Development of transgenic hybrid sweetgum (Liquidambar styraciflua x L-formosana) expressing γ-glutamylcysteine synthetase or mercuric reductase for phytoremediation of mercury pollution

Using Agrobacterium-mediated gene transfer, we generated transgenic hybrid sweetgum (Liquidambar styraciflua x L. formosana) overexpressing two types of genes to enhance plant remediation of mercury-contaminated soil and water: bacterial gamma-glutamylcysteine synthetase gene (ECS), the first and most important enzyme in phytochelatin synthesis, or various genes encoding a mercuric ion reductase (merA9, merA18, merA77). Hybrid sweetgum proembryogenic masses (PEMs) constitutively overexpressing ECS were able to grow in the presence of 50 mu M HgCl2, which inhibited wild-type PEMs, but plantlets regenerated from the PEMs had abnormal form and did not survive for more than a few weeks following germination. In contrast, mature somatic embryos generated from PEMs constitutively overexpressing merA9 and merA18 converted to normal plantlets on germination medium containing 25 mu M HgCl2, while control embryos were killed on 25 mu M Hg(II)-medium. Transgenic merA plantlets displayed enhanced resistance to Hg(II) and released Hg(0) two to three times more efficiently than the wild-type plantlets.