MicroRNA regulation and physiological adaptation of Brassica juncea to cadmium toxicity

Brassica juncea is a promising candidate for the phytoremediation of heavy metal - contaminated soils. MicroRNAs (miRNAs) act as key regulators in plant heavy metal stress responses and detoxification. However, the physiological adaptation mechanisms and the specific roles of miRNAs in B. juncea under cadmium (Cd) stress remain to be fully elucidated. This study aimed to investigate the physiological changes and expression profiles of five candidate miRNAs in leaves and roots under varying Cd concentrations and exposure durations.

Physiological traits and the expression of five miRNAs (miR408, miR397, miR156, miR398a, and miR398b/c) were analyzed in B. juncea leaves and roots under Cd treatment at three concentrations (10, 80, 200μM) for three durations (6h, 48h, 7d). Cd accumulation was significantly higher in roots than in leaves and increased with concentration and duration. DAB, NBT and Evans blue staining demonstrated that higher Cd concentrations and longer exposure triggered more reactive oxygen species production and cell death. The activities of five antioxidant enzymes fluctuated across different treatments. Leaves exhibited higher activities of most antioxidant enzymes except POD than roots. MiR408, miR397, miR398a and miR398b/c reached their highest levels in both tissues at 7 d under 10μM Cd, while miR156 peaked at 48 h under 80μM Cd. Overall, miRNA expression levels were generally higher in roots than in leaves.

Cd induces tissue-specific, concentration-dependent, and time-dependent changes in antioxidant enzyme activities and miRNA expression in B. juncea. The distinct upregulation patterns of miR156, miR397, miR398a, miR398b/c, and miR408 in roots suggest their involvement in Cd detoxification and tolerance. These findings provide novel insights into miRNA-mediated regulatory mechanisms underlying Cd adaptation in B. juncea.