The increasing demand for precious metals such as palladium and gold for industrial applications has led to the exploration of sustainable environmental-friendly technologies to capture and recycle these metals from mine wastes. Phytomining is an emerging technology that makes use of the ability of plants to extract and accumulate metals from soil and water. Chapter 3 discusses the potential of phytomining to recover palladium from mine waste materials. These studies determined that willow (Salix sp.) and miscanthus (Miscanthus giganteus) were able to accumulate high levels of palladium in the aerial tissues when grown on synthetic media containing palladium as well as on mine waste materials. The use of chemical lixiviants improved the uptake and translocation of palladium in both willow and miscanthus. The potential of palladium nanoparticle formation in plants as plant-based catalysts was investigated but no palladium nanoparticles were detected when the plants were grown on synthetic mine waste. Chapter 4 evaluates the potential of merA gene for mercuric reductase in Arabidopsis as a genetic engineering approach to improve tolerance to gold and palladium in plants. In contrast to previously published findings merA expression did not increase tolerance of the transgenic plants to toxic levels of gold and palladium. Inhibition studies on purified mercuric reductase further revealed that gold and palladium inhibited the activity of MerA with ionic mercury. In Chapter 5, the potential of synthetic biology strategy was also investigated where the expression of synthetic short peptides, which are shown to be responsible in the formation of various sizes of metal nanoparticles in vitro, were found to increase the formation of smaller sized gold nanoparticles (<10 nm diameter) compared to wild type plants when expressed in Arabidopsis. Chapter 6 describes the transcriptional response of Arabidopsis to precious metals and investigates the potential involvement of heavy metal transporter 5 (AtHMA5) in the detoxification mechanism for gold and palladium. AtHMA5 was found to be strongly up regulated in response to gold and palladium. However, studies with Arabidopsis hma5-1 mutant knockout lines and yeast heterologous expression studies demonstrated that gold and palladium is not a substrate for AtHMA5 suggesting that AtHMA5 is not involved in gold and palladium detoxification. Overall, this work is the first to describe a holistic approach in searching for suitable field applicable plant candidates for phytomining of precious metals such as palladium and gold as well as strategies to improve its uptake, tolerance and nanoparticle formation in plants
