Summary• Uptake kinetics and translocation characteristics of cadmium and zinc are presented for two contrasting ecotypes of the Cd/Zn hyperaccumulator Thlaspi caerulescens , Ganges (southern France) and Prayon (Belgium).• Experiments using radioactive isotopes were designed to investigate the physiology of Cd and Zn uptake, and a pressure-chamber system was employed to collect xylem sap.• In contrast to similar Zn uptake and translocation, measurements of concentrationdependent influx of Cd revealed marked differences between ecotypes. Ganges alone showed a clear saturable component in the low Cd concentration range; maximum influx V max for Cd was fivefold higher in Ganges; and there was a fivefold difference in the Cd concentration in xylem sap. Addition of Zn to the uptake solution at equimolar concentration to Cd did not decrease Cd uptake by Ganges, but caused a 35% decrease in Prayon.• There is strong physiological evidence for a high-affinity, highly expressed Cd transporter in the root cell plasma membranes of the Ganges ecotype of T. caerulescens . This raises evolutionary questions about specific transporters for non-essential metals. The results also show the considerable scope for selecting hyperaccumulator ecotypes to achieve higher phytoextraction efficiencies.
Analysis of rice plants exposed to a broad range of relatively low and environmentally realistic Cd concentrations showed that the root capacity to retain Cd ions rose from 49 to 79%, corresponding to increases in the external Cd
ZmST1;1, a putative high-affinity sulfate transporter gene expressed in maize (Zea mays) roots, was functionally characterized and its expression patterns were analyzed in roots of plants exposed to different heavy metals (Cd, Zn, and Cu) interfering with thiol metabolism. The ZmST1;1 cDNA was expressed in the yeast (Saccharomyces cerevisiae) sulfate transporter mutant CP154-7A. Kinetic analysis of sulfate uptake isotherm, determined on complemented yeast cells, revealed that ZmST1;1 has a high affinity for sulfate (K m value of 14.6 6 0.4 mM). Cd, Zn, and Cu exposure increased both ZmST1;1 expression and root sulfate uptake capacity. The metal-induced sulfate uptakes were accompanied by deep alterations in both thiol metabolism and levels of compounds such as reduced glutathione (GSH), probably involved as signals in sulfate uptake modulation. Cd and Zn exposure strongly increased the level of nonprotein thiols of the roots, indicating the induction of additional sinks for reduced sulfur, but differently affected root GSH contents that decreased or increased following Cd or Zn stress, respectively. Moreover, during Cd stress a clear relation between the ZmST1;1 mRNA abundance increment and the entity of the GSH decrement was impossible to evince. Conversely, Cu stress did not affect nonprotein thiol levels, but resulted in a deep contraction of GSH pools. Our data suggest that during heavy metal stress sulfate uptake by roots may be controlled by both GSH-dependent or -independent signaling pathways. Finally, some evidence suggesting that root sulfate availability in Cd-stressed plants may limit GSH biosynthesis and thus Cd tolerance are discussed.To minimize the detrimental effects of heavy metal accumulation, plants have evolved detoxification mechanisms, mainly based on chelation and subcellular compartmentalization (Clemens, 2001). The efficiency of these processes might result in the natural heavy metals tolerance and their basic understanding might be crucial for improving plant performances in phytoextraction of heavy metals from polluted soils (Salt et al., 1998;Pilon-Smits, 2005).Chelation of heavy metals is a ubiquitous detoxification strategy described in a wide variety of plants (Zenk, 1996;Clemens, 2001). One of the principal classes of heavy metal chelators known in plants is phytochelatins (PCs), a family of Cys-rich peptides with the general structure (g-Glu-Cys) n -Gly (n 5 2-11). PCs are synthesized nontranslationally from reduced glutathione (GSH) in a transpeptidation reaction catalyzed by the enzyme PC synthase (PCS; Rea et al., 2004). Their synthesis is induced within minutes following exposure to different metals or metalloids; among these, Cd is the strongest inducer, whereas other metals such as Cu, Zn, Pb, and Ni are less effective and require higher external levels for induction (Grill et al., 1987;Maitani et al., 1996). Interestingly, some inducers, such as Zn, do not seem to serve as substrates for chelation (Clemens, 2001;Souza and Rauser, 2003).The mechanism of heavy metal detoxif...
The effect of cadmium (Cd) on high-affinity sulfate transport of maize (Zea mays) roots was studied and related to the changes in the levels of sulfate and nonprotein thiols during Cd-induced phytochelatin (PC) biosynthesis. Ten micromolar CdCl 2 in the nutrient solution induced a 100% increase in sulfate uptake by roots. This was not observed either for potassium or phosphate uptake, suggesting a specific effect of Cd 2ϩ on sulfate transport. The higher sulfate uptake was not dependent on a change in the proton motive force that energizes it. In fact, in Cd-treated plants, the transmembrane electric potential difference of root cortical cells was only slightly more negative than in the controls, the external pH did not change, and the activity of the plasma membrane H ϩ -ATPase did not increase. Kinetics analysis showed that in the range of the high-affinity sulfate transport systems, 10 to 250 m, Cd exposure did not influence the K m value (about 20 m), whereas it doubled the V max value with respect to the control. Northern-blot analysis showed that Cd-induced sulfate uptake was related to a higher level of mRNA encoding for a putative high-affinity sulfate transporter in roots. Cd-induced sulfate uptake was associated to both a decrease in the contents of sulfate and glutathione and synthesis of a large amount of PCs. These results suggest that Cd-induced sulfate uptake depends on a pretranslational regulation of the high-affinity sulfate transporter gene and that this response is necessary for sustaining the higher sulfur demand during PC biosynthesis.
: A pressure-chamber technique was used to study the root uptake and xylem translocation of some fungicides, herbicides and an insecticide from di †er-ent chemical classes in detopped soybean roots. Physiological parameters such as K`leakage from roots, K`concentrations in the xylem sap, and protein and ATP levels in the root cells were measured so as to evaluate any potential damage of this technique to the root system. HPLC was used to quantify the compounds in the xylem sap. The pressure-chamber technique has proved useful to study the root uptake and translocation of pesticides, does not damage the root system, and allows one to obtain appreciable volumes of xylem sap that can be analysed directly by HPLC, thus avoiding dependence on the availability of radio-labelled compounds. The concentration of each pesticide in the xylem sap showed a steady-state kinetic proÐle. Non-linear regression analysis was used to calculate the steady-state concentration and the time required to achieve 50% of the steady-state concentration was well correlated with log (TSSC 50 ). TSSC 50 the more lipophilic the compound the more time was required to reach the K ow ; steady-state concentration. The efficiency of translocation was assessed by the transpiration stream concentration factor (TSCF) and a non-linear relationship between TSCF and log was observed. The highest TSCF values were mea-K ow sured for those compounds with log values around 3, a lipophilicity value K ow similar to that reported earlier in an analogous experiment with detopped soybean plants but slightly higher than that reported in earlier experiments with intact barley plants. Lower TSCF values were obtained with chemicals with log values below as well as above 3. K ow
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