As important suppliers of dietary minerals for humans and animals, plants form a bridge between the soil elemental composition and the food chain. Consequently, contaminated soils with potentially toxic elements such as Cd and Cu may affect crop production and the food chain and, hence, human health (Cuypers et al. 2011a).
1.1.1 Uptake of Excess Cu and Cd by the Plant Is Unavoidable
To avoid Cu deficiency, plants have high-affinity Cu transporters belonging to the COPT family to ensure Cu+-uptake under low Cu availability, after reduction of Cu2+ by ferric reductase (FRO). The uptake systems when plants are exposed to excess Cu have not yet been revealed. These could be COPT3-5, which are not affected by Cu levels, unlike COPT1 and COPT2 that are down regulated by excess Cu. Otherwise, Cu2+, which is the most abundant form present in soil, could be taken up by ZIP (zinc-regulated transporter/iron-regulated transporter-like or ZRT/ IRT-like) transporters (Burkhead et al. 2009; Palmer and Guerinot 2009). However, these bivalent cation transporters are also important uptake systems for non-essential elements such as Cd. This is due to the chemical similarity of Cd2+ ions with the ions of these essential elements (Clemens 2006; Perfus-Barbeoch et al. 2002; Verbruggen et al. 2009). Therefore, when growing on soils contaminated with excess metals, unspecific metal uptake seems unavoidable and plants need to have systems in place for chelation and sequestration of these excess metals.
Due to their redox-active properties, free Cu-ions in the cell are very toxic. Therefore, Cu is bound to chaperones that deliver it directly to the particular protein where its redox-active properties will be used in cellular functioning. For example, CCS chaperones deliver Cu to CuZnSOD in the chloroplast, COX chaperones deliver Cu to cytochrome c oxidase in mitochondria, and even for recycling of Cu from senescing tissues, CCH is used as a Cu chaperone (Palmer and Guerinot 2009; Robinson and Winge 2010). Besides these chaperones that are essential in normal functioning, detoxification of metals is achieved by specific chelators, which form non-toxic complexes with metals and facilitate their sequestration away from sensitive sites in cells. In this way, chelation prevents interaction of free metal ions with physiologically important proteins. The favoured ligands of the bivalent cations Cd2+ and Cu2+ are thiols (SH-groups), which are present on cysteine residues of glutathione (GSH), phytochelatins (PCs) and metallothioneins (MTs).
Glutathione (g-glu-cys-gly) is the major low molecular mass peptide in plants and is present at the millimolar level within cells. It protects potentially susceptible protein thiol groups from binding free metal ions and consecutively hampering their function (Foyer and Noctor 2011; Herbette et al. 2006; Verbruggen et al. 2009). Phytochelatins (PCs) (g-(glu-cys)n-gly) are a polymerisation of 2-11 GSH molecules catalysed by phytochelatin synthase (PCS). These polymerised forms are more efficient in chelating several metal ions, because of multiple thiol-binding sites. Whereas Cd bound to GSH can be transported into the vacuole by an ABC transporter, PCs facilitate vacuolar sequestration of Cd (Noctor and Foyer 1998; Verbruggen et al. 2009). Arabidopsis mutants, deficient in PCs, are hypersensitive to Cd because they lack this high affinity metal chelator (Howden et al. 1995). Moreover, it has also been shown that PCs are involved in root-to-shoot transport of Cd2+ in order to alleviate metal accumulation in root cells (Gong et al. 2003; Van Belleghem et al. 2007).
Metallothioneins (MTs) are small gene-encoded proteins with many thiol groups due to their high cysteine content (Cobbett and Goldsbrough 2002; Guo et al. 2008). Their gene expression is strongly induced by Cu exposure in a number of plant species, and a correlation between MT gene expression and Cu tolerance has been inferred from studies of Arabidopsis ecotypes and Silene populations (Guo et al. 2008, and references therein). Metallothioneins have been mainly associated with Cu homeostasis. However, expression of a Brassica juncea MT gene in Arabidopsis increased not only Cu but also Cd tolerance. Zhigang et al. (2006) and Guo et al. (2008) provided evidence that MTs and PCs may have overlapping functions as they contribute both to Cu and Cd tolerance.
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