HMA4 and IRT3 as indicators accounting for different responses to Cd and Zn by hyperaccumulator Arabidopsis halleri ssp. gemmifera

Using plants as bioremediation agents are rising worldwide as an environmentally friendly tool to remove contaminants such as heavy metals. Arabidopsis halleri ssp. gemmifera is able to hyperaccumulate Cd/Zn, however, the commonality between Cd/Zn accumulation pathways are still vague. This study focused on HMA4 and IRT3 as indicators and investigated their transcription in A. halleri ssp. gemmifera under Cd/Zn treatments. In addition to the known Cd/Zn xylem loading functions in roots, HMA4 showed a potential role in Cd/Zn redistribution or metal efflux from the vascular bundle in leaves due to increased transcription by 3-fold after 7 d under all treatments. On the other hand, IRT3 as a Zn homeostasis gene was expressed continuously in both roots and leaves under single Cd or Zn treatment. However, IRT3 transcription in leaves plunged to 50-fold lower under combined Cd and Zn treatment. The result indicated an increase of intracellular free Zn ion, implying a new unreported mechanism of Zn homeostasis in leaves under the presence of both metals. Furthermore, no competition between Cd/Zn uptake and accumulation suggested the existence of independent transport system for Cd and Zn in A. halleri ssp. gemmifera, which is differed from what have been reported until now.