**2. Arbuscular mycorrhizal fungi (AMF)**

AMF are significant elements of the soil–plant system [13]. Mycorrhizae from the Greek "*myco*" for fungus and "*rhize*" for root essentially refers to the symbiotic association between fungi and plants' roots. AMF constitute 5 to 50% of the microbial biomass of soils. Mycorrhizal hyphae biomass can vary from 54 to 900 kg per hectare [39], or nearly 200 meters of hyphae per gram of soil [40]. AMS is the most widespread and oldest terrestrial symbiosis [41], formed by more than 80–90% of terrestrial vascular plants [14]. AMF inhabit all continents, from the subarctic islands to the Antarctic Peninsula [19, 42]. According to Wang et al. [9], AMF has co-evolved with plants for at least 400 million years, allowing plants the colonization of lands by plants through improved hydro-mineral nutrition.

AMF belong to the phylum of Glomeromycota [43], with a taxonomic classification of AMF originally based on morpho-anatomical observations of spores [44]. However, the advances in biomolecular tools have allowed the use of Polymerase Chain Reaction (PCR) in the classification of AMF through amplification of ribosomal regions (18S), which permits a better definition of species or even molecular taxa [45]. More than 250 species of Glomeromyceta are currently described, with a constantly updated taxonomy updated, resulting in new species and higher taxa being regularly introduced [46, 47]. AMF are obligate symbiont because of their inability to develop without a host plant [48], with a reproductive system that can be either clonal or asexual through spore or coenocyte formation [49]. Several studies have shown the existence of hyphal fusions, called anastomoses which lead to exchanges of nuclei and cytoplasm between species of the same genus [50–52], hence participating in the conservation of diversity and the complex genetics of AMF [53]. Some AMF species are homokaryotic, with identical nuclei in each spore; thus, the genetic variation is present in each spore, resulting in several different copies of the same gene [54]. Heterokaryotic species are characterized by different nuclei in each spore, resulting in the distribution of the genetic variation among the different nuclei inside a spore. In other words, the existence of several different genomes in each spore helps AMF to adapt to different environments [55]. Furthermore, there are four phases in the life cycle of AMF [56]: the spore germination and hyphae growth in the rhizosphere phase, the root infection phase

by hyphae, stimulated by carbon dioxide (CO2), and root exudates, which propagates in the root, the phase of root colonization with the formation of arbuscules and vesicles, and the phase of the development of external hyphae, resulting in an increased volume of soil explored by the roots and production of spores [57].
