Exogenous selenium restricts cadmium accumulation in edible shoots of flowering Chinese cabbage via the bHLH39-CAD5-mediated root lignification pathway

Cadmium (Cd), a highly toxic heavy metal, poses significant threats to agricultural productivity and human health by accumulating in the food chain. Selenium (Se), an essential micronutrient, has shown promise in mitigating Cd toxicity in plants. However, the underlying molecular mechanisms remain largely unknown. This study elucidates how exogenous selenite (Na 2 SeO 3 ) reduces Cd accumulation in the edible parts of flowering Chinese cabbage. Our results demonstrate that Se application (2.5 µM) dramatically mitigated Cd-induced growth inhibition, recovering shoot and root biomass to 96.3% and 90.7% of those of the control levels, respectively, while significantly decreasing Cd concentration in the shoots. Mechanistically, Se orchestrates a multi-layered defense strategy in the roots under Cd stress. Ultrastructural and physiological analyses revealed that, Se reduced the average net Cd 2 + influx by 24.3%, increased root cell wall thickness by 40%, and significantly boosted cell wall lignification under Cd stress. Furthermore, Se promoted Cd compartmentalization by shifting Cd from metabolically sensitive organelles to the soluble fraction under Cd stress. At the molecular level, transcriptome analysis combined with Weighted Gene Co-expression Network Analysis (WGCNA) identified a potential regulatory module. The transcription factor bHLH39 acts as a key repressor, directly binding to the promoter of the lignin biosynthesis gene CAD5 . Se co-application suppresses the Cd-induced expression of bHLH39 , thereby de-repressing CAD5 expression to promote lignification and physically restrict Cd translocation. This study identified the bHLH39 - CAD5 regulatory module as a potential mechanism underlying Se-mediated Cd detoxification, providing a strategy for improving low-Cd leafy vegetables crops to ensure food safety.