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Expression of BjMT2, a metallothionein 2 from Brassica juncea, increases copper and cadmium tolerance in Escherichia coli and Arabidopsis thaliana, but inhibits root elongation in Arabidopsis thaliana seedlings.

Zhigang A, Cuijie L, Yuangang Z, Yejie D, Wachter A, Gromes R, Rausch T

Northeast Forestry, University Key Laboratory of Forest Plant Ecology, Ministry of Education, Hexing Road 26, Harbin 150040, PR China.

The protective function of a plant type-2 metallothionein was analysed after expression in Escherichia coli and in Arabidopsis thaliana seedlings. BjMT2 from Brassica juncea was expressed in E. coli as a TrxA::BjMT2 fusion protein. After affinity chromatography and cleavage from the TrxA domain, pure BjMT2 protein was obtained which strongly reacted with the thiol reagent monobromobimane. Escherichia coli cells expressing the TrxA::BjMT2 fusion were more tolerant to Cu(2+) and Cd(2+) exposure than control strains. Likewise, when BjMT2 cDNA was expressed in A. thaliana under the regulation of the 35S promoter, seedlings exhibited an increased tolerance against Cu(2+) and Cd(2+) based on shoot growth and chlorophyll content. Analysis of transiently transformed cells of A. thaliana and tobacco leaves by confocal laser scanning microscopy (CLSM) revealed exclusive cytosolic localization of a BjMT2::EGFP (enhanced green fluorescent protein) fusion protein in control and heavy metal-exposed plant cells. Remarkably, ectopic expression of BjMT2 reduced root growth in the absence of heavy metal exposure, whereas in the presence of 50 or 100 microM Cu(2+) root growth in control and transgenic lines was identical. The results indicate that in A. thaliana, root and shoot development are differentially affected by ectopic expression of BjMT2.

Published 7 September 2006 in J Exp Bot.
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