Submitted to: Journal of Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/10/1996
Publication Date: N/A
Interpretive Summary: Heavy metal (Pb, Zn, Cd, Cu, Cr and Ni) contamination of soils from industrial practices poses serious problems to both human and agriculture in some locations in the US. Current engineering-based technologies used to remediate soils (e.g., removal of top soil for storage in landfills) are quite costly, and often dramatically disturb the landscape. Recently, there ehas been considerable interest focused on the use of terrestrial plants to absorb heavy metals from the soil and concentrate them in the easily harvestable shoot tissues as an alternative remediation technology. A small number of very interesting plant species have been identified that can grow in soils containing high levels of heavy metals, and will also accumulate these metals to high concentrations in the shoot. Despite the intense interest in these hyperaccumulator plants, very little is known about mechanisms of heavy metal transport, translocation and sequestration involved in heavy meatl hyperaccumulation in plants. Therefore, in this study we conducted a physiological characterization of Zn accumulation in the heavy metal species hyperaccumulating plants, Thlaspi caerulescens. It was found that this plant species expresses a greatly stimulated Zn uptake into the roots, as well as a strongly enhanced Zn transport from the root to the shoot. The results indicate that metal hyperaccumulation is a complex trait involving a number of different transport sites within the plant. This fundamental information will help us gain a better understanding of metal hyperaccumulation, and this information will be used to develop more effective remediating plant species and agronomic practices that enhance phytoremediation.
Technical Abstract: Radiotracer techniques were employed to characterize 65Zn2+ influx into the root symplasm and translocation to the shoot in Thlaspi caerulescens, a zinc hyperaccumulator, and Thlaspi arvense, a species found not to express hyperaccumulating properties. A protocol was developed that allowed us to quantify unidirectional 65Zn2+ influx across the root-cell plasma membrane. .Concentration-dependent Zn2+ influx in both Thlaspi species yielded nonsaturating kinetic curves that could be resolved into a linear and a saturable component. The linear kinetic component was shown to be cell wall-bound Zn2+ remaining in the root after desorption, while the saturable component was due to Zn2+ influx across the root cell plasma membrane. This saturable component followed Michaelis-Menten kinetics with similar apparent Km values for T. caerulescens and T. arvense (8 and 6 yM, respectively). However, the Vmax for Zn2+ 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, 10-fold more 65Zn was translocated to the shoot of T. caerulescens compared to T. arvense. This indicates that transport sites other than entry into the root symplasm are also stimulated in T. caerulescens. We suggest that while increased root Zn2+ influx is a significant component, transport across plasma membrane and tonoplast of leaf cells must also be critical sites for zinc hyperaccumulation in T. caerulescens.