Plants have a range of potential mechanisms at the cellular level that might be involved in the detoxification and thus tolerance to heavy metal stress (Hall 2002). Cell wall binding and vacuolar sequestration are the two essential detoxification mechanisms that play a vital role in hyperaccumulation of heavy metals (Cosio et al. 2004).
The cell wall is the "first line of defense" against negative factors from the external environment. When captured by root cells, metals are first bound by the cell wall, by an ion exchange of comparatively low affinity and low selectivity (Clemens et al. 2002). Boominathan and Doran (2003) reported that T. caerulescens root hairs contained most of the Cd in the cell wall. In roots of A. halleri grown hydroponically, Zn and Cd were accumulated in the cell wall of the rhizosphere as Zn/Cd phosphates (Kupper et al. 2000). Cell wall binding can prevent Cd from being transported across the plasma membrane. This delay in transmembrane uptake may represent an important factor in the defense against Cd poisoning in T. caerulescens, allowing time for activation of intracellular mechanisms for heavy metal detoxification (Nedelkoska and Doran 2000).
The vacuole, in turn, is generally considered to be the main storage site for metals in yeast and plant cells (Salt and Rauser 1995). Compartmentalization of metals in the vacuole is also part of the tolerance mechanism of some metal hyperaccumulators (Tong et al. 2004). In T. caerulescens, Cd has been found in the apoplast and the vacuole (Vazquez et al. 1992). A 100% and more than 90% Cd in the leaf protoplast were localized in the vacuoles of Ganges ecotype of T. caerulescens Ma et al. (2005) and P. griffithii (Qiu et al. 2011), respectively. These results clearly indicated that internal detoxification of Cd is achieved by vacuolar compartmentation.
After Cd enters the cytosol, some mechanisms may play roles to inactivate it through chelation or conversion to a less toxic form. Phytochelatins could bind Cd2+ in the cytoplasm, and Cd transport into the vacuole is an effective way to reduce the levels of Cd toxicity in the cytosol. No doubt, an efficient tonoplast transport of Cd is very important. Several families of transporters may take part in the process of vacuole sequestration, such as YCF1, MTP1, and CaCA (see Sect. 2.2 for details).
It is generally assumed that low molecular weight (LMW) complexes are formed in the cytosol and subsequently transported into the vacuole where more Cd2+ and sulfide are incorporated to produce the high molecular weight (HMW) complex, which represents the main storage form of Cd (Clemens 2000). The first molecular insight into the vacuolar sequestration of Cd came from the cloning of HMT1, which complemented a S. pombe mutant deficient in the formation of the HMW complex (Ortiz et al. 1995). A constitutively high concentration of malate in the vacuoles, and the formation of the Cd-malate complex may lead to a decrease in subsequent Cd efflux to the cytoplasm (Ueno et al. 2005).
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