PBDEs in Rice Plants: Bioaccumulation, Translocation, and the Role of LHT Protein

This study investigated the bioaccumulation and translocation of five PBDE congeners (BDE-47, BDE-77, BDE-99, BDE-153, and BDE-209) in rice plants. The bioaccumulation of these PBDEs reached an apparent equilibrium in 10 days, with the average bioaccumulation concentration decreasing with increasing bromine substitution number, molecular weight, and octanol-water partition coefficient. Interestingly, the translocation behavior of PBDEs in rice plants exhibited a distinct time-dependent pattern. The root-to-shoot translocation factor (TF) values showed a continuous increasing trend over 21 days of exposure. This suggests that the translocation process from the underground part to the aboveground part was gradually facilitated, leading to continuous accumulation of PBDEs in shoots. Furthermore, the growth of rice shoots was significantly inhibited as the exposure duration prolonged, while the inhibition rates of root biomass peaked at 10 days, indicating tissue-related growth inhibition caused by PBDEs.

To identify the key transport proteins associated with PBDE translocation, RNA-seq-based transcriptomics was employed. Differentially expressed genes (DEGs) related to transport proteins, particularly amino acid transporters (AATs), were found to be associated with PBDE exposure. Among these, OsLHT1, OsAAP10, and OsLHT9 showed positive correlations with PBDE concentrations in shoots and roots, as well as TF values. The expression of these AAT genes was significantly enhanced in rice plants exposed to PBDEs, with a positive relationship observed between the TF of PBDEs and the expression of these genes, especially for OsLHT1 and OsLHT9. This suggests that LHT protein, with OsLHT1 as the key coding gene, could be the potential transporter for PBDEs.

To validate the involvement of LHT protein in PBDE translocation, a series of in vitro experiments were conducted. Fluorescence spectroscopy revealed that PBDEs caused conformational changes in LHT protein, affecting its physiological activity. Circular dichroism (CD) analysis further confirmed the conformational changes induced by PBDEs, with BDE-77 showing the strongest affinity to LHT. Sorption equilibrium experiments demonstrated that BDE-77 exhibited the highest sorption fraction and affinity to LHT, consistent with its remarkable translocation in plants.

Finally, OsLHT1 knockout rice plants were used to examine the effects of LHT protein on PBDE translocation. Compared to wild type plants, the knockout of OsLHT1 resulted in a significant reduction in PBDE accumulation in shoots and a decrease in root-to-shoot translocation, confirming the prominent carrier role of LHT for PBDEs.

To further understand the mechanisms underlying the difference in affinity between LHT and the five PBDE congeners, molecular docking and quantitative structure-activity relationship (QSAR) studies were performed. Molecular docking revealed that PBDEs, phenylalanine (Phe), and tyrosine (Tyr) all inserted into the same cavity of LHT, suggesting competitive binding. The binding free energy analysis showed that BDE-77 had a higher binding capacity with LHT than BDE-153 and BDE-99, indicating a limitation of hydrophobicity in interpreting ligand-receptor affinity. QSAR analysis identified the oxygen atom in PBDEs as a key structural parameter influencing their translocation. A strong positive correlation was found between the energy contribution of the oxygen atom in PBDEs and their TF, particularly for BDE-77, highlighting its crucial role in the energy matching of PBDEs and LHT.

In conclusion, this study provides compelling evidence that LHT protein acts as a carrier for the translocation of PBDEs in rice plants. The findings suggest that OsLHT1 could be a potential target for reducing PBDE accumulation in rice plants. Further research is needed to explore the molecular mechanisms underlying LHT-mediated PBDE transport and to develop strategies for mitigating PBDE contamination in rice and other food crops.