The cation-efflux transporter BjCET2 mediates zinc and cadmium accumulation in <Emphasis Type="Italic">Brassica juncea</Emphasis> L. leaves

Brassica juncea L. is a Zn/Cd accumulator. To determine the physiological basis of its metal accumulation phenotype, the functional properties and role of the metal efflux transporter BjCET2 were investigated using transgenic technology. Heterologous expression of BjCET2 in the double mutant yeast strain Δzrc1Δcot1 enhanced the metal tolerance of the yeast strain and led to decrease in Zn or Cd accumulation. Detection of green fluorescence from green fluorescent protein (GFP) in the root tip of transgenic tobacco further revealed that BjCET2::GFP is localized at the plasma membrane. Semi-quantitative RT-PCR analysis showed that BjCET2 was most abundant in the root and was weakly expressed in the stem and leaves. The expression of BjCET2 was up-regulated by heavy metals. However, exposure to low temperature, salt and drought did not affect the expression of BjCET2. Overexpression of BjCET2 in transgenic B. juncea plants conferred heavy metal tolerance and increased Cd/Zn accumulation in the leaves. BjCET2-deficient B. juncea mediated by antisense RNA resulted in hypersensitivity to heavy metals and decreased Zn/Cd accumulation in the plants. These results suggest that the heavy metal efflux of BjCET2 plays important roles in the metal tolerance of B. juncea and in Zn/Cd accumulation in B. juncea.