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Molecular Signal Processing
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Biochemistry of Plant Interactions
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Transition metals such as copper are essential for many physiological processes yet can be toxic at elevated levels. Other metals (e.g. lead) are nonessential and potentially highly toxic. Plants – like all other organisms – possess homeostatic mechanisms to maintain the correct concentrations of essential metal ions in different cellular compartments and to minimize the damage from exposure to nonessential metal ions. A regulated network of metal transport, chelation, trafficking and sequestration activities functions to provide the uptake, distribution and detoxification of metal ions. Some of the components of this network have now been identified: a number of uptake transporters have been cloned as well as candidate transporters for the vacuolar sequestration of metals. Chelators and chaperones are known, and evidence for intracellular metal trafficking is emerging. This recent progress in the molecular understanding of plant metal homeostasis and tolerance is reviewed.
Publications
Certain plant species and genotypes are able to accumulate large quantities of heavy metals in their shoots. Based on this trait the concept of phytoremediation was developed, i.e. the use of metal hyperaccumulating plants for the cleansing of contaminated soils and water. In order to more efficiently use this capacity, an engineering of plants might be needed. However, very little is known about the underlying molecular mechanisms. Our work is focussing on the identification and characterization of plant genes involved in plant metal uptake, tolerance and accumulation. Phytochelatins are small glutathione-derived metal-binding peptides which are part of the plant metal detoxification system. Genes encoding phytochelatin synthases have been cloned and are now being studied with regard to their regulation, biochemistry and biotechnological potential. Another project aimes at the dissection of metal responses in the metallophyte Arabidopsis halleri. This plant, a close relative to the model plant Arabidopsis thaliana, is Cd hypertolerant and Zn hyperaccumulating. It grows,for instance, on medieval mining sites in the Harz mountains in Germany and in many other metal-contaminated sites in Central Europe. We have isolated metal-regulated genes from A.halleri and molecularly analyzed interesting candidate genes with regard to their function and involvement in metal accumulation and tolerance.
Publications
The formation of phytochelatins, small metal‐binding glutathione‐derived peptides, is one of the well‐studied responses of plants to toxic metal exposure. Phytochelatins have also been detected in some fungi and some marine diatoms. Genes encoding phytochelatin synthases (PCS) have recently been cloned from Arabidopsis , wheat and Schizosaccharomyces pombe . Surprisingly, database searches revealed the presence of a homologous gene in the Caenorhabditis elegans genome, DDBJ/EMBL/GenBank accession no. 266513. Here we show that C. elegans indeed expresses a gene coding for a functional phytochelatin synthase. CePCS complements the Cd2+ sensitivity of a Schizosaccharomyces pombe PCS knock‐out strain and confers phytochelatin synthase activity to these cells. Thus, phytochelatins may play a role for metal homeostasis also in certain animals.
Publications
Phytochelatins represent a major detoxifying pathway for heavy metals in plants and many other organisms. The Arabidopsis thaliana CAD1 (=AtPCS1 ) gene encodes a phytochelatin synthase and cad1 mutants are phytochelatin deficient and cadmium hypersensitive. The Arabidopsis genome contains a highly homologous gene, AtPCS2 , of which expression and function were studied in order to understand the apparent non‐redundancy of the two genes. Low constitutive AtPCS2 expression is detected in all plant organs analyzed. The AtPCS2 gene encodes a functional phytochelatin synthase as shown by expression in Saccharomyces cerevisiae and the complementation of a Schizosaccharomyces pombe phytochelatin synthase knockout strain.