Superfund Research Program
Resistance to Heavy Metals - a Possible Tool for Phytoremediation
Several species of plants are able to survive in environments contaminated with high concentrations of metals. An understanding of the mechanisms that enable these species to accumulate and tolerate heavy metals could lead to cost effective approaches for the remediation of heavy metal-laden soils and waters. Dr. Julian Schroeder at the University of California, San Diego (UCSD) is part of an SBRP-funded research project designed to identify, isolate and characterize genes and physiological mechanisms that result in heavy metal accumulation and detoxification by plants.
In general, toxic ions are removed from eukaryotic cells by chelation and sequestration. Following exposure to metals, in plants, fungi and worms, the enzyme phytochelatin synthase (PCS) synthesizes the peptide phytochelatin (PC) from glutathione. Phytochelatins can form complexes with arsenic, cadmium, lead, and mercury, and these peptide-metal complexes are transported into plant lysosomal vacuoles, effectively isolating the toxic metals from various metal-sensitive enzymes in the plant cell cytoplasm. Often, metal concentrations can be significantly higher in plant roots than in the shoots or leaves. Prior to this research, it was hypothesized that phytochelatins function mainly in detoxifying metals within the cells where metals are taken up, and that phytochelatins cannot undergo long distance transport in plants. Therefore, it was thought that targeted expression of phytochelatin synthase genes to plant roots would cause sequestration of heavy metals in roots. For phytoremediation purposes, it would be optimal if the heavy metals were transported to the shoots and leaves before sequestration into vacuoles because the aerial parts of the plant can be easily harvested.
Dr. Shroeder's group is using transgenic plants to investigate questions concerning the root-to-shoot transport of phytochelatin synthase, phytochelatins and/or heavy metals. Starting with mutant Arabidopsis plants (cad1-3) that do not produce detectable levels of phytochelatins, they designed test systems to evaluate long-distance transport during heavy metal detoxification. They cloned a wheat PCS gene (TaPCS1) and inserted it into cad1-3 mutant plants, resulting in plants with TaPCS1 activity targeted in either the roots only or alternatively in stems, rosette leaves and roots (ectopic expression). Transgenic Arabidopsis (root-specific and ectopic), wild type Arabidopsis and cad1-3 mutant plants were then exposed to cadmium, mercury and arsenic . The Schroeder lab found that:
- both root-specific and ectopic transgenic expression of TaPCS1 suppressed the heavy metal sensitivity of cad1-3 mutant plants to arsenic, mercury and cadmium.
- in plants expressing the phytochelatin synthase enzyme in roots only, phytochelatins were detected in roots and interestingly also in rosette leaves and stems.
- both root-specific and ectopic transgenic expression of TaPCS1 reduced cadmium accumulation in roots - indicating that phytochelatin-dependent long distance transport plays a role in maintenance of low cadmium levels in the roots.
- both root-specific and ectopic transgenic expression of TaPCS1 significantly enhanced long distance cadmium transfer and accelerated cadmium accumulation in stems and rosette leaves - much more cadmium was transported from roots to shoots in transgenic plants compared to wild type plants.
These findings demonstrate that root-to-shoot transport of phytochelatins does occur and indicate that phytochelatins can provide an important mechanism for regulating long-distance cadmium transport in Arabidopsis. To follow-up on these findings, Dr. Schroeder is beginning additional studies to determine which vascular transport pathways mediate long distance phytochelatin transport (xylem or phloem), and to identify the molecular mechanisms underlying vascular phytochelatin loading.
Various studies indicate that removal of heavy metals from soils by plants would be one to two orders of magnitude less costly than excavation, transport and burial. This work highlights the real possibility that biological engineering could be used to produce transgenic plants to maximize the uptake of heavy metals. Dr. Schroeder emphasizes that for engineering of highly efficient heavy metal hyperaccumulator plants, several processes and genes would likely need to be enhanced in parallel in plants. As a first step, the rate-limiting genes and mechanisms need to be characterized. Dr. Schroeder is working closely with the UCSD SBRP Outreach Program to move these advances into practical applications at hazardous waste sites in San Diego.
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To learn more about this research, please refer to the following sources:
- Lee DA, Chen A, Schroeder JI. 2003. Ars1, an Arabidopsis mutant exhibiting increased tolerance to arsenate and increased phosphate uptake. Plant J 35(5):637-646. PMID:12940956
- Thomine S, Lelievre F, Debarbieux E, Schroeder JI, Barbier-Brygoo H. 2003. AtNRAMP3, a multispecific vacuolar metal transporter involved in plant responses to iron deficiency. Plant J 34(5):685-695. PMID:12787249
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