Physiological Characterization of Root Zn2+ Absorption and Translocation to Shoots in Zn Hyperaccumulator and Nonaccumulator Species of Thlaspi

Abstract

Radiotracer techniques were employed to characterize "Zn2+ influx into the root symplasm and translocation to the shoot i n Thlaspi caerulescens, a Zn hyperaccumulator, and Thlaspi arvense, a nonaccumulator. A protocol was developed that allowed us to quantify unidirectional 65Zn2+ i nflux across the root-cell plasma membrane (20 min of radioactive uptake followed by 15 min of desorption in a 100 p~ ZnCI, + 5 mM CaCI, solution).Concentration-dependent ZnZ+ influx in both Thlaspi species yielded nonsaturating kinetic curves that could be resolved into linear and saturable components. The linear kinetic component was shown to be cell-wall-bound Zn2+ remaining in the root after desorption, and the saturable component was due to Zn2+ influx across the root-cell plasma membrane. This saturable component followed Michaelis-Menten kinetics, with similar apparent Michaelis constant values for T. caerulescens and T. arvense (8 and 6 p~, respectively). However, the maximum initial velocity for ZnZ+ influx in T. caerulescens root cells was 4.5-fold higher than for T.arvense, indicating that enhanced absorption into the root is one of the mechanisms involved in Zn hyperaccumulation. After 96 h 1 O-fold more "Zn was translocated to the shoot of T. caerulescens compared with T. arvense. This indicates that transport sites other than entry into the root symplasm are also stimulated in T. caerulescens. We suggest that although increased root Zn2+ influx is a significant component, transport across the plasma membrane and tonoplast of leaf cells must also be critical sites for Zn hyperaccumulation in T. caerulescens.Recently, there has been an increased interest in the use of plants to decontaminate heavy-metal-polluted soils. In this process, called phytoremediation, higher plants are used to absorb contaminants from the soil into their roots and translocate them to shoots. Pollutants are subsequently removed by harvesting the above-ground tissues. This research area is lacking in basic information regarding the fundamental mechanisms employed by some plants to accumulate heavy metals in shoots.There is a small number of plant species capable not only of growing in soils containing high levels of heavy metals, '

Organic acids enhance the uptake of lead by wheat roots

“…Our results for the three uptake experiments with CaCl 2 desorption resemble previous findings by Lasat et al (1996). The linear nature of Pb uptake by de- sorbed wheat roots suggests that unidirectional Pb influx into the root symplast occurred for at least 120 min (Fig.…”

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

“…Lasat et al [3] have found that hyperaccumulator Thlaspi caerulescens had bigger capacity for Zn 2+ than its relative T. arvense. And such gap is caused by different amounts of Zn transporter proteins [5], which indicates that transporter proteins play a crucial role.…”

“…Different from other organic pollutants, hazardous heavy metals are indestructible, as they cannot be chemically or biologically degraded. 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].…”

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

“…However, the maximum capacity for Zn transport across the plasma membrane into the cytosol was 4.5-fold greater in the Znaccumulator T. caerulescens compared with the Znnonaccumulator Thlaspi arvense L. These results indicate that one characteristic of T. caerulescens is a greater capacity for Zn absorption from soil solution into root cells. Enhanced Zn uptake into the root symplasm of T. caerulescens was also associated with a greater capacity for Zn translocation to the shoot (Lasat et al, 1996). For example, after 96 h of exposure to a 65 Zn-labeled uptake solution, 10-fold more 65 Zn accumulated in the shoots of T. caerulescens compared with T. arvense, and 1.2-fold more 65 Zn accumulated in the roots of T. arvense compared with T. caerulescens.…”

“…In a previous study we reported that Zn influx into root symplasm of Zn-hyperaccumulator and -nonaccumulator species of Thlaspi was mediated by a saturable component with similar affinities for Zn (Lasat et al, 1996). However, the maximum capacity for Zn transport across the plasma membrane into the cytosol was 4.5-fold greater in the Znaccumulator T. caerulescens compared with the Znnonaccumulator Thlaspi arvense L. These results indicate that one characteristic of T. caerulescens is a greater capacity for Zn absorption from soil solution into root cells.…”