## 4 Evolution of Cadmium Transporters and Their Functions in Cadmium Accumulation
### Function of Cd Transporters
#### 4.2.8 Other Transporters Involved in the Uptake and Intervascular Transfer of Cd  
In addition to the well-known transporter families like NRAMPs, HMAs, ABCCs, ZIPs, CDFs, CAXs and OPTs, there are other transporters also described to be involved in Cd transport. OsLCT1, a PM-localized protein that is strongly expressed in nodes and leaf blades during the reproductive stage of rice, mediates the efflux of Cd\({}^{2+}\) from phloem into grains [90]. It has been estimated that the RNA interference-mediated knockdown of _OsLCT1_ resulted in 30-50% reduction in grain Cd of rice plants grown in contaminated soil [90]. Recently, _OsCd1_, a member of the major facilitator superfamily, was shown to be involved in the root uptake and grain accumulation of Cd in rice [70]. Furthermore, phylogenetic analysis using 950 rice accessions demonstrated the natural variation in _OsCd1_ diverged between _indica_ (genotype _OsCd1_[149]) and _japonic_ (genotype _OsCd1_[149]) subspecies, and genotype _OsCd1_[149] displayed a relatively lower Cd accumulation in rice grain than the genotype _OsCd1_[149], suggesting the great potential of _OsCd1_[149] in the breeding of low-Cd rice [70]. In addition, [PERSON] et al. [152] found that Cd accumulation in leaf 1 (_CALI_), which encodes a defensin-like protein, is preferentially expressed in root exodermis and xylem parenchyma cells and acts by chelating Cd in the cytosol and facilitating Cd secretion into the apoplast, thus reducing the cytosolic Cd concentration.  
Given the similarities in charge and ionic radius between Ca\({}^{2+}\) and Cd\({}^{2+}\), it is possible for Cd\({}^{2+}\) to enter the plant symplast via passive transport through channel proteins transporting Ca\({}^{2+}\)[67; 68; 153]. Indeed, several kinds of calcium-permeable channels, such as depolarization-activated calcium channels (DACCs), hyperpolarization-activated calcium channels (HACCs), and voltage-insensitive cation channels (VICCs), are capable of transporting Cd\({}^{2+}\). Increasing evidence obtained using channel blockers and flux measurements confirms the effects of Ca on Cd uptake and accumulation in plants [67; 68; 153]. However, the function of these channels in the facilitation of Cd transport is poorly understood. In a recent study on rice, the expression of the genes belonging to the two Ca channel families, annexins and glutamate receptors (GLRs), was shown to co-segregate with Cd\({}^{2+}\) influx and uptake by root cells [67], thereby suggesting the possibility of identifying the candidate channels responsible for Cd transport.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Family**} & \multirow{2}{*}{**Gene Symbol**} & **Expression Organ and** & **Plant** & \multirow{2}{*}{**Possible Properties**} & \multirow{2}{*}{**References**} \\  & & **Localization** & & & \\ \hline
**NRAMPs** & _AtNRAMP3/4_ & root, leaf (tonoplast) & _Arabidopsis thaliana_ & Cd, Fe, Mn & [103; 154] \\  & _N\&NRAMP1_ & root, shoot (PM, & _Noccana carelessness_ & Cd & [110] \\  & _T\&NRAMP3/4_ & root, shoot (tonoplast) & _Thalspi earuslescens_ & Cd, Fe, Mn & [155] \\  & OsNRAMP1_ & root, shoot (PM) & _Oryza sativa_ & Cd & [104] \\  & OsNRAMP5 & root (PM) & _Oryza sativa_ & Cd, Fe, Mn & [156; 157] \\  & _H\&NRAMP5_ & root (PM) & _Hordeum vulgare_ & Cd, Mn & [109] \\  & _At\&_n_, shoot (chloroplast & _Arabidopsis thaliana_ & Cd, Zn, Cu & [158; 159] \\
**HMAs** & _AtHMA1_ & envelope) & _Arabidopsis thaliana_ & Cd, Zn, Pb, Co & [160; 161; 162] \\  & _AtHMA3_ & (tonoplast) & _Arabidopsis thaliana_ & Cd, Zn, Pb, Co & [163; 164; 165; 166] \\  & _AtHMA2_/4 & root, stem, leaf (PM) & _Arabidopsis thaliana_ & Cd, Zn & [163; 164; 165; 166] \\  & OsHMA2_ & root (PM) & _Oryza sativa_ & Cd, Zn & [161; 162] \\ \hline \hline \end{tabular}
\end{table}
Table 2: The identified transporters mediating cadmium (Cd) uptake, translocation, sequestration, and distribution in plants.