Physiology and Molecular Biology of Trace Element Hyperaccumulation

Abstract

Metals (trace elements) are essential for plants but become toxic at high concentration. Remarkably, about 700 species worldwide are able to accumulate large quantities of metals in their leaves and are therefore called metal hyperaccumulators. In the context of sustainable development, there is renewed interest in understanding the mechanisms of metal hyperaccumulation that may become instrumental for improved metal phytoextraction from contaminated soils and for making metals available at lower environmental cost. In addition, studying the molecular mechanisms of hyperaccumulation in diverse plant species is necessary in order to understand the evolution of this extreme and complex adaptation trait in plants. Our current knowledge of metal hyperaccumulation is based mostly on the analysis of few species from the Brassicaceae family and suggests that the underlying mechanisms result from an exaggeration of the basic mechanisms involved in metal homeostasis. However, the development of Next Generation Sequencing technologies enables the study of new hyperaccumulator species and therefore the revealing of greater diversity in these mechanisms. The goal of this chapter is to provide background information on metal hyperaccumulation and give an instantaneous picture of what is currently known about the molecular mechanisms involved in this trait. We also attempt to outline for the reader the future scientific challenges that this field of research is facing.

The Different Faces of Arabidopsis arenosa—A Plant Species for a Special Purpose

“…The first stage is the uptake and transport of a metal by the roots. The primary role in this process is played by members of the ZIP family (the zinc-regulated transporter/iron-regulated transporter-like proteins), whose expression is high in the roots and/or shoots of hyperaccumulating plants [ 45 , 46 , 67 ]. ZIP19 and ZIP23 transporters are mainly responsible for the uptake of zinc by the roots of A. halleri and N. caerulescens [ 46 ].…”

“…ZIP19 and ZIP23 transporters are mainly responsible for the uptake of zinc by the roots of A. halleri and N. caerulescens [ 46 ]. In non-hyperaccumulator species, the expression of several ZIP genes is low and increases during Zn deficiency [ 46 , 67 ]. In addition, it was found that Cd treatment induces the higher expression of genes related to Cd uptake and transport in roots ( IRT1 , ZIF1 ) and shoot ( ZIF1 and YSL3 ), as well as Cd vacuolar sequestration ( HMA3 ) [ 68 ].…”

“…In addition, it was found that Cd treatment induces the higher expression of genes related to Cd uptake and transport in roots ( IRT1 , ZIF1 ) and shoot ( ZIF1 and YSL3 ), as well as Cd vacuolar sequestration ( HMA3 ) [ 68 ]. IRT1 (iron-regulated transporter 1) encoding a low selective Fe-uptake transporter in the root epidermis is also involved in Zn, Cd and/or nickel (Ni) uptake [ 67 ]. Therefore, it has been shown that different expression levels of these genes in A. halleri are associated with the differential accumulation of these metals in shoots [ 68 , 69 ].…”

“…Due to the presence of Casparian strips, the further transport of Zn is only possible via ZIP4/ZNT1 transporters. The increased expression of genes encoding these transporters was demonstrated in the roots of both A. halleri and N. caerulescens [ 46 , 67 , 71 ]. The next stage is the loading of metals into the xylem, which is mediated by the HM transporter, the HMA4 protein, a plasma membrane ATPase [ 40 , 72 ].…”

Intraspecific Variation in Nickel Tolerance and Hyperaccumulation among Serpentine and Limestone Populations of Odontarrhena serpyllifolia (Brassicaceae: Alysseae) from the Iberian Peninsula

“…The physiological mechanisms underlying hyperaccumulation have been intensively studied and current knowledge is summarized in recent reviews representing many perspectives, including genetic, molecular, and evolutionary biology [13][14][15][16][17][18][19][20]. In very broad terms, hyperaccumulation involves the two suites of processes that can be termed metal accumulation and metal tolerance.…”