Altered Zn Compartmentation in the Root Symplasm and Stimulated Zn Absorption into the Leaf as Mechanisms Involved in Zn Hyperaccumulation in Thlaspi caerulescens

Abstract

We investigated Zn compartmentation in the root, Zn transport into the xylem, and Zn absorption into leaf cells in Thlaspi caerulescens, a Zn-hyperaccumulator species, and compared them with those of a related nonaccumulator species, Thlaspi arvense. 65 Zncompartmental analysis conducted with roots of the two species indicated that a significant fraction of symplasmic Zn was stored in the root vacuole of T. arvense, and presumably became unavailable for loading into the xylem and subsequent translocation to the shoot. In T. caerulescens, however, a smaller fraction of the absorbed Zn was stored in the root vacuole and was readily transported back into the cytoplasm. We conclude that in T. caerulescens, Zn absorbed by roots is readily available for loading into the xylem. This is supported by analysis of xylem exudate collected from detopped Thlaspi species seedlings. When seedlings of the two species were grown on either low (1 M) or high (50 M) Zn, xylem sap of T. caerulescens contained approximately 5-fold more Zn than that of T. arvense. This increase was not correlated with a stimulated production of any particular organic or amino acid. The capacity of Thlaspi species cells to absorb 65 Zn was studied in leaf sections and leaf protoplasts. At low external Zn levels (10 and 100 M), there was no difference in leaf Zn uptake between the two Thlaspi species. However, at 1 mM Zn 2؉ , 2.2-fold more Zn accumulated in leaf sections of T. caerulescens. These findings indicate that altered tonoplast Zn transport in root cells and stimulated Zn uptake in leaf cells play a role in the dramatic Zn hyperaccumulation expressed in T. caerulescens.Phytoextraction is an emerging technology that involves the use of vascular plants to remediate soils contaminated with heavy metals and/or radionuclides (Nanda Kumar et al., 1995). This approach is based on the ability of higher plants to absorb contaminants from the soil and translocate them to their shoots. The identification of several metalhyperaccumulator plant species (Baker and Brooks, 1989;Baker et al., 1998) demonstrates that the genetic potential exists for successful phytoremediation of contaminated soils. One of the best-known metal hyperaccumulators is Thlaspi caerulescens J&C Presl, which has been reported to have a great potential for extraction of Zn and Cd from metalliferous soils (Reeves and

Transition metal transport

“…Thus, AtFRD3 is needed to maintain root-to-shoot Fe mobility [12,61]. It is possible that in A. halleri and T. caerulescens, FRD3 contributes to Fe homeostasis and, in particular, to Fe mobility in the presence of xylem Zn concentrations that are likely to be substantially higher than in non-accumulator species like A. thaliana [62]. The need for a higher metal chelation capacity in the xylem could also explain the high transcript levels of FRD3 in the hyperaccumulators.…”

Hyperaccumulation of Cadmium and Zinc in Thlaspi caerulescens and Arabidopsis halleri at the Leaf Cellular Level

“…The percentage of viable protoplasts in the stock was determined by staining with FDA (Fluka) as described by Lasat et al (1998). A stock solution of 7.2 mm FDA dissolved in acetone was prepared.…”

“…An important trait of hyperaccumulating species might be enhanced translocation of the absorbed metal to the shoot. Time course studies of Zn accumulation revealed that T. caerulescens exhibited a 10-fold greater Zn translocation to the shoot as compared with Thlaspi arvense (Lasat et al, 1996), which was correlated with a 5-fold increase of Zn in xylem sap (Lasat et al, 1998). These authors have performed compartmentation studies and have found that 65 Zn uptake by leaf protoplasts is also stimulated in T. caerulescens compared with T. arvense.…”

Forms of Zinc Accumulated in the HyperaccumulatorArabidopsis halleri

“…The genetics and the biochemical processes involved in metal uptake, transport, and storage by hyperaccumulating plants are still poorly understood, although this basic information is fundamental for the improvement of the technique (Van Der Lelie et al, 2001). Zn is one of the most important metal contaminant in industrialized countries (Nriagu and Pacyna, 1988), and numerous studies have been conducted on the species Thalspi caerulescens (Vazquez et al, 1992(Vazquez et al, , 1994Pollard and Baker, 1996;Lasat et al, 1998Lasat et al, , 2000Kü pper et al, 1999;Salt et al, 1999;Frey et al, 2000;Assunçaõ et al, 2001) and, to a lesser extent, on Arabidopsis halleri (Macnair et al, 1999;Bert et al, 2000;Kü pper et al, 2000;Zhao et al, 2000). This latter species is of particular interest because it is one of the closest relatives to Arabidopsis (Koch et al, 2001), whose genome is entirely sequenced (Meinke et al, 1998;Kaul et al, 2000).…”