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

Abstract

Vacuolar compartmentalization or cell wall binding in leaves could play a major role in hyperaccumulation of heavy metals. However, little is known about the physiology of intracellular cadmium (Cd) sequestration in plants. We investigated the role of the leaf cells in allocating metal in hyperaccumulating plants by measuring short-term 109 Cd and 65 Zn uptake in mesophyll protoplasts of Thlaspi caerulescens "Ganges" and Arabidopsis halleri, both hyperaccumulators of zinc (Zn) and Cd, and T. caerulescens "Prayon," accumulating Cd at a lower degree. The effects of low temperature, several divalent cations, and pre-exposure of the plants to metals were investigated. There was no significant difference between the MichaelisMenten kinetic constants of the three plants. It indicates that differences in metal uptake cannot be explained by different constitutive transport capacities at the leaf protoplast level and that plasma and vacuole membranes of mesophyll cells are not responsible for the differences observed in heavy metal allocation. This suggests the existence of regulation mechanisms before the plasma membrane of leaf mesophyll protoplasts. However, pre-exposure of the plants to Cd induced an increase in Cd accumulation in protoplasts of "Ganges," whereas it decreased Cd accumulation in A. halleri protoplasts, indicating that Cd-permeable transport proteins are differentially regulated. The experiment with competitors has shown that probably more than one single transport system is carrying Cd in parallel into the cell and that in T. caerulescens "Prayon," Cd could be transported by a Zn and Ca pathway, whereas in "Ganges," Cd could be transported mainly by other pathways.Cadmium (Cd) and zinc (Zn) are two widespread harmful heavy metals, but there is no cost-effective mean to remove them from the soil. Although phytoextraction using hyperaccumulator plants is seen as a promising technique, a lack of understanding of the basic physiological, biochemical, and molecular mechanisms involved in heavy metal hyperaccumulation prevents the optimization of the phytoextraction technique and its further commercial application. Therefore, a research priority is to gain basic information on the dynamics of metal movement into the cells, their final allocation, and their sink capacities in hyperaccumulating species.Thlaspi caerulescens and Arabidopsis halleri are both plants able to hyperaccumulate Zn and Cd (Robinson et al., 1998; Bert et al., 2000). In T. caerulescens, Zn seems to be sequestrated preferentially in vacuoles of epidermal cells in a soluble form (Kü pper et al., 1999; Frey et al., 2000). In A. halleri leaves, Zn was found to be predominantly coordinated to malate (Sarret et al., 2002) and accumulated in the mesophyll cells (Kü pper et al., 2000;Zhao et al., 2000). 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 a...

Phytofiltration of arsenic and cadmium by using an aquatic plant, Micranthemum umbrosum: Phytotoxicity, uptake kinetics, and mechanism

“…Further confirmation by measuring the content of amino acids concluded that only 2.13% of the amino acids present in the Cd treated leaf samples were thiol containing cysteic acid. Cosio et al (2004) found that Cd accumulation increases in T. caerulescens 'Ganges' and decreases in A. halleri protoplasts indicating that Cd-permeable transport proteins are differentially regulated. They also concluded that Cd could be transported by a Zn and Ca pathway in Thlaspi caerulescens 'Prayon', whereas in 'Ganges', Cd was transported by other pathways (Cosio et al, 2004).…”

“…Cosio et al (2004) found that Cd accumulation increases in T. caerulescens 'Ganges' and decreases in A. halleri protoplasts indicating that Cd-permeable transport proteins are differentially regulated. They also concluded that Cd could be transported by a Zn and Ca pathway in Thlaspi caerulescens 'Prayon', whereas in 'Ganges', Cd was transported by other pathways (Cosio et al, 2004). Thus, Cd uptake mechanisms might vary from one species of plant to another.…”

AtOSA1, a Member of the Abc1-Like Family, as a New Factor in Cadmium and Oxidative Stress Response

“…First, the mesophyll protoplasts were prepared from leaves according to the protocol described in Cosio et al (2004) and subsequently, the intact chloroplasts were obtained according to the method of Fitzpatrick and Keegstra (2001). The collected protoplast pellet was resuspended briefly in 300 mM sorbitol, 20 mM Tricine-KOH, pH 8.4, 10 mM EDTA, 10 mM NaHCO 3 , 0.1% (w/v) bovine serum albumin, and forced twice through 20-and 11-mm nylon mesh.…”

Ecophysiological adaptation and metal accumulation in water hyacinth from two tropical rivers

“…The formation of Fe plaques on aquatic plants roots has been also reported (Povidisa et al, 2009) and this probably difficult the exactitude in quantifying this metal. When another hyperaccumulator (Arabidopsis halleri) was submitted to growth in a hydroponic solution containing Cd and Zn, the cell walls of epidermal cells retained practically all these ions, suggesting that accumulation of these metals may be due to the precipitation of Cd and Zn phosphate in the cell wall of (Cosio et al, 2004). However, when metals enter in the cell, in general they are restrained in vacuole, in a process crucial of detoxification.…”