Laccaria bicolor Mobilizes both Labile Aluminum and Inorganic Phosphate in Rhizosphere Soil of Pinus massoniana Seedlings Field-grown in a Yellow Acidic Soil

Plant growth is often limited by high-activated aluminum (Al) and low available phosphorus (P) in acidic soil. Ectomycorrhizal (ECM) fungi could improve their host plants Al-tolerance by increasing P availability while decreasing Al activity in vitro or in hydroponic/sand culture systems. However, the role of ECM fungi on inorganic P (IP) and labile Al in acidic soil in the field, particularly under Al treatment, remains poorly understood. The present study aimed to determine the influence of ECM fungal association on the mobilization of IP and labile Al in rhizosphere soil of the host plant grown in the field under external Al treatment and the underlying nutritional mechanism in plant Al-tolerance. In doing so, 4-week-old Pinus massoniana seedlings were inoculated with three ECM isolates (Laccaria bicolor 270, L. bicolor S238A and L. bicolor S238N), respectively, and grown in a Haplic Alisol field with or without Al treatment for 12 weeks. Results showed that L. bicolor association enhanced the available P depletion and facilitated the mobilization of IP and labile Al, in turn improving the capacity of host plant to use Al-bound P (Al-P), Ca-bound P (Ca-P), and occluded P (O-P), particularly when P. massoniana seedlings were inoculated with L. bicolor S238A. Inoculation with L. bicolor isolates also enhanced the solubility of labile Al and facilitated the conversion of acid-soluble Al into exchangeable Al. Our findings suggested that ECM inoculation could enhance plant Al-tolerance in the field by mobilizing IP to improve the P bioavailability, but not by decreasing Al activity. IMPORTANCE Here, we reveal the underlying nutritional mechanism in plant Al-tolerance conferred by ectomycorrhizal (ECM) fungus inoculation in the field and screen a promising ECM isolate to assist the phytoremediation and afforestation using Pinus massoniana in acidic soil in south China. This study advances our understanding of the contribution of ECM fungi in the plant-ECM fungus symbiosis and highlights the vital role of ECM fungus inoculation in plant Al-tolerance. In addition, the results described in the present study confirm the importance to carry out studies in field compared to the previous results from in vitro study. Our findings strengthen the role of ECM fungus association in exploring, utilizing and transporting the unavailable nutrients in the soil to enhance the host plant growth and adaptability in response to adverse habitats.