Expression of Arabidopsis phytochelatin synthase 2 is too low to complement an AtPCS1-defective Cad1-3 mutant

Phytochelatins play an important role in heavy metal detoxification in plants as well as in other organisms. The Arabidopsis thaliana mutant cad1-3 does not produce detectable levels of phytochelatins in response to cadmium stress. The hypersensitivity of cad1-3 to cadmium stress is attributed to a mutation in the phytochelatin synthase 1 (AtPCS1) gene. However, A. thaliana also contains a functional phytochelatin synthase 2 (AtPCS2). In this study, we investigated why the cad1-3 mutant is hypersensitive to cadmium stress despite the presence of AtPCS2. Northern and Western blot analyses showed that expression of AtPCS2 is weak compared to AtPCS1 in both roots and shoots of transgenic Arabidopsis. The lower level of AtPCS2 expression was confirmed by RT-PCR analysis of wild type Arabidopsis. Moreover, no tissue-specific expression of AtPCS2 was observed. Even when AtPCS2 was under the control of the AtPCS1 promoter or of the cauliflower mosaic virus 35S promoter (CaMV 35S) it was not capable of fully complementing the cad1-3 mutant for cadmium resistance.