Gold uptake and tolerance in Arabidopsis

Gold is a precious metal with a variety of industrial uses. The gold mining process is often incomplete, and discarded mine tailings contain quantities of gold uneconomical for conventional extraction. Previous work has indicated the potential to use plants for gold phytoextraction from these sites. Additionally, it has been demonstrated that gold nanoparticles can form within the tissues of live plants. These small particles have many uses in catalysis and medical applications but are expensive to synthesise chemically. Plants have the potential to be used as a low cost method of gold nanoparticle manufacture.
Whilst gold uptake and nanoparticle formation in plants has been studied, little research has investigated mechanisms of gold uptake. The aims of this study were therefore to characterise gold uptake and nanoparticle formation in the model plant species Arabidopsis thaliana and subsequently investigate the transcriptional response to gold to identify mechanisms of uptake and tolerance.
A number of approaches were used to investigate the gold tolerance and uptake by Arabidopsis. Growth studies determined that gold is toxic to plants and inhibits plant growth. Methods were developed to investigate and measure gold uptake in a variety of conditions. Arabidopsis was found to remove gold from soil and contained similar concentrations to those in previously tested species. Gold uptake from liquid media was also investigated. These studies determined that Arabidopsis contained significant quantities of gold in the root tissues, some of which was translocated to aerial tissues.
Electron microscopy analysis showed that gold nanoparticles form in the roots of Arabidopsis plants in a variety of growth conditions. However, gold nanoparticles were never observed in the aerial tissues of Arabidopsis but were found in the aerial tissues of alfalfa, demonstrating a difference between these two species. Gold nanoparticles were not taken up by Arabidopsis seedlings, an interesting observation because of the potential impacts of increasing numbers of nanoparticles present in the environment.
A microarray study of the transcriptional response of Arabidopsis to gold identified approximately 800 genes upregulated more than two-fold in the presence of gold. Many of these are normally upregulated in a general response to stress including cytochromes P450 and glucosyl transferases, further demonstrating the toxicity of gold to plants. Over 800 genes were downregulated more than two-fold in response to gold, including various aquaporins and transition metal transporters. Many of the most downregulated genes are controlled by the FIT1 transcription factor. FIT1 is involved in the Arabidopsis response to iron, suggesting that gold interacts in this pathway.
COPT2 (encoding a copper transporter) was one of the most downregulated genes in the presence of gold (24-fold). The other five members of the COPT family did not have altered expression in the presence of gold. A putative Arabidopsis COPT2 knockout mutant was obtained and tested for increased gold tolerance. No obvious phenotypic differences were observed when copt2-1 and wild-type seedlings were compared, suggesting that COPT2 is not involved in plant gold tolerance.
This work is the first investigation of the transcriptional response of plants to gold and indicates targets to further study for involvement in gold uptake and tolerance.