The Comprehensive Effects of Rhizophagus intraradices and P on Root System Architecture and P Transportation in Citrus limon L.

Both arbuscular mycorrhizal fungi (AMF) and phosphorus (P) collectively influence the root system architecture (RSA), but whether the combination of the two affects RSA, particularly lateral root formation, is unknown. In the present study, a pot experiment was conducted to evaluate the effects of an arbuscular mycorrhizal fungus (Rhizophagus intraradices) on the RSA of lemon (Citrus limon L.) seedlings under 0 (P0) and 50 mg/kg (P50) P levels. Moreover, P and carbohydrate content; acid phosphatase activity; and the expression of P transporter genes (PTs), phosphatase genes (PAPs), and lateral-root-related genes; were determined. Our results show that root mycorrhizal colonization and mycorrhizal dependency of lemon plants are significantly higher under P0 than under P50 conditions. AMF significantly promoted the plant growth performance of lemon, irrespective of substrate P levels. The RSA parameters of AMF plants, including total root length, projected area, surface area, average diameter, volume, and second- and third-order lateral root numbers, were distinctly increased under the two P levels compared to those of non-AMF plants. Mycorrhizal treatment also induced higher carbohydrate (sucrose, glucose, and fructose) and P contents, along with a higher activity of root acid phosphatase. The expression of P-related genes, including ClPAP1, ClPT1, ClPT3, ClPT5, and ClPT7, as well as the expression of lateral-root-related genes (ClKRP6, ClPSK6, and ClRSI-1), was dramatically upregulated by AMF inoculation, irrespective of substrate P levels. Principal component analysis showed that root P and carbohydrate contents, as well as the expression of ClKRP6 and ClPSK6, were positively correlated with RSA traits and lateral root development. Our study demonstrates that mycorrhizas accelerate the P acquisition and carbohydrate accumulation of lemon plants by upregulating the expression of lateral-root-related genes, thereby positively improving the RSA. Furthermore, AMF had a greater impact on the RSA of lemon than substrate P levels.