Effects of Ag nanoparticles on plant growth, Ag bioaccumulation, and antioxidant enzyme activities in Phragmites australis as influenced by an arbuscular mycorrhizal fungus

Ag nanoparticles (AgNPs) are considered an emerging contaminant in recent years, and their harmful effects on plants pose new concerns, especially in coexistence with soil microorganisms. Arbuscular mycorrhizal fungi (AMF), as mutualistic fungi with most terrestrial plants, may contribute to alleviating nanotoxicity in plants. Herein, AgNP toxicity of different concentrations (1, 5, 10, 50, 100 mg/kg) on reed (Phragmites australis (Cav.) Trin. ex Steudel) as influenced by mycorrhizal inoculation with Funneliformis mosseae was investigated. The results revealed that concentration is the main factor influencing the AgNP phytotoxicity; AgNP dose had biphasic effects on AMF colonization, plant biomass, and antioxidant enzyme activities. Thereinto, different antioxidant enzymes had different tolerances to AgNP stress, and the turning point of their activities was respectively the following: POD-5 mg/kg < SOD-10 mg/kg < CAT-50 mg/kg. The growth configuration (root:shoot ratio) of Phragmites australis increased firstly and then decreased to cope with the increasing AgNP concentration. Additionally, the Ag accumulation and translocation of AgNP-exposed plants were relatively lower than that of equivalent Ag+-exposed plants. However, AMF inoculation improved plant antioxidant capability and biomass growth in response to AgNP-induced toxicity. Meanwhile, AMF effectively regulated the root:shoot ratio to accommodate AgNP stress. The linear model fittings and heat maps showed that the mycorrhizal plants exhibited a higher Ag accumulative rate and root partitioning (Ag organ distribution: root > stem > leaf) than the non-inoculated plants. Overall, our results demonstrated that AMF could diminish the negative effects induced by AgNPs and promote Ag immobilization in plant roots so as to alleviate AgNP-posed environmental risks.