Uptake and Transport of Cd by Plants

Cadmium accumulation by higher plants can occur through foliar or root uptake. However, the primary point of entry for Cd into plants is through the roots. Cadmium uptake by plants grown in contaminated soils has been extensively studied, particularly in sludge-amended soils (e.g. Jackson and Alloway 1991; McLaughlin et al. 2006; Singh and Agrawal 2007; Speir et al. 2003) and in soils treated with Cd-enriched phosphate fertilizers (Crews and Davies 1985; He and Singh 1994a, b; Huang et al. 2003). In general, metals have to be in an available form to be taken up by plants. Alternatively plants must have mechanisms to make the metals available. The degree to which higher plants are able to take up Cd depends on its concentration in the soil and its bioavailability. Cadmium bioavailablity in soils is modulated by the presence of organic matter, pH, redox potential, temperature, light intensity, cation exchange capacity and concentrations of other elements (Greger 1999; He and Singh 1993; Sanita di Toppi and Gabbrielli 1999). In particular, Cd ions seem to compete with other micro and macronutrients such as calcium and zinc for the same transmembrane carriers (Sanita di Toppi and Gabbrielli 1999) , which might lead to plant nutrient deficiencies (Krupa et al. 2002) . As is the case for other metals, Cd uptake tends to be reduced at low pHs because of competition with H+ ions at root uptake sites; however, Cd bioavailability increases with decreasing pH in soil (Greger 1999). The presence of colloids from which there is a release of metals at low pH, increases the metal concentration in pore water and thus also in the roots (Greger 1999) . For instance, acid rain and the resulting acidification of soils and surface waters are known to increase the geochemical mobility of Cd (Campbell 2006) . Cadmium uptake also appears to be decreased in the presence of dissolved organic matter because ligands on the organic matter effectively bind Cd ions (He and Singh 1993 ; Prasad 1995). Chloride levels would also be expected to affect Cd availability as soil sodium chloride has an antagonistic effect on metal toxicity (Bhartia and Singh 1994).

Cadmium is believed to enter the root through the cortical tissue till the stele either by apoplas-tic and/or a symplastic pathway (Sanita di Toppi and Gabbrielli 1999). The apoplast continuum of the root epidermis and cortex is readily permeable to solutes. The cell walls of the endodermal cell layer act as a barrier for apoplastic diffusion into the vascular system. In general, solutes have to be taken up into the root symplasm before they can enter the xylem (McLaughlin 2002). The cell membrane plays a key role in metal homeostasis, preventing or reducing entry into the cell. However, examples of exclusion or reduced uptake mechanisms in higher plants are limited (Benavides et al. 2005). The mechanism for metal transport across the plasma membrane to the stele is still not completely understood (McLaughlin 2002) . For all cationic metals, such as Cd2+, the main route for uptake across the plasma membrane is the large negative electrochemical potential produced as a result of the membrane H+ translocating adenosine triphospatase (ATPases) (Krämer 2010; McLaughlin 2002). For instance, Costa and Morel (1994) reported that in lettuce grown in hydroponic solution with Cd concentrations from 0.05 mM to 5 mM, high amounts of Cd in roots were correlated with high contributions from H+-ATPase in the active process of Cd uptake. Other authors contend, however, that the main route for uptake of divalent metals is via ion channels, such as Cd2+ and Mg2+ channels (McLaughlin 2002 and references therein). Subsequent to metal uptake into the root sym-plasm, three processes govern the movement of metals from the root into the xylem: sequestration of metals inside root cells, symplastic transport into the stele and release into the xylem (Clemens et al. 2002).

During their transport through the plant, metals become bound to cell walls, which can explain why normally Cd2 + ions are mainly retained in the roots, and only small amounts are translocated to the shoots (Cataldo et al. 1983; Greger 1999) . But once loaded in the xylem sap, Cd is translocated to the aerial parts of plants through the transpiration stream, where they might be present as a divalent ion (Greger 1999) or com-plexed by several ligands, such as amino acids, organic acids and/or phytochelatins (Briat and

Lebrun 1999; Gong et al. 2003; Salt et al. 1995; Sanita di Toppi and Gabbrielli 1999).

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