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We have previously identified an ecotype of the hyperaccumulator Thlaspi caerulescens (Ganges), which is far superior to other ecotypes (including Prayon) in Cd uptake. In this study, we investigated the effect of Fe status on the uptake of Cd and Zn in the Ganges and Prayon ecotypes, and the kinetics of Cd and Zn influx using radioisotopes. Furthermore, the T. caerulescens ZIP (Zn-regulated transporter/Fe-regulated transporter-like protein) genes TcZNT1-G and TcIRT1-G were cloned from the Ganges ecotype and their expression under Fe-sufficient and -deficient conditions was analyzed. Both short- and long-term studies revealed that Cd uptake was significantly enhanced by Fe deficiency only in the Ganges ecotype. The concentration-dependent kinetics of Cd influx showed that the V(max) of Cd was 3 times greater in Fe-deficient Ganges plants compared with Fe-sufficient plants. In Prayon, Fe deficiency did not induce a significant increase in V(max) for Cd. Zn uptake was not influenced by the Fe status of the plants in either of the ecotypes. These results are in agreement with the gene expression study. The abundance of ZNT1-G mRNA was similar between the Fe treatments and between the two ecotypes. In contrast, abundance of the TcIRT1-G mRNA was greatly increased only in Ganges root tissue under Fe-deficient conditions. The present results indicate that the stimulatory effect of Fe deficiency on Cd uptake in Ganges may be related to an up-regulation in the expression of genes encoding for Fe(2+) uptake, possibly TcIRT1-G.
Lombi et al. (Mon,) studied this question.