Ecological Role of Cadmium Hyperaccumulation

Since the amount of heavy metal content accumulated in the plant can easily be controlled by the concentration of the metal in the growth medium [39], 

Another alternative hypothesis about the biological role of hyperaccumulation was the increase of osmotic pressure for increased tolerance to the drought stress that often characterizes the natural habitats of hyper accumulators. This hypothesis, however, was falsified by a study with the Ni hyperaccumulator Alyssum murale and the Cd/Zn hyperaccumulator T. caerulescens [53]. 

Besides all the aforementioned benefits hyperaccumulated metals (may) have for the plants, there is also one recent study that strongly suggests a much more direct role in metabohsm - it may be an alternative active center in carboanhydrase of T. caerulescens, which is normally a Zn-dependent enzyme [54]. Such a functional replacement of Zn by Cd, leading to growth increase upon addition of Cd, was previously found in the marine alga Thalassiosira weissflogii [55], from which Cd-carboanhydrase was even purified and its three-dimensional structure resolved [56]. This case is described in detail in Chapter 16 of this book. 

Whatever the main ecological benefit of hyperaccumulation is, plants doing it treat the accumulated metal as something valuable, which becomes obvious during leaf senescence. As it is generally known for metals that are essential plant nutrients (e.g., [57]), also hyperaccumulators recycle beneficial metals, which seem to include the hyperaccumulated heavy metals Cd concentrations were found to be lower in senescent compared to mature and young leaves of T. caerulescens [58,59]. Furthermore, roots of T. caerulescens have been found to grow towards rather than away from heavy metals [60]. 

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