Compartmentation and complexation of metals in hyperaccumulator plants

Abstract

Hyperaccumulators are being intensely investigated. They are not only interesting in scientific context due to their “strange” behavior in terms of dealing with high concentrations of metals, but also because of their use in phytoremediation and phytomining, for which understanding the mechanisms of hyperaccumulation is crucial. Hyperaccumulators naturally use metal accumulation as a defense against herbivores and pathogens, and therefore deal with accumulated metals in very specific ways of complexation and compartmentation, different from non-hyperaccumulator plants and also non-hyperaccumulated metals. For example, in contrast to non-hyperaccumulators, in hyperaccumulators even the classical phytochelatin-inducing metal, cadmium, is predominantly not bound by such sulfur ligands, but only by weak oxygen ligands. This applies to all hyperaccumulated metals investigated so far, as well as hyperaccumulation of the metalloid arsenic. Stronger ligands, as they have been shown to complex metals in non-hyperaccumulators, are in hyperaccumulators used for transient binding during transport to the storage sites (e.g., nicotianamine) and possibly for export of Cu in Cd/Zn hyperaccumulators [metallothioneins (MTs)]. This confirmed that enhanced active metal transport, and not metal complexation, is the key mechanism of hyperaccumulation. Hyperaccumulators tolerate the high amount of accumulated heavy metals by sequestering them into vacuoles, usually in large storage cells of the epidermis. This is mediated by strongly elevated expression of specific transport proteins in various tissues from metal uptake in the shoots up to the storage sites in the leaf epidermis. However, this mechanism seems to be very metal specific. Non-hyperaccumulated metals in hyperaccumulators seem to be dealt with like in non-hyperaccumulator plants, i.e., detoxified by binding to strong ligands such as MTs.

NaCl protects against Cd and Cu-induced toxicity in the halophyte Atriplex halimus

“…Based on their ability to cope with heavy metals in the medium where they grow, plants can be divided into three groups: i) indicator plants, sensitive to heavy metals; ii) excluder plants, being able to tolerate high heavy metal concentrations in the soil by preventing its accumulation the cytosol, and iii) hyperaccumulator plants, that tolerate high concentrations of specific trace elements in the medium, with an active uptake and specific accumulation of those elements (reviewed in Leitenmaier & Küpper, 2013).…”

SOIL FERTILITY AFFECTS ELEMENTAL DISTRIBUTION IN NEEDLES OF THE CONIFER Araucaria angustifolia: A MICROANALYTICAL STUDY

“…For a wide range of species that accumulate metals in leaves, Leitenmaier and Küpper (2013) observed that the principal accumulation site is the epidermis. Under the most common condition (accumulation in the epidermis), it is likely that metals cannot be discharged from the rib xylem to the mesophyll symplastically, thus, they are carried by rib extensions to the epidermis apoplastically (KARLEY et al, 2003).…”

Oxalic Acid/Oxalates in Plants:From Self-Defence to Phytoremediation

“…by hyperaccumulators [38][39][40][41] , which are being utilized in phytoremediation of soils affected by toxicity of these heavy metals. This toxicity in soils may be caused by the continuous use of sewage sludge 42 or closeness to zinc or lead smelters [43][44][45] or due to other natural causes.…”