Arsenic pollution is an environmental problem affecting the health of millions of people worldwide. Unfortunately, conventional remediation technologies for this toxic pollutant are costly and environmentally destructive. An alternative to conventional remediation methods is phytoremediation, the use of plants to extract pollutants from contaminated soil, water and air. Recent studies demonstrated that increasing the thiolsinks in transgenic plants by over-expressing the bacterial -glutamylcysteine synthetase gene resulted in a higher tolerance and accumulation of arsenic. To further explore the potential of transgenic plants to remove arsenate from polluted soil, we genetically engineered eastern cottonwood (Populus deltoides) trees to over-express -ECS and, we also established an in vitro propagation system for another phytoremediation candidate, Salix nigra. Our results show that eastern cottonwood trees over-expressing the -ECS gene were able to grow normally on toxic levels of arsenate. We also established an in vitro regeneration system for Salix nigra from immature inflorescence explants.