Transcriptional regulation of Arabidopsis thaliana phytochelatin synthase (AtPCS1) by cadmium during early stages of plant development

Transcriptional regulation of Arabielopsis thaliana (L.) Heynh. phytochelatin synthase (AtPCS1) by cadmium (Cd) was analyzed at various stages of plant development using transgenic Arabidopsis and wild-type plants. Histochemical analysis of beta-glucuronidase (GUS) activity in transgenic lines carrying a uidA gene driven by a 2.0-kb AtPCS1 promoter revealed higher GUS activities in 5-day-old seedlings subjected to 50 muM Cd treatment for 5 days, beginning at seed germination, than in non-treated plants. This high level of GUS activity gradually decreased as plants continued their growth until no differences were observed between Cd-treated and non-treated transgenic plants. The observed GUS activity due to Cd treatment during the early stage of plant development corresponded with induction of AtPCS1 mRNA as confirmed by RNA blot analysis of wild-type Arabidopsis. The steady-state level of AtPCS1 mRNA increased by 2-fold in 5-day-old Cd-treated wild-type Arabidopsis compared to non-treated seedlings. Moreover, AtPCS1 protein levels increased following Cd treatment as observed in western blot analysis of transgenic Arabidopsis lines carrying a C-terminal FLAG-tagged AtPCS1 genomic DNA driven by a 2.0-kb 4tPCS1 promoter. The transcriptional regulation of AtPCS1 by Cd during, the early stage of seedling development seemed to be correlated with a higher Cd sensitivity in cad2, in Arabidopsis mutant deficient in phytochelatin synthesis, during early stages of plant development. This was supported by our finding that Cd sensitivity in cad2 was reduced as plants continued their growth, and was comparable to that of wild-type plants.