Mechanisms of Cadmium Hyperaccumulation

The mechanisms by which plants hyperaccumulate heavy metals in their shoots and prevent phytotoxicity of these metals have been the subject of many studies. Nonetheless, many of these mechanisms are still under debate. 

While the following sections deal with mechanisms of hyperaccumulation on the cellular and molecular level, it should also be noted that hyperaccumulation is modified by interactions between plants and arbuscular mycorrhizal fungi [61]. In this study, the colonisation of the plant roots with these fungi reduced Cd uptake and thus increased metal tolerance of the plants, but in studies on other plants 

A rather comprehensive study of interactions between uptake of different metals in T. caerulescens was done by Assungao et al. [67]. Via binary metal combinations they tested interactions between Cd and Zn, Cd and Ni, and Ni and Zn, which confirmed that the Ganges population expresses a highly Cd-specific high-affinity uptake system, which was not found in the other T. caerulescens populations. The other populations only had a high-affinity Zn-uptake system (present also in the Ganges population) and a low-affinity Cd/Zn/Ni-uptake system. 

An enhanced uptake of metals into the root symplasm was found in T. caerulescens compared to the related non-accumulator, T. arvense [68,69], and a reduced sequestration into the root vacuoles was associated with the higher root to shoot translocation efficiency of T. caerulescens [49,69,70]. Xylem loading and xylem transport are key steps in Cd hyperaccumulation, as it will be discussed in detail in the section about transport proteins below, and as it was commented by White et al. already 10 years ago [71]. Also in non-hyperaccumulators, the degree of Cd accumulation in above-ground tissues mainly depends on xylem transport [72]. 

While metal uptake through the root is the first important step in hyperaccumulation, most of the metal is stored in the above-ground parts. Studies of cellular metal compartmentation have shown that in most hyperaccumulators the metal is sequestered preferentially into compartments where they can not damage metabolic processes, e.g., photosynthesis as a very cadmium-sensitive vital function of plants (see Chapter 13). Therefore, it is important for hyperaccumulators to keep the metal concentration in the cytoplasm of mesophyll cells as low as possible. 

---