Abstract/Summary

Cadmium (Cd) is one of the most hazardous contaminants in the environment and it is often associated with zinc (Zn) in polluted soils, a nutrient that can also cause toxicity at high concentrations. Among soil remediation techniques, phytoremediation – the use of plants to immobilise and/or extract contaminants from soils - is a promising technique, considered to be less harmful to the environment. This thesis studies Cd and Zn fungi- and phytotoxicity, and the biotechnological potential of different organisms (ecto- and arbuscular mycorrhizal fungi, trees and yeast) in environmental remediation. The experiments conducted in this project aimed to investigate the potential of poplar trees (Populus trichocarpa) in Cd and Zn phytoremediation, and the use of mycorrhizal symbiosis (Rhizophagus irregularis) to enhance metal extraction and sequestration in the host plant. Another aim was to understand some of the physiological and molecular processes by which poplar trees withstand Cd and Zn toxicity, and to provide additional knowledge on the metal uptake process in mycorrhizal poplars. Transgenic yeast carrying a poplar gene (PtMT2b) was also studied for its potential in Cd bioremediation from contaminated solutions. Results showed that P. trichocarpa is highly tolerant to Cd stress, and has a considerable accumulation capacity of Cd and Zn; under both Cd and Zn exposure, poplar shoots reached hyperaccumulator levels. Mycorrhizal symbiosis increased Cd sequestration in roots, and Zn accumulation in leaves, supporting their use for Cd phytostabilisation and Zn phytoextraction. Gene expression assessment indicated mainly the involvement of PtHMA4 and PtZIP1 in Cd and Zn transport. Expression of PtMT2b was associated with mycorrhizal colonisation and its role in Cd tolerance was demonstrated in transgenic yeast assays. A mutated version of the MT2b gene (PtMT2b ‘Y’) promoted high Cd tolerance and accumulation in transgenic yeast showing promising results for bioremediation of Cdcontaminated wastewater. This thesis offers new opportunities for this possibly sustainable soil remediation technique; the knowledge gathered in this work may serve as basis for the genetic engineering of poplars or other organisms for heavy metal remediation or further research in refining and enhancing this technique.