Transcriptome insights into root adaptation of Erigeron canadensis to combined cadmium and lead stress

The invasive plant species, Erigeron canadensis L., exhibits a propensity to colonize heavy metal-contaminated areas. However, its molecular adaptation mechanisms to heavy metals remain poorly understood. This study explored its adaptive mechanisms under combined cadmium (Cd) and lead (Pb) stress by investigating growth performance, heavy metal accumulation, and root molecular responses. Aboveground growth of E. canadensis was unaffected by any treatment, whereas root biomass was reduced only under high-concentration combined Cd and Pb stress. Cd and Pb were predominantly accumulated in roots instead of aboveground tissues. Notably, a translocation factor (TF) > 1 for Cd implied that E. canadensis actively accumulated Cd in roots and translocated it to aerial parts. However, a TF < 1 for Pb suggested that this species employed an exclusion strategy to mitigate Pb stress. Differentially expressed genes (DEGs) in E. canadensis roots were primarily involved in transport, signal transduction, antioxidant defence, transcription regulation, and ribosome function under heavy metal stress. Zinc transporter ZntB was verified to mediate Cd uptake and tolerance, while natural resistance-associated macrophage protein 3 and cyclic nucleotide-gated ion channel 2 were shown to mediate Cd and Pb uptake and tolerance by heterologous expression in yeast. Up-regulated amino acid, sugar, and auxin transporters together with ribosomal proteins may contribute to the maintenance of nutrient supply, auxin homeostasis, and protein synthesis for normal root growth. High-level metal stress triggered more extensive down-regulation of ROS-associated genes in E. canadensis roots. These findings provide root-specific molecular insights into the E. canadensis adaptation to heavy metal stress.