Subcellular localization and chemical speciation of Cd in Arabidopsis halleri ssp. gemmifera to reveal its hyperaccumulating and detoxification strategies

The study involving low-level Cd using Arabidopsis halleri ssp. gemmifera is likely suitable for realistic Cd phytoremediation since the available Cd in the contaminated soil is considered low. However, the comprehensive evidence on how this plant undergoes hyperaccumulating-detoxification strategies under low Cd levels in its root, stem, and leaves remained questionable. Therefore, Cd uptake and distribution, intracellular localization, Cd speciation, and metal chelation compounds, glutathione (GSH) and phytochelatins (PCs), were assessed in a time-course analysis to portray their involvement in the Cd hyperaccumulating mechanisms. The results showed that under low Cd stress, 0.3 µmol/L of Cd was completely absorbed by this plant during 336 h, indicating a high Cd uptake system. At the same time, more Cd content accumulated in the stem and leaves compared to Cd content in the root. Excessive Cd being transported to the upper part was supported by the availability of more than 80% of Cd speciation categorized as mobile fractions (ethanol-and H2O-extractable forms). The highest Cd soluble fraction found in the root during initial and middle exposure promoted Cd intracellular translocation to stem and leaves. At the end of exposure (336 h), we predicted Cd was compartmentalized in the less metabolite sites, cell wall and vacuole due to less toxicity effect, which was also found in high Cd level (30 µmol/L). Furthermore, GSH and PC as metal complexation compounds owned a minor role even under high Cd exposure level. Therefore, regardless of the Cd supply level, elevating of metal transport and sequestration in the shoots were preferred over thiol complexed forms as common hyperaccumulating strategies in A. halleri ssp. gemmifera.