The term “hyperaccumulator” describes a number of plants that belong to distantly related families,
but share the ability to grow on metalliferous soils and to accumulate extraordinarily high amounts of
heavy metals in the aerial organs, far in excess of the levels found in the majority of species, without
suffering phytotoxic effects. Three basic hallmarks distinguish hyperaccumulators from related
non-hyperaccumulating taxa: a strongly enhanced rate of heavy metal uptake, a faster root-to-shoot
translocation and a greater ability to detoxify and sequester heavy metals in leaves. An interesting
breakthrough that has emerged from comparative physiological and molecular analyses of hyperaccumulators
and related non-hyperaccumulators is that most key steps of hyperaccumulation rely on
different regulation and expression of genes found in both kinds of plants. In particular, a determinant
role in driving the uptake, translocation to leaves and, finally, sequestration in vacuoles or cell walls of
great amounts of heavy metals, is played in hyperaccumulators by constitutive overexpression of genes
encoding transmembrane transporters, such as members of ZIP, HMA, MATE, YSL and MTP families.
Among the hypotheses proposed to explain the function of hyperaccumulation, most evidence has supported
the “elemental defence” hypothesis, which states that plants hyperaccumulate heavy metals as a
defence mechanism against natural enemies, such as herbivores. According to the more recent hypothesis
of “joint effects”, heavy metals can operate in concert with organic defensive compounds leading to
enhanced plant defence overall.
Heavy metal contaminated soils pose an increasing problem to human and animal health. Using plants
that hyperaccumulate specific metals in cleanup efforts appeared over the last 20 years. Metal accumulating
species can be used for phytoremediation (removal of contaminant from soils) or phytomining
(growing plants to harvest the metals). In addition, as many of the metals that can be hyperaccumulated
are also essential nutrients, food fortification and phytoremediation might be considered two sides of the
same coin. An overview of literature discussing the phytoremediation capacity of hyperaccumulators to
clean up soils contaminated with heavy metals and the possibility of using these plants in phytomining
is presented.