Abstract
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.
Highlights
Root systems are the primary part of a plant; they perceive changes in the soil environment and are the major absorptive organ responsible for supporting shoot growth and development, as they acquire mineral nutrients and water from the soil [1]
Mycorrhizal colonization was not observed in the roots of non-arbuscular mycorrhizal fungi (AMF)-inoculated seedlings
AMF upregulated the expression of LR-related genes, contributing to LR formation and root system architecture (RSA) improvement
Summary
Root systems are the primary part of a plant; they perceive changes in the soil environment and are the major absorptive organ responsible for supporting shoot growth and development, as they acquire mineral nutrients and water from the soil [1]. Roots are important for plants, providing access to available resources, as evidenced by their accelerated growth and increased length in nutrient-rich areas, as well as by the spatial distribution of the length, root angle, and branching of the well-constructed root system architecture (RSA) [2]. The addition of P at low concentrations has been observed to stimulate the root growth and development of tobacco, dependent on plant genotypes and substrate P levels [11]
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