Overexpression of poplar GSTU51 confers selective tolerance to both mercury and methyl viologen but not to CDNB or cadmium in transgenic poplars

Glutathione S-transferases belong to a large ancient gene family and are thought to be one of the effective detoxification systems. To elucidate the function of the gene in poplar, a tau class gst gene (PatgGSTU51) was cloned from poplar cell suspension cDNA library and its expression was examined. The gene was not expressed in normal conditions, but significantly induced by toxic heavy metals like cadmium and inorganic mercury. However, the highest expression was observed when treated with its synthetic substrate, 1-chloro-2,4-dinitrobenzene (CDNB). Several transgenic poplar lines harboring a chimeric p35S-PatgGSTU51 were developed to understand its function in plant defense. Real-time quantitative PCR, western blot and cellular GST activities consistently showed the transgene was highly expressed in the transgenic lines. The transgenic lines showed increased tolerance to methyl viologen as well as to inorganic mercury but not to cadmium. Furthermore, they did not show any significant tolerance against CDNB, the electrophilic substrate of GST isozymes. Thus, the results suggest that, although PatgGSTU51 is induced by a number of stress agents, it confers a selective tolerance to the toxicity of heavy metal mercury over those of other stress agents and also to oxidation stress caused by MV. Tolerance to CDNB that induces the gene to high level may require such an extra supplemental gene action as vacuolar sequestration.