Abstract: Mycorrhizal fungi are crucial components of plant microbiota and key players in terrestrial biogeochemical cycles. Arbuscular mycorrhizal (AM) fungi can form symbiotic relationships with more than 70% of plants and their origin can be traced back to the Devonian period 460 million years ago. They play an important role in the transition of plants from aquatic to terrestrial and profoundly affect the growth performance of plants and ecosystem functions. In the peri-arbuscular space of AM symbiosis, plants provide carbon fixed by photosynthesis in exchange for minerals, especially phosphorus and nitrogen. In the hyphosphere, many bacteria are involved in AM symbiosis. They obtain carbon from extraradical hyphal exudates and compensate for the limited saprophytic capacity of AM fungi by mineralizing organic compounds to enhance fungal mineral availability. Therefore, plants, AM fungi and hyphospheric bacteria form a continuum and are accompanied by top-down carbon flows and bottom-up nutrient flows. In this review, we first introduce the latest research progress on how plants, AM fungi and their related hyphospheric bacteria exchange carbon from host plants and minerals from the soil. These exchanges provide energy for microbial partners and deliver nutrients for plants that are necessary for growth and development. Secondly, we analyze in detail the mechanism by which the plant-AM fungi-bacterial continuum maintains cross-kingdom cooperation, which is conducive to a better understanding of the complex ecological relationships among plants, AM fungi and soil bacteria and their evolutionary significance.

Key words: Plants, arbuscular mycorrhizal fungi, hyphospheric bacteria, carbon and mineral flow, cross-kingdom cooperation