Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/8/2001
Publication Date: 9/29/2001
Citation: EBBS, S., LAU, I., AHNER, B., KOCHIAN, L.V. PHYTOCHELATIN SYNTHESIS IS NOT RESPONSIBLE FOR CD TOLERANCE IN THE ZN/CD HYPERACCUMULATOR THLASPI CAERULESCENS. PLANTA. 2001. Interpretive Summary: Heavy metal contamination of soils poses serious problems to both human health & agriculture in the U.S. Current engineering-based technologies used to remediate soils are quite costly, & often dramatically disturb the landscape. There has been considerable interest focused on the use of terrestrial plants to absorb heavy metals from the soil & concentrate them in the easily harvestable shoot tissues as an alternative remediation technology. A small number of interesting plant species have been identified that can grow in soils containing high levels of heavy metals & will also accumulate these metals to high concentrations in the shoot. Despite intense interest in these hyperaccumulator plants, little is known about mechanisms of heavy metal transport, translocation & sequestration involved in heavy metal hyperaccumulation in plants. Therefore, in this study we investigated a possible mechanism of heavy metal tolerance in the heavy metal hyperaccumulating plant, Thlaspi caerulescens. This plant accumulates & tolerates very high levels of the heavy metals cadmium & zinc in the leaves. We investigated whether cadmium tolerance was associated with enhanced synthesis of metal binding peptides known as phytochelatins. These peptides very effectively bind & detoxify cadmium & are believed to be involved in plant metal tolerance. In this study we found that Thlaspi caerulescens did not exhibit enhanced levels of phytochelatins compared with a non-accumulator Thlaspi species, when grown on a range of cadmium levels in the growth media. Thus metal tolerance in Thlaspi caerulescens must be by some other mechanism. This information will help us gain a better understanding of metal hyperaccumulation, & this information will be used to develop more effective remediating plant species.
Technical Abstract: Phytochelatins are metal binding peptides synthesized in response to heavy metal in plants. Phytochelatin synthase is activated primarily by cadmium, although other metals do stimulate synthesis. Phytochelatins are believed to participate in Cd tolerance, forming a Cd-PC complex that is sequestered in vacuoles. Thlaspi caerulescens is a heavy metal hyperaccumulator that accumulates metals such as Zn and Cd to high concentrations (40,000 and 4,000 mg kg-1 DW, respectively) without phytotoxicity. The mechanism of Cd tolerance in this species has not been characterized but reportedly involves vacuolar sequestration. The role of PCs in metal tolerance in Thlaspi caerulescens and the related non-accumulator Thlaspi arvense was examined. Although PCs were produced by both species in response to Cd exposure, these peptides do not appear to be involved in metal tolerance in the hyperaccumulator. Leaf and root PC levels for both species showed a similar positive correlation with tissue Cd, but total PC levels in leaves of the hyperaccumulator were generally lower, despite the correspondingly higher tissue metal concentrations. The lack of a role for PCs in the hyperaccumulator's response to metal stress suggests that another ligand might be involved in Cd tolerance. The lower level of leaf PCs in T. caerulescens, despite higher tissue Cd levels, also implies that Cd complexed in the root is transported to the shoot and sequestered in a compartment or form that does not elicit a PC response.