Carbon-phosphorus reciprocal mechanism for plants to acquire nutrients through the root/mycorrhizal pathway

Plants have evolved multiple strategies for obtaining nutrients from soil. They can absorb mineral nutrients directly via root epidermal cells and root hairs (the root pathway), and also obtain mineral nutrients, especially P, by forming mutualistic symbionts with arbuscular mycorrhizal (AM) fungi (the mycorrhizal pathway). The hyphosphere is defined as the narrow region of the soil around the hyphae where the physical, chemical, and biochemical conditions are different from the bulk soil due to the influence of hyphal exudates. Many microbes with phosphate-solubilizing function colonize in the hyphosphere and help AM fungi to mineralize soil organic phosphorus and solubilize non-soluble inorganic phosphorus, improving the efficiency of P uptake via the mycorrhizal pathway by plants. To effectively obtain P, plants allocate a large amount of photoassimilate to roots or mycorrhizal fungi. Therefore, plants need to make trade-off between the root pathway and the mycorrhizal pathway based on carbon input and P benefits. In this review, we systematically reviewed the carbon-phosphorus reciprocity of nutrient acquisition via mycorrhizal pathway by plants. The effects of P supply levels in soil on plant carbon allocation strategies during roots/mycorrhizal access to phosphorus, and the response of AM fungi extraradical hyphae to soil nutrient heterogeneity are discussed. The molecular mechanism of the regulation of phosphorus homeostasis in the hyphae and the mycorrhizal phosphorus supply mechanism of AM fungi recruiting P-solubilizing microbes through hyphal exudates in the hyphosphere are summarized. Future research should use new technologies and methods to quantify carbon allocation and P uptake in the root/mycorrhizal pathway, and to clarify the functions and roles of key components of hyphal exudates in regulating the bacterial community in the hyphosphere.