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.