Origin and cohabitation of arbuscular mycorrhizal fungi matter: forest-derived Gigaspora species are promising candidates for bioinoculant development in forest tree production

Arbuscular mycorrhizal fungi (AMF) are potential bioinoculants to grow healthy plants in healthy soils. Although they are generalist, plant – AMF associations work along a mutualism-to-parasitism continuum where currently unresolved host – symbiont compatibility overrides the versatility of AMF functions. Here, we hypothesized that habitat, origin, and cohabitation of AMF determine compatibility and functionality in the associations. To test the hypothesis, we established two forest-inhabiting AMF isolates under Cryptomeria japonica (Cupressaceae) and inoculated them into C. japonica seedlings grown in a controlled environment. We identified the isolates as Gigaspora rosea LFB-4 and G. margarita LFB-A1 using morphological, molecular, and phylogenetic analyses. They simultaneously developed multiple germ tubes during germination, showed pre-symbiotic sporulation, erratic root colonization, and produced spores inside host root cells. When compared to controls, G. rosea and G. margarita significantly promoted the growth of C. japonica seedlings, with synergistic effects in their cohabitation. While G. rosea boosted water uptake and height growth, G. margarita improved biomass production. Together, they encouraged carbon release into the soil and delayed root growth, increasing shoot-to-root biomass ratio for faster seedling growth. We concluded that, despite erratic root colonization, forest-derived G. rosea and G. margarita worked synergistically to improve the growth of C. japonica seedlings by modulating root development. We proved that beyond taxonomic affiliation, habitat, origin, and cohabitation of AMF matter in plant – AMF compatibility for mutual benefits. Our findings imply that forests are home to potent AMF bioinoculants such as beneficial Gigaspora species and diversity improves plant – AMF associations.