## 5 Conclusions and Perspectives
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Family** & **Gene Symbol** & **Expression Organ and** & **Plant** & **Possible Properties** & **References** \\ \hline \multirow{9}{*}{**ABCCs**} & _OsHMA3_ & root (tonoplast) & _Oryza sativa_ & Cd, Zn & [118; 167; 168] \\  & _OsHMA9_ & vascular bundle and & _Oryza sativa_ & Cd, Cu, Zn, Pb & [169] \\  & _ThHMA2_ & root, shoot (PM) & _Tritcium aestivum_ & Cd, Zn & [170] \\  & _GmHMA3_ & root (ER) & _Chycine max_ & Cd, Zn & [171] \\  & _ThHMA3_ & root, shoot (tonoplast) & _Thalsyi carelessness_ & Cd & [172] \\  & _SaHMA3_ & root, shoot (tonoplast) & _Sedium alfredii_ & Cd & [173] \\  & _SpHMA3_ & root, shoot (tonoplast) & _Sedium plumbiniczicola_ & Cd, Zn & [115] \\  & _AtABCCs1/2_ & root, shoot (tonoplast) & _Arabidopsis thaliana_ & Cd-PC, Hg-PC, & [82; 174] \\  & _AtABCCs3_ & root, shoot (tonoplast) & _Arabidopsis thaliana_ & Cd,PC & [143] \\  & _AtPDR8_ & root, shoot (PM) & _Arabidopsis thaliana_ & Cd & [144] \\  & _OsAC36_ & root, shoot (PM) & _Arabidopsis thaliana_ & Cd & [146] \\
**CDFs** & _OsMTP1_ & root, leaf (tonoplast) & _Oryza sativa_ & Cd, Ni, Fe & [132] \\  & _TgMTP1_ & root, leaf (tonoplast) & _Thalsyi goseisugense_ & Cd, Zn, Co, Ni & [133; 175] \\  & _CiMTP1_ & root, leaf (tonoplast) & _Citrus sinensis_ & Cd, Zn, Mn, Cu & [129] \\  & _CsMTP1/4_ & cotyled, petiole, leaf & _Cucumis sativos_ & Cd, Mn, Zn & [134] \\  & & (tonoplast) & leaf blade and sheath, & & \\
**OPTs** & _ZmYS1_ & crown, seminal root & _Zan mays_ & Cu, Ni, Cd, Fe, Zn, & [66] \\  & & (PM) & & Mn & \\  & & (PM) & & Fe(II)-Na, Mn-NA, & \\  & _OsYSL2_ & shoot phloem (PM) & _Oryza sativa_ & Cd-NA & [138; 176] \\  & _AtOPT3_ & root, shoot (PM) & _Arabidopsis thaliana_ & Cd, Zn, Fe & [177] \\  & _SnYSL3_ & root, shoot (PM) & _Sedium nigerum_ & Fe(II)-NA, Mn-NA, & [137] \\  & _ZIPs_ & _OsZIP1_ & root, shoot (ER, PM) & _Oryza sativa_ & Cd, Zn, Cu & [178] \\  & _TeIRT1_ & root (PM) & _Thalsyi carelessness_ & Cd, Zn, Fe(II, III); Mn & [179] \\  & _TeZNT1_ & root, shoot (PM) & _Thalsyi carelessness_ & Cd, Zn & [126; 127] \\
**CAXs** & _AtCAX2/4_ & root (tonoplast) & _Arabidopsis thaliana_ & Cd, Zn, Mn & [81; 148] \\  & _AtCAX1_ & root, shoot (tonoplast) & _Arabidopsis hulleri_ & Cd & [150] \\  & _SnCAX2_ & root, shoot (tonoplast) & _Sedium alfredii_ & Cd & [151] \\
**Others** & _OnLCT1_ & leaf, node, phloem & _Oryza sativa_ & Cd & [90; 180] \\  & & parenchyma (PM) & & & \\  & _OsLCD_ & root, shoot (cytoplasm, & & \\  & nucleus) & _Oryza sativa_ & Cd & [181] \\  & _OsCd1_ & root (PM) & _Oryza sativa_ & Cd & [70] \\  & _CAL1_ & root, leaf sheath, & & \\  & _CAL1_ & _Oryza sativa_ & Cd & [152] \\ \hline \hline \end{tabular} Note: PS: phytosiderophore; PC: phytotchedatin; NA: nicotianaamine; FM: plasma membrane; ER: endoplasmic reticulum; CW: cell wall; NRAMPs: natural resistance-associated macrophage proteins; HMAs: heavy metal P1B-ATPases; ABCCs: adenosine triphosphate-binding cassette subfamily C proteins; ZIPs: zinc/iron-regulated transporter-like proteins; CDFs: cation diffusion facilitators; CAXs: Ca\({}^{2+}\) exchangers; OPTs: oligopeptide transporters.  
\end{table} 
Table 2: _Cont._gests that Cd accumulation in plants is a derived and polyphyletic trait that has evolved convergently several times. During the course of evolution from algae to angiosperms, membrane transporter families, such as NRAMPs, HMAs, ABCCs, ZIPs, CDFs, and CAXs, have been conserved throughout the evolutionary lineage of entire plant kingdom (ranging from Rhodophytes to eudicots), thereby indicating that their functions are evolutionarily conserved for metal homeostasis. However, the OPT protein family is absence in all algal species of Strepthytes, Chlorophyta and Rhodophyta, thereby suggesting that this family may have evolved after the emergence of land plants.