The rhizospheres of hyperaccumulator plants may have some influence on the availability and solubility of metals by both acidification of the rhizosphere and exudation of carboxylates (Whiting et al. 2000). However, several studies have indicated that rhizosphere acidification and root exudates are not responsible for improvement of metal capture (Whiting et al. 2001a, b; Zhao et al. 2001).
On the contrary, the release of root exudates by some hyperaccumulators can bind and sequester heavy metals in soil (Schwab et al. 2005). This mechanism could protect roots from the toxic effects of heavy metals (Liao and Xie 2004) and may also decrease Cd uptake by plants (Sarwar et al. 2010). Microbes are ubiquitous even in soils with high heavy metal concentrations. Rhizosphere microorganisms have also been shown to influence metal availability and mobility by reducing soil pH or by producing chelators and siderophores (Abou-Shanab et al. 2003; Wenzel et al. 2003; Whiting et al. 2001a, b). The existence of active rhizospheric and microbial metal-mobilizing mechanisms has not conclusively been demonstrated.
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