The molecular physiology of heavy metal transport in the Zn/Cd hyperaccumulatorThlaspi caerulescens

Abstract

An integrated molecular and physiological investigation of the fundamental mechanisms of heavy metal accumulation was conducted in Thlaspi caerulescens, a Zn͞Cd-hyperaccumulating plant species. A heavy metal transporter cDNA, ZNT1, was cloned from T. caerulescens through functional complementation in yeast and was shown to mediate high-affinity Zn 2؉ uptake as well as lowaffinity Cd 2؉ uptake. It was found that this transporter is expressed at very high levels in roots and shoots of the hyperaccumulator. A study of ZNT1 expression and high-affinity Zn 2؉ uptake in roots of T. caerulescens and in a related nonaccumulator, Thlaspi arvense, showed that alteration in the regulation of ZNT1 gene expression by plant Zn status results in the overexpression of this transporter and in increased Zn influx in roots of the hyperaccumulating Thlaspi species. These findings yield insights into the molecular regulation and control of plant heavy metal and micronutrient accumulation and homeostasis, as well as provide information that will contribute to the advancement of phytoremediation by the future engineering of plants with improved heavy metal uptake and tolerance. R ecently, there has been considerable interest in the use of terrestrial plants as a green technology for the remediation of surface soils contaminated with toxic heavy metals. This technology, termed phytoremediation, uses plants to extract heavy metals from the soil and to concentrate them in the harvestable shoot tissue (1, 2). A major factor behind the interest in phytoremediation of metal-polluted soils has been the growing awareness of the existence of a number of metalaccumulating plant species. These plant species, called hyperaccumulators, are endemic to metalliferous soils and can accumulate and tolerate high levels of heavy metals in the shoot (3, 4). Among the best known hyperaccumulators is Thlaspi caerulescens. This member of the Brassicaceae family has attracted the interest of plant biologists for over a century because of its ability to colonize calamine and serpentine soils containing naturally elevated levels of heavy metals such as Zn, Pb, Cd, Ni, Cr, and Co.

Transition metal transport

“…In both hyperaccumulators, A. halleri and T. caerulescens, the genes ZIP4, ZIP10 and IRT3 are highly expressed in roots under conditions of sufficient Zn supply [44][45][46][51][52][53]. Nevertheless, the expression of these genes remains responsive to changes in Zn supply and decreases with increasing external Zn in both species.…”

A critical review on the bio-removal of hazardous heavy metals from contaminated soils: Issues, progress, eco-environmental concerns and opportunities

“…Even worse, some heavy metals can concentrate along the food chain and eventually accumulate in human body because we are at the top of the food chain [1][2][3]. Therefore increasing attention has been paid in recent years to the remediation of polluted soils, among which the use of plants and microbes to remove hazardous metal ions is particularly emphasized [4][5][6].…”

Isolation of Zn-responsive genes from two accessions of the hyperaccumulator plant Thlaspi caerulescens

“…The ZNT1 gene, a member of the ZIP (ZRT/IRT-like proteins) transporter gene family, mediates high-aYnity Zn uptake in T. caerulescens roots (Pence et al 2000). The expression of ZNT1 is higher in T. caerulescens than in the non-accumulator Thlaspi arvense, and is not down-regulated by elevated Zn, thus potentially resulting in the increased Zn uptake (Pence et al 2000;Assunção et al 2001). …”