Root uptake of Cd2+ typically exhibits two phases: apoplastic binding and symplastic uptake (Hart et al. 1998; Zhao et al. 2002). Cadmium transport across the plasma membrane of root cells has been shown to occur via a concentration-dependent process exhibiting saturable kinetics in many species, suggesting that Cd is taken up via a carrier-mediated system (Verbruggen et al. 2009a; Zhao et al. 2006). For such a non-essential element, it is likely that Cd enters cells through Ca channels or broad-range metal (e.g., Zn or Fe) transporters (Fig. 1b). Ca transport pathway might be involved in the uptake of Cd. Adding La3+ (a potent Ca channel inhibitor) or increasing the concentration of Ca appeared to suppress the metaboli-cally dependent Cd uptake substantially in the Prayon ecotype of the hyperaccumulator T. caerulescens (Zhao et al. 2002). Except for Ca channelmediated uptake, abundant evidence suggests that Fe may also lead to the upregulation of genes encoding Fe transporter proteins such as IRT1, which also mediate Cd uptake (Lombi et al. 2002; Zhao et al. 2006). However, Plaza et al. (2007) have reported that although TcIRT1-G may be involved in Cd hyperaccu-mulation in the Ganges ecotype of T. caerulescens, the transporter expressed in yeast seems to be incapable of Cd transport in contrast to AtIRT1. Therefore, the unique Cd accumulating ability of the T. caerulescens Ganges ecotype may be correlated with the expression of other transporter proteins or interacting proteins. Cadmium and Zn uptake are genetically correlated and competitive as they may be transported, at least partly, by the same transporter(s) or are controlled by common regulators because of their similar electronic structure (Baker et al. 1994; Benavides et al. 2005). In some cases, Zn was found to depress Cd uptake in T. caerulescens (Lombi et al. 2000, 2001). ZNT1 which is a Zn transporter cloned from the Prayon ecotype of T. caerulescens can also mediate Cd transportation with low affinity (Pence et al. 2000).
Most studies have shown that the main site of Cd accumulation in roots is the apoplast, particularly cell walls. Using energy-dispersive X-ray microanalysis, Cd was determined in cortex parenchyma cells, endodermis, parenchyma cells of the central cylinder, and xylem vessels in T. caerulescens (Wojcik et al. 2005). After treatment with 20 ppm Cd, about 13% of the total Cd in T. caerulescens roots was associated with organic acids (Boominathan and Doran 2003). Nedelkoska and Doran (2000) have found retention of almost the whole pool of Cd taken up in the fraction of cell wall of Thlaspi roots, and then diffused into the symplast after 7-10 days of exposure to 200 p.M Cd. The authors also suggested that such a delay in Cd transport through membranes inside cells is an important defense mechanism against Cd toxicity, enabling simultaneous activation of intracellular detoxification mechanisms of Cd such as chelation and antioxidative defense.
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