Several studies have suggested that root-to-shoot Cd translocation via the xylem, presumably mainly as free Cd, is the major and common physiological process in hyperaccumulating plants (Lu et al. 2009; Ueno et al. 2008; Uraguchi et al. 2009).
Metals could reach the xylem through either the symplast or the apoplast (White et al. 2002) (Fig. 1c). Symplastic bypass is considered as an energy-consuming (positive) pathway involving metal transporter proteins. Apoplastic bypass is known to be a passive pathway which is usually correlated to transpiration. Transpiration as a controller of Cd accumulation in shoots has been investigated either by manipulating transpiration or by observing unperturbed transpiration rates (Salt et al. 1995; Van der Vliet et al. 2007). However, results have led to equivocal conclusions. Liu et al. (2010b) reported that transpiration has an important role in Cd accumulation in shoots of the metal accumulator Phytolacca americana. Their argument is based mainly on a comparison of literature data on Zn influx in roots and accumulation in shoots. Meanwhile, as admitted by White et al. (2002), their calculation excluded the possible contribution by low-affinity transporters, which would be more prominent in the range of high external Zn concentrations. In T. caerulescens, EDX microanalysis revealed the presence of Cd in leaf along the path of water migration from the vascular cylinder to epidermal cells, which is in line with passive Cd transport by the transpiration stream (Wojcik et al. 2005). However, some current hypotheses suggest a dominant role of the symplastic pathway in the transport of Cd to xylem in several plant species. Xing et al. (2008) have reported that the Cd translocation efficiency did not correlate with the apoplastic bypass flow in 11 Thlaspi accessions, indicating that Cd and Zn translocation is not controlled by the apoplastic pathway. In the study by Ueno et al. (2008), xylem loading of Cd by A. halleri is an energy-dependent and rapid process, which appears to be partly shared with the Zn and Fe pathway. In addition, the metabolic inhibitor significantly reduced the Cd concentration in xylem sap, while the transpiration rate had no effect on Cd accumulation in shoots of the hyperaccumulating ecotype (HE) of S. alfredii. This suggests that Cd translocation to the xylem is an energy-driven process, and apoplastic bypass makes a very small contribution to Cd hyperaccumulation in shoots of HE of S. alfredii (Lu et al. 2009).
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