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.

Arbuscular mycorrhizal fungi (AMF) have the function of promoting water absorption for the host plant, whereas the role of easily extractable glomalin-related soil protein (GRSP), an N-linked glycoprotein secreted by AMF hyphae and spores, is unexplored for citrus plants. In this study, the effects on plant growth performance, root system characteristics, and leaf water status, along with the changes of mineral element content and relative expressions of tonoplast intrinsic protein (TIP) genes in lemon (Citrus limon L.) seedlings, were investigated under varying strengths of exogenous EE-GRSP application under potted conditions. The results showed that 1/2, 3/4, and full-strength exogenous EE-GRSP significantly promoted plant growth performance, as well as increased the biomass and root system architecture traits including root surface area, volume, taproot length, and lateral root numbers of lemon seedlings. The four different strengths of exogenous GRSP displayed differential effects on mineral element content: notably increased the content of phosphorus (P) and iron (Fe) in both leaves and roots, as well as magnesium (Mg) and zinc (Zn) content in the roots, but dramatically decreased the content of calcium (Ca) and manganese (Mn) in the roots, as well as Zn and Mn in the leaves. Exogenous EE-GRSP improved leaf water status, manifesting as decreases in leaf water potential, which was associated with the upregulated expressions of tonoplast intrinsic proteins (TIPs), including ClTIP1;1, ClTIP1;2, ClTIP1;3, ClTIP2;1, ClTIP2;2, ClTIP4;1, and ClTIP5;1 both in leaves and roots, and TIPs expressions exhibited diverse responses to EE-GRSP application. It was concluded that exogenous EE-GRSP exhibited differential responses on plant growth performance, which was related to its strength, and the effects were associated with nutrient concentration and root morphology, especially in the improvement in water status related to TIPs expressions. Therefore, EE-GRSP can be used as a biological promoter in plant cultivation, especially in citrus.