The transport of metals across plasma membranes by means of primary and secondary active transporters is of central importance in the metal homeostasis network in plants (Kotrba et al. 2009). With the use of genetic and molecular
Fig. 1 Molecular and physiological mechanisms involved in Cd hyperaccumulation and detoxification (a-d), and their evolutionary significance for Cd hyperaccumulators (e-g). (a) Root foraging for Cd is a strategy that hyperaccumulator species employ to acquire high levels of Cd in Cd-enriched patches in a heterogeneous substrate, (b) In the root, uptake of Cd ion is mediated by various metal transporters residing in the plasma membrane, (c) Cd is transported to
Fig. 1 Molecular and physiological mechanisms involved in Cd hyperaccumulation and detoxification (a-d), and their evolutionary significance for Cd hyperaccumulators (e-g). (a) Root foraging for Cd is a strategy that hyperaccumulator species employ to acquire high levels of Cd in Cd-enriched patches in a heterogeneous substrate, (b) In the root, uptake of Cd ion is mediated by various metal transporters residing in the plasma membrane, (c) Cd is transported to techniques such as sequence comparison to identify transporters, functional complementation of yeast mutants, and plant transformation to regulate gene activities, a wide range of gene families have now been identified in plants that are likely to be involved in transition metal transport (Hall and Williams 2003). Although a picture of Zn hyperaccumulation is beginning to emerge, which attributes a central role to metal transport proteins and improves our mechanistic understanding of Zn hyperaccumulation and homeostasis, the molecular basis of Cd uptake into plant cells has not yet been elucidated clearly (Fig. 1b, d).
The heavy metal-transporting P1B-ATPases belong to the superfamily of P-type ATPases which use energy from ATP hydrolysis to translocate cations across biological membranes (Williams and Mills 2005). The first identified transporter involved in Cd transportation from roots to shoots is the P1B-type ATPase AtHMA4 (heavy metal-transporting ATPase4), which could mediate efflux of Zn and Cd from the xylem parenchyma cells to the xylem vessels, and therefore plays a key role in their transport from roots to shoots in A. thaliana (Mills et al. 2003; Verret et al. 2004). It has been reported that T. caerulescens and A. halleri have a much higher level of expression of the P1B-type ATPase genes, especially TcHMA4 or AhHMA4 (Hanikenne et al. 2008; Papoyan and Kochian 2004). The authors have reported that TcHMA4 expression in roots of the Prayon accession of T. caerulescens is significantly induced by Zn deficiency and exposure to high Zn or Cd levels. The characterization of TcHMA4 protein has shown that its C-terminus contains numerous His and Cys repeated residues, which may participate in heavy metal binding (Papoyan and Kochian 2004). Additionally, HMA4 has been shown to be a strong candidate in determining the Cd hyperaccumulator phenotype: results from a cross between A. halleri and A. lyrata show how a major Cd-tolerance QTL colocalized with AtHMA4 (Courbot et al. 2007). By contrast, difference between Ganges and Prayon accessions in the root-to-shoot translocation of Cd and Zn was not attributable to different expression levels of TcHMA4 (Xing et al. 2008).
Fig. 1 (continued) the shoot via the xylem through either the symplastic or the apoplastic way. HMA4 is responsible for Cd transfer from the xylem parenchyma to the vessel. (d) In the shoot, Cd is preferentially sequestered in the trichomes, epidermis, and mesophyll mediated by various metal transporters located at the plasma membrane and tonoplast. Cell wall binding and vacuole sequestration are suggested to be the main pathways of Cd detoxification. (e) According to the metal defense hypothesis, elevated Cd concentrations in plant tissues protect hyperaccumulators from herbivores or pathogens. (f) The enhanced photosynthesis by addition of suitable Cd dosage results from the stimulation of the activity of carbonic anhydrase (CA) and amount of Rubiso in the Calvin cycle. (g) The growth of hyperaccumulators is stimulated by a suitable range of Cd dosage but inhibited at higher dosages, suggesting a higher requirement of Cd for optimal growth
ABC (ATP-Binding Cassette) Family
The ATP-binding cassette (ABC) family is one of the largest protein families in living organisms (Higgins 1992). They have been shown to be implicated in a range of processes that encompass polar auxin transport, and xenobiotic and metal detoxification. Among the plant ABC transporters, AtMRP3 (Bovet et al. 2003) and AtATM3 (Kim et al. 2006) have been suggested to transport Cd. Overexpression of AtATM3 in Arabidopsis plants showed enhanced Cd and Pb resistance compared to wild type plants, whereas AtATM3 knockout plants showed Cd-sensitive phenotypes (Kim et al. 2006). Another ABC transporter in Arabidopsis, AtPDR8, is not only involved in the pathogen resistance in A. thaliana (Kobae et al. 2006), but it also acts as an efflux pump of Cd2+ or Cd conjugates at the plasma membrane of Arabidopsis cells (Kim et al. 2007). Regarding yeast and fission yeast, in which Cd is able to form complexes with either glutathione (GSH) or phytochelatins (PCs) subsequently transported into vacuoles via ABC transporters, it is also very likely that some plant ABC transporters are able to transport GS2-Cd or PC-Cd complexes into sub-cellular compartments of or outside the cell (Bovet et al. 2005).
CDF (Cation Diffusion Facilitator) Family
Cation diffusion facilitator (CDF) proteins are a family of heavy metal transporters implicated in the transport of Zn, Cd, and Co that has been identified in bacteria and eukaryotes (Williams et al. 2000). Certain members of the CDF family are known to mediate vacuolar sequestration, storage and transport of metal ions from the cytoplasm to the outer compartment, and some are found in plasma membranes, while others are in intracellular membranes (Paulsen and Saier 1997). MTP transporters in the CDF family have been reported as efflux transporters for the sequestration of Zn to enhance the capability of metal tolerance in plants (Kramer 2005). Additionally, a constitutively high expression of MTP1 in A. halleri (Becher et al. 2004; Chiang et al. 2006) and in an interspecies cross between A. halleri and A. lyrata has also been reported (Drager et al. 2004). However, there is no conclusive evidence to date for a functional difference between hyperaccumulator and non-accumulator MTP1 proteins.
ZIP (ZRT1/IRT1 -Like Protein) Family
ZRT1/IRT1-like protein (ZIP) generally contributes to metal ion homeostasis through the transport of cations into the cytoplasm (Colangelo and Guerinot 2006). It is noteworthy that the involvement of ZIP genes in metal accumulation by hyperaccumulators has previously been reported. High TcZNT1 expression is associated with increased Zn uptake in roots of T. caerulescens (Pence et al. 2000). As one of the best characterized members of the ZIP family, AtIRTl which transports a wide range of divalent transition metal cations, including Cd, is known as the primary root iron uptake system of A. thaliana (Korshunova et al. 1999). IRT-overexpressing A. thaliana has shown to accumulate more Zn and Cd in roots under Fe-deplete conditions which stabilize IRT1 protein (Connolly et al. 2002). Such a role of IRT1 is in line with the observation from physiological studies on a number of plant species that Fe limitation leads to an increase in Cd accumulation. A contribution of other ZIP transporters to Cd uptake has also been suggested. TcZNT1 mediates high-affinity Zn uptake and low-affinity Cd uptake when expressed in Saccharomyces cerevisiae zhy3 cells (Pence et al. 2000).
Natural resistance-associated macrophage protein (NRAMP) defines a novel family of related proteins that have been implicated in the transport of divalent metal ions, especially in Fe homeostasis (Curie et al. 2000). Initial work on AtNRAMP1, 3, and 4 has shown that they confer Cd uptake activity when expressed in S. cerevisiae (Thomine et al. 2000). The authors have shown that NRAMP genes in plants encode metal transporters and AtNRAMPs transport both Fe and Cd (Thomine et al. 2000). NRAMP3 and NRAMP4 are responsible for Cd2+ efflux from the vacuole and are found to be overexpressed in both shoots and roots of T. caerulescens (Oomen et al. 2009; Thomine et al. 2003). It has been indicated that AtNRAMP3 and AtNARMP4 function redundantly in the mobilization of Fe from the vacuole (Lanquar et al. 2005). Also, NRAMP6, a new member of the NRAMP family, is functional and potentially involved in Cd tolerance (Cailliatte et al. 2009). Additionally, the protein shown in yeast to be targeted to intracellular vesicles may mediate the transport of Cd from internal pools.
Among the transporters thought to be capable of pumping Cd into plant vacuoles, cation/H+ exchangers (CAXs) have been well characterized (Hirschi et al. 2000). CAX antiporters are a group of proteins that export cations out of the cytosol to maintain ion homeostasis across biological membranes (Pittman et al. 2002). All of the CAXs (CAX1-CAX6) found in Arabidopsis have been demonstrated to be capable of transporting Cd (Korenkov et al. 2007a, b). Korenkov et al. (2007a) have suggested that CAX antiporters are not negatively impacted by high Cd and that supplementation of the tonoplast with AtCAX somewhat compensates for a reduced number of tonoplast proton pumps and proton leak, and thereby results in sufficient vacuolar Cd sequestration to provide higher tolerance in tobacco. A recent report has shown that an Arabidopsis CAX1 mutant (CAXcd) conferring enhanced Cd transport in yeast is ectopically expressed in Petunia which can significantly increase Cd tolerance and accumulation (Wu et al. 2010).
So far numerous evidences have shown that Cd is taken up into plant cells by Fe, Ca, and Zn transporters/channels transporting a broad range of substrates. However, a potential Cd-specific uptake system has also been suggested in hyperaccumulator plants (Lombi et al. 2001; Zhao et al. 2002), even though it has not been identified. Therefore, there are still lots of unclear areas that need to be investigated in the future.
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