ABA Enhances Cadmium Tolerance in Egeria densa via Modulating Transport, Metabolism, and Antioxidant Defense

Cadmium (Cd) contamination severely threatens aquatic ecosystems and human health. Aquatic macrophytes offer a sustainable phytoremediation strategy due to their high biomass and metal accumulation capacity, but their intrinsic stress sensitivity limits efficiency. While abscisic acid (ABA) enhances heavy metal tolerance in terrestrial plants, its role in aquatic macrophytes remains unclear. This study investigated the mechanisms underlying ABA-induced Cd tolerance in the submerged macrophyte Egeria densa, a species with known phytoremediation potential. Exogenous ABA application (2 μM) significantly reduced Cd accumulation in E. densa by 63.7% and restored photosynthetic function. Multi-omics analyses revealed that ABA orchestrates a coordinated defense network: it transcriptionally reprograms metal transporters to limit uptake (downregulating ZIP1 and HIPP26 by 2.4- and 2.4-fold, respectively) and enhance efflux/vacuolar sequestration (upregulating ABCG and ABCC by 14.6- and 3.7-fold, respectively). Metabolically, ABA activates the pentose phosphate pathway for defense synthesis, restarts the TCA cycle for energy supply, and strategically reshapes amino acid profiles to conserve resources. Concurrently, ABA potently enhances the antioxidant system, boosting the activities of superoxide dismutase (SOD) by 2.9-fold, catalase (CAT) by 1.7-fold, and peroxidase (POD) by 1.3-fold, while increasing glutathione (GSH) levels by 2.7-fold compared to the Cd-only treatment. Our findings establish ABA as a key regulator coordinating metal homeostasis, metabolic adaptation, and antioxidant defense in aquatic plants, advancing our understanding of phytohormone-mediated Cd stress adaptation. This study provides a mechanistic foundation for developing hormone-assisted strategies to improve the efficiency of macrophyte-based phytoremediation in Cd-contaminated waters.