Inter- and intraspecific fungal diversity in the arbuscular mycorrhizal symbiosis

The 450-million-year-old arbuscular mycorrhizal (AM) symbiosis plays a critical role for the nutrient uptake and abiotic (drought, salinity, and heavy metals) and biotic stress resistance of the majority of land plants. The fungal extraradical mycelium takes up nutrients, such as phosphate and nitrogen, and delivers them to the intraradical mycelium, where the fungus exchanges these nutrients against carbon from the host. It is known for decades that AM fungi can improve the nutrient acquisition of many important crops under low input conditions and are able to increase plant productivity in stressful environments. However, despite their application potential as biofertilizers and bioprotectors, AM fungi have so far not been widely adopted. This is mainly due to the high variability and context dependency of mycorrhizal growth and nutrient uptake responses that make benefits by AM fungal communities difficult to predict. In this review, we summarize our current understanding of interspecific and intraspecific fungal diversity in mycorrhizal growth benefits and discuss the role of fungal genetic variability and host and fungal compatibility in this functional diversity. A better understanding of these processes is key to exploit the whole potential of AM fungi for agricultural applications and to increase the nutrient acquisition efficiency and productivity of economically important crop species.