Auxin signaling related to H+-ATPase synthesis and antioxidant enzyme activities regulates fluoranthene uptake by ryegrass roots

Previous research has shown that fluoranthene (Flu) exhibits dual uptake behavior in ryegrass. At low concentrations (1-10 mg/L), Flu uptake is higher, whereas at higher concentrations (20-40 mg/L), uptake appears to decrease. Furthermore, indole-3-acetic acid (IAA) content and antioxidant enzyme activity play distinct roles in this process. However, the molecular mechanisms underlying these behaviors remain unclear. To address this, we exposed ryegrass to different Flu concentrations (0, 5, and 20 mg/L) and conducted a combined transcriptomic and physiological analysis of the root system to elucidate the specific mechanisms of Flu uptake. Our results revealed that under 5 mg/L Flu treatment, ryegrass has a higher bioconcentration factor (BCF). The genes involved in IAA synthesis (TAA1, ALDH, and AAO1/2) were upregulated, which led to an increase in IAA content. Elevated IAA levels, in turn, promoted the expression of genes encoding H⁺-ATPase (ATP5A1, ATP5B, ATP5H, and ATP6E) and the ABC transporter protein (ABCB1), resulting in enhanced H⁺-ATPase activity, and facilitated the active transport of Flu. In contrast, the 20 mg/L Flu treatment resulted in a lower BCF. The downregulation of IAA synthesis genes (amiE and YUCCA) decreased IAA content. The downregulation of the H⁺-ATPase gene (ATP6C) and the ABC transporter protein gene (ABCG2), resulting in decreased H⁺-ATPase activity and inhibited Flu transport. Moreover, the promoted expression of redox-related genes (POD1, SOD1 and SOD2) further reduced Flu uptake. Elucidating the molecular mechanisms underlying Flu uptake in ryegrass may provide a theoretical foundation for developing strategies to regulate Flu accumulation in plants.