformospora indica belongs to the genus Piriformospora, the family Sebacinaceae, the order Sebacinales, the class Hymenomycetes, and the phylum Basidiomycota (Waller et al., 2005; Hibbett et al., 2007). It is a root endophytic fungus that was isolated by Indian scientists from the rhizosphere of woody shrubs while collecting spores of the arbuscular mycorrhiza (AM) fungus Glomus mosseae in the interior desert of Rajasthan, India (Verma et al., 1998). It is named P. indica due to its pear-shaped chlamydospores (Varma et al., 2012). It shares similarities with some AM fungi, such as G. mosseae and Gigaspora margarita (Chen et al., 2007; Venice et al., 2020). They are both endophytic fungi that can coexist with the host plant (Singh et al., 2000; Kesharwani et al., 2022), promote the absorption of mineral elements and cycling substances in the soil, and enhance the plant's resistance to biotic and abiotic stress (Xu et al., 2018; Bandyopadhyay et al., 2022). Compared to the AM fungus, P. indica has notable advantages, such as its ability to obtain nutrition from non-living sources and its wide range of host plants (Varma et al., 2001; Deshmukh et al., 2006; Oelmüller et al., 2009). Therefore, it has gained widespread attention since its discovery (Kaldorf et al., 2005; Gill et al., 2016; Su et al., 2017). P. indica can interact with various plant species, including Gymnospermae and Angiospermae (Khalid et al., 2019), as well as lower plants such as Bryophyta and Pteridophyta (Varma et al., 1999; Manzar et al., 2021a). Currently, it is known to colonize over 200 plants, including monocotyledonous plants such as Oryza sativa (Ghorbani et al., 2021), Triticum aestivum (Li et al., 2022), Hordeum vulgare (Alikhani et al., 2013), and Zea mays (Xu et al., 2017), and dicotyledonous plants like Nicotiana tabacum (Hui et al., 2015), Passiflora edulis (Yan et al., 2021), Solanum tuberosum (Fakhro et al., 2010), and Cicer arietinum (Narayan et al., 2017). It can even colonize Arabidopsis thaliana, a cruciferous model plant that AM fungi cannot colonize (Abdelaziz et al., 2017; Manzar et al., 2021b). Many scholars have studied the colonization of P. indica in various plants, focusing on its effects on promoting host plant growth, improving disease resistance, and enhancing stress tolerance, including resistance to drought, cold, heat, salt, and flooding stresses (Ghabooli et al., 2013; Amani et al., 2021). However, research on the basic biological characteristics of P. indica, such as its growth dynamics on different culture media, has been limited and has only been focused on screening suitable media at a macro level. There have been no reports on the micro-metabolic phenotype profiling of P. indica up to now.

The metabolic phenotypes include the ability to utilize carbon, nitrogen, phosphorus, and sulfur sources, as well as growth tests under different osmotic pressure and pH conditions (Mazur et al., 2013; Narware et al., 2023). The Phenotype Microarray (PM) technology, developed by Biolog Company in the United States, can be used to determine various metabolic phenotypes of microorganisms. The PM is a novel technology that is parallel to genomics and proteomics and is widely used in the study of various microorganisms (Bochner, 2003). It involves fixing a dried cell culture medium and a colorimetric substance in each well of a 96-well microplate (PM plate). After adding microbial cell suspension and incubation, the cell metabolic phenotype can be observed through color changes. Finally, the dynamics of phenotypic reactions are obtained, and the strength of phenotypic reactions can be quantitatively measured (Li and Lu, 2007).

Based on these premises, this study aimed at screening the most suitable growth medium and sporulation medium for P. indica by analyzing its growth dynamics on five commonly used artificial culture media. The goal is to quickly provide the required biomass and spore quantities for production or scientific research. Additionally, the metabolic phenotype of P. indica was assessed using the Biolog PM technology, aiming to identify its characteristic nutrients and reveal its adaptability under different osmotic pressure and pH environments. This information will help to clarify the physiological reasons behind its enhancement of plant stress resistance and provide a theoretical basis for the development and utilization of P. indica as a biocontrol agent.

P. indica possesses most of the functions of AM fungi for host plants, but the key difference is its ability to be cultured in vitro. In this study, we monitored the growth dynamics of P. indica on five common microbial culture media, including four fungal culture media and one bacterial culture medium. According to a regression equation that was derived to describe the relationship between colony diameter and growth days on different culture media, P. indica showed the fastest hyphal growth rate in the PDA culture medium. Previous studies have mainly used PDA medium and Kaefer medium (Kumar et al., 2011; Kashyap et al., 2023) for P. indica cultivation. However, the Kaefer medium, despite being considered optimal for growth, sporulation, and preservation of P. indica, is complex to prepare and expensive due to its requirement of more than 20 components. This poses challenges for its widespread application in laboratory or production settings (Attri and Varma, 2018; Kumar et al., 2023). Our study findings indicate that although PDA culture medium could induce spore production of P. indica, the sporulation time was long and the spore yield was low, making it difficult to meet the needs of strain preservation, metabolic phenotype testing, or other basic research. In contrast, this study demonstrates that the OA culture medium is the most suitable for sporulation, with the shortest sporulation time and the highest spore yield. The optimum medium for sporulation is coincident with some fungi, for instance, Epicoccum latusicollum (Li et al., 2023a) and Valdensinia heterodoxa (Zhao and Shamoun, 2006), but it is contrary to other fungi, including Alternaria alternata (Masangkay et al., 2000). OA culture medium, made from simple and inexpensive raw materials, contains a suitable carbon-to-nitrogen ratio and abundant vitamins and minerals. At the same time, the nutrients, especially nitrogen sources, in OA are not as abundant as those in PDA, YD, AEA, etc., making it favorable for the spore production of P. indica. The most suitable medium for P. indica can be used to provide the required biomass and spore quantity quickly for production or scientific research.

As a consequence, we employed an OA culture medium for subsequent Biolog PM metabolic phenotype experiments on P. indica. Compared with traditional microbial metabolism measurement methods, this high-throughput metabolic phenomics technology offers the advantages of high throughput, fast speed, and high accuracy (Khatri et al., 2013). It can dynamically reflect the biochemical processes of microbial cell metabolism in real-time (Kashyap et al., 2021). Biolog PM metabolic phenotype experiments have been used in the metabolic phenotype research of various microorganisms, including Helicobacter pylori (Lee et al., 2017), A. alternate (Wang H. C. et al., 2015), and Phytophthora parasitica (Wang M. S. et al., 2015). Our study obtained metabolic phenotype information from P. indica on various metabolic substrates and different environmental conditions, revealing a wide metabolic range of available carbon sources (87.89%), which is much higher than that of some plant pathogens, such as Botrytis cinerea (Wang et al., 2018), Rhizopus oryzae (Li et al., 2023b), and E. latusicollum (Li et al., 2023a). The available carbon sources accounted for 17%, 54.21%, and 61.58% individually. This characteristic allows P. indica to be co-cultivated with plant seedlings, colonizing their roots during the seedling stage. At this stage, due to the limited types of carbon sources in plants, some pathogenic bacteria are difficult to grow normally. However, P. indica can utilize a wider range of carbon sources, which enhances its growth and spore production. After successful colonization, P. indica has great potential to enhance the plant's disease resistance. In the carbon sources of PM1-2, it is interesting that some of the carbon sources that cannot be metabolized by P. indica are isomers of metabolizable substrates, for example, α-Methyl-D-Galactoside cannot be utilized, while β-Methyl-D-Galactoside can be moderately utilized. However, isomers of some substrates, such as α-Methyl-D-Glucoside and β-Methyl-D-Glucoside, can be moderately utilized.

Furthermore, P. indica exhibits a relatively extensive range of metabolizable nitrogen sources (87.63%), including amino acids, dipeptides, or tripeptides, which is higher than that of some plant pathogens, such as B. cinerea (63% of available amino acid nitrogen substrates and 80% of available peptide nitrogen substrates) (Wang et al., 2018). Its metabolic ability on phosphorus sources (96.61%) and sulfur sources (100%) is significantly higher than that of certain plant pathogens. For instance, Pseudomonas syringae showed active metabolism with 10.17% phosphorus sources and metabolized none of the tested sulfur sources (Guo et al., 2017; Manzar et al., 2022a). It is worth noting that the sporulation rate of P. indica was 70.66% among all available carbon sources, while among all available nitrogen sources, only specific substances, such as Adenosine, D, L-Lactamide, Xanthosine, Ethanolamine, Ethylamine, and N-Butylamine induced spore production. This may be attributed to excessive nitrogen sources in the nitrogen source metabolism experiments, resulting in promoting mycelium growth but hindering sporulation. Therefore, it is necessary to select suitable types of nitrogen sources and control the appropriate carbon-to-nitrogen ratio to facilitate the production of P. indica spores in cultivation.

P. indica demonstrated a wide range of tested biosynthetic pathways, reflecting its ability to utilize various substrates to produce complex products or to metabolize complex substrates into simpler products. These biosynthetic pathways may play a crucial role in its life activities. For example, they are involved in the production of energy, such as the tricarboxylic acid cycle, and the synthesis of substances such as amino acids, nucleotides, and fatty acids. The diversity of biosynthetic pathways allows microbes to utilize substrates and energy more efficiently, thereby increasing resilience and competitiveness. The wide range of biosynthetic pathways in P. indica coincides with its powerful capability of substrate utilization. P. indica can grow in various tested osmotic environments, but it cannot grow under the conditions of 20–200 mM sodium benzoate (pH 5.2). The osmotic pressure adaptability of P. indica is stronger than that of some pathogens, such as R. oryzae could not survive in 3–6% sodium formate, 4–7% urea, and 20–200 mM sodium benzoate (pH 5.2). The extensive osmotic pressure adaptability of P. indica allows it to grow normally under various conditions of dryness and watering (Wang H. C. et al., 2015). P. indica had a wide range of pH tolerance, with the activity of decarboxylase being higher than that of deaminase. Decarboxylase of the microbe generates alkaline amines by the catabolism of amino acids, which help counteract the low pH condition (Maurer et al., 2005). Deaminase generates ketonic acid through the catabolism of amino acids, which helps counteract high pH. The deaminase activity is significantly higher than that of A. alternata (Wang H. C. et al., 2015). Additionally, P. indica could produce spores in most of the growable osmotic pressure and pH conditions. Obviously, P. indica has a wide range of environmental adaptability, which is consistent with its ability to colonize a variety of plant hosts (Qiang et al., 2012; Manzar et al., 2022b; Mahawer et al., 2023).

The growth dynamics and the Biolog PM metabolic phenotype information obtained in this study for P. indica can be used to optimize the culture conditions, spore formation, and germination conditions of artificial culture media for P. indica. These findings aid in understanding its colonization ability on various plants and enhance the host's resilience, enabling better cultivation and utilization of its colonization on various plants and maximizing its function in enhancing resistance to plant diseases and stress after colonization. Simultaneously, it provides a reference for the metabolic phenotype of other species within the genus Piriformospora.

This study indicates that among the five commonly used microbial media tested for P. indica, PDA was found to be the most suitable medium for mycelium growth, while OA was the most suitable medium for spore production. Biolog PM1-10 was used to assess the metabolic phenotype of P. indica, revealing its ability to metabolize 87.89% of the tested carbon sources, 87.63% of the tested nitrogen sources, 96.61% of the tested phosphorus sources, and 100% of the tested sulfur sources, with 92 kinds of tested biosynthetic pathways. P. indica could grow under 92 kinds of test osmotic pressures and 88 kinds of test pH conditions, indicating a wide range of substrate metabolism in terms of carbon, nitrogen, phosphorus, and sulfur sources, as well as a wide range of environmental adaptability. This study provides a theoretical basis for screening more suitable growth and sporulation conditions for P. indica, elucidating its adaptive mechanisms in symbiosis with plants, understanding the physiological reasons behind enhancing the resistance of host plants, and establishing preferable application strategies.

The original contributions presented in the study are included in the article/Supplementary material, further inquiries can be directed to the corresponding authors.

J-rH: Conceptualization, Funding acquisition, Software, Writing – original draft, Writing – review & editing. J-mL: Conceptualization, Writing – original draft. H-yW: Conceptualization, Writing – original draft, Software. M-lS: Data curation, Writing – original draft. C-yH: Conceptualization, Writing – original draft. H-cW: Writing – original draft, Methodology, Conceptualization.