Methods for assessing the quality of AM fungal bio-fertilizer: Retrospect and future directions

In the recent past, the mass production of arbuscular mycorrhizal (AM) fungi has bloomed into a large biofertilizer industry. Due to their obligate symbiotic nature, these fungi are propagated on living roots in substrate-based pot cultures and RiTDNA in in vitro or root organ culture systems. The quality assessment of AM inocula remains critical for the production and efficacy evaluation of AM fungi. The vigour of AM inocula are assessed through microscopic methods such as inoculum potential, infectivity potential/infection units, most probable number (MPN) and spore density. These methods marginally depend on the researcher's skill. The signature lipids specific to AM fungi, e.g. 16:1ω5cis ester-linked, phospholipid, and neutral lipid fatty acids provide more robustness and reproducibility. The quantitative real-time PCR of AM fungal taxa specific primers and probes analyzing gene copy number is also increasingly used. This article intends to sensitize AM fungal researchers and inoculum manufacturers to various methods of assessing the quality of AM inocula addressing their merits and demerits. This will help AM producers to fulfil the regulatory requirements ensuring the supply of high-quality AM inocula to end-users, and tap a new dimension of AM research in the commercial production of AM fungi and its application in sustainable plant production systems.