Early Cd response and sequestration strategies in Arabidopsis halleri ssp. gemmifera elucidated by transcriptomic analysis

Arabidopsis halleri ssp. gemmifera is a promising Cd phytoremediation agent, however, the metal uptake and accumulation mechanism remain poorly understood. This study focused on the 2-hours early responses associated with Cd uptake and temporary Cd retention in roots. To distinguish Cd-specific responses from shared divalent metal responses, transcriptomic analyses were performed on roots exposed to Cd compared to excess Zn. Cd exposure induced a clearly larger number of differentially expressed genes than higher concentration of Zn exposure, indicating a distinct early response to Cd. Genes encoding transporters such as PCR2, DTX1, PDR8, PDR12, CAX4, MHX1, and ABCC2 were highly upregulated during the early exposure phase. Cd retention in roots may be mediated by these transporters, which could contribute to Cd efflux into the apoplast or vacuolar sequestration. Further, genes involved in intracellular Cd chelation, including those encoding glutathione, HIPPs, and HMPs protein, were upregulated rather than genes encoding phytochelatins. Additionally, upregulation of genes involved in cell wall biosynthesis and remodeling was observed, suggesting a structural modification occurs during early Cd exposure, contributing to reinforcement and temporary Cd storage before translocation. This hypothesis is supported by increased lignification in root tissues and the accumulation of Cd in the apoplastic region, indicating that cell wall serves as sequestration site in A. halleri.