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