Co-expression of alnA and bphC in alfalfa enhances PCB phytoremediation by coupling bioavailability modulation and intracellular detoxification.

Phytoremediation of polychlorinated biphenyls (PCBs)-contaminated soils is hindered by low PCB bioavailability and insufficient plant detoxification. Here, we engineered transgenic alfalfa co-expressing alnA (encoding an emulsifying protein) and bphC (encoding a ring-cleavage dioxygenase) to address these barriers. Five representative congeners spanning a range of chlorination degrees were selected to evaluate the system's efficacy. The transgenic AB lines achieved sustained rhizosecretion of AlnA, enhancing PCB solubility by 2.0- to 3.2-fold relative to wild-type (WT) plants. While enhanced bioavailability typically exacerbates phytotoxicity, endogenous BphC cleaved the biphenyl ring, preventing toxic intermediate accumulation. This extracellular solubilization-intracellular detoxification synergy enabled AB alfalfa to maintain improved growth under PCB stress, increasing root accumulation by 3.7-fold and overall removal by 2-fold compared to WT alfalfa. Metagenomic and structural equation modeling revealed that while microbial genes act as "proximate executors" directly degrading PCBs, the transgenic plant functions as the "primary driver". The AB lines enriched specific degraders (Pseudomonas and certain genera within the order Burkholderiales) and assembled a functional network for aerobic PCB degradation. This integrated plant-microbiome system offers a low-maintenance strategy for in situ remediation of PCB-contaminated sites.