Insights into the synergistic mechanisms of citric acid in detoxifying cadmium in tall fescue.

Tall fescue (Festuca arundinacea) can potentially be utilized as an accumulator species for cadmium (Cd) phytoremediation, whereas Cd toxicity can be alleviated by citric acid (CA). This study systematically explored the mechanisms of CA in reducing the toxicity of Cd to tall fescue from morph-physiological, biochemical, ecological and bioinformatics perspectives. Results showed that CA enhanced energy via the tricarboxylic acid cycle and increased antioxidants through the phenylpropanoid and amino acids metabolic pathways, thereby stabilizing photosynthesis and improving Cd tolerance in tall fescue. Morphologically, CA widened root cell wall by increasing lignin, hemicellulose and pectin, leading to enhanced Cd adsorption and restricted Cd migration. Fungal nutrient types were specifically recruited, including 34 Cd-resistant fungi contributing to detoxify Cd. CA-altered soil environments interacted with root exudates, which chelated Cd and showed a strong correlation with Cd-resistant fungi. Synergistic interactions through five pathways for CA-mediated Cd detoxification among root exudates, soil factors, and microbes were disclosed, with fungi contributing the most (17.6 %). Collectively, this study revealed the CA alleviated Cd toxicity through a synergistic interaction network across multiple dimensions. The identified metabolites, microbes and soil factors could serve as indicators for future research on Cd detoxification in other crops or perennial grasses.