Heavy metals including cadmium are absorbed through the plant roots by mass flow, diffusion and root interception. The processes depends on the concentrations or levels of the metals in soil.
Cadmium is transported into the root xylem via apoplastic and symplastic pathways.
It may be via the symplastic/apoplastic pathways after absorption through specific Cd transporters or relevant ABC transporters.
As a non-essential metal it is assumed that there are no specific input mechanisms for cadmium. Among the proteins responsible for the entry of cadmium into the cell include the specific calcium transporter LCT1 (Clemens et al. 1998), and the IRT1 protein, belonging to the Zn and Fe (ZIP) family of transporters (Guerinot 2000). Other family of conveyors involved is the Nramp, located in the vacuole membrane, so it probably has a role in mobilizing metal rather than in getting it to the root (Thomine et al. 2003). Once inside the cell cadmium can be coordinated with S-ligands such as glutathione (GSH) or phytochelatines (PCs) and organic acids such as citrate (Clemens 2006; Domínguez-Solís et al. 2004). Other possible molecules responsible for Cadmium chelators are small cysteine-rich proteins called metalothionines (MTs).
In this way, the Cd-ligand complexes can be transported into the vacuole or other cells (Shah and Nongkynrh 2007). In the plant, cadmium accumulates preferably in the root sequestered in the vacuole of the cells, and only a small part is transported to the aerial part of the plant concentrating in decreasing order on stems, leaves, fruits and seeds (Chan and Hale 2004). In yeast cells it has been shown that cadmium enters the vacuole bound to phytochelatines (PCs) through an ABC type conveyor (Ortiz et al. 1995). Another possible mechanism of entry of cadmium into the vacuole is by means of a Cd2+/H+ co-transporter located in the membrane of the same (Salt and Wagner 1993). The conveyors of CAX cations, involved in the transport of calcium to the vacuole, also transport other metals such as cadmium (Park et al. 2005). Once in the root, the cadmium can pass into the xylem through the apoplast and/or through the simplast forming complexes (Clemens et al 2002). Figure 3 shows a scheme of cadmium input through the cells of the root, until it reaches the xylem.
Figure 3 (attached in pdf) presents a schematic representation of the mechanisms of entry, abduction and translocation of the Cd into roots (adapted from Clemens, 2006). Two are represented cells, the one on the right in contact with the rhizosphere, and the adjacent one in contact with xylem. The ability of the metal to sequester in the root cells plays a key role in the translocation of it to the air side. Another important factor is the accessibility and mobilization of the seized metal, the efficiency of the radial passage through the simple and of the endodermis, and finally the flow to the xylem. Abbreviations: GSH; glutathione, possible ligand of the Cd, with which it would form the bisglutationato-Cd complex (GS2-Cd2+). PC, phytochelatine; CAX2, possible Arabidopsis protein, responsible for the transport of H+/Cd2+; ABC, ABC type conveyor; HMA4, possible Arabiopsis CD pump; ZIP, ZIP conveyors; Nramp3, Nramp family conveyor; HMW and LMW high and low molecular weight Cd-PC complex, respectively.
"Cadmium enters plants from the soil solution. It traverses the root through symplasmic or apoplasmic pathways before entering the xylem and being translocated to the shoot"
Article Root responses to cadmium in the rhizosphere: A review
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