Chloroplast-targeted BrMT1 (Brassica rapa type-1 metallothionein) enhances resistance to cadmium and ROS in transgenic Arabidopsis plants

Metallothioneins (MTs) are low-molecular-weight, cysteine-rich proteins that bind to heavy metals. Type-1 MTs function under various abiotic stresses, including exposure to the cadmium ion. We have now isolated the Brassica rapa type-1 metallothionein gene (BrMT1) using yeast systems, and have found that it confers resistance to Cd in otherwise Cd-sensitive yeast. Using a constitutive CaMV35S promoter and an RbsS transit peptide, we successfully targeted BrMT1 to the chloroplasts of Arabidopsis. Overexpression in either the chloroplasts or the cytosol effectively detoxified cadmium and H2O2 stresses in transgenic Arabidopsis. in particular, the chloroplast-targeted BrMT1 was associated with a significant reduction in paraquat-induced chlorosis and the accumulation of H2O2. This is the first report regarding the effects of type-1 MT1 targeted to chloroplasts. Our results suggest that this may be applicable to the development of plants with enhanced tolerance against environmental stresses.