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ENVIRONMENTAL STRESS AND ADAPTATION TO STRESS

Examining the Specific Contributions of Individual Arabidopsis Metallothioneins to Copper Distribution and Metal Tolerance^1,[OA]

Department of Botany and Plant Pathology, Purdue University, West Lafayette, Indiana 47907

Metallothioneins (MTs) are small cysteine-rich proteins found in various eukaryotes. Plant MTs are classified into four types based on the arrangement of cysteine residues. To determine whether all four types of plant MTs function as metal chelators, six Arabidopsis (Arabidopsis thaliana) MTs (MT1a, MT2a, MT2b, MT3, MT4a, and MT4b) were expressed in the copper (Cu)- and zinc (Zn)-sensitive yeast mutants, cup1 and zrc1 cot1, respectively. All four types of Arabidopsis MTs provided similar levels of Cu tolerance and accumulation to the cup1 mutant. The type-4 MTs (MT4a and MT4b) conferred greater Zn tolerance and higher accumulation of Zn than other MTs to the zrc1 cot1 mutant. To examine the functions of MTs in plants, we studied Arabidopsis plants that lack MT1a and MT2b, two MTs that are expressed in phloem. The lack of MT1a, but not MT2b, led to a 30% decrease in Cu accumulation in roots of plants exposed to 30 µM CuSO₄. Ectopic expression of MT1a RNA in the mt1a-2 mt2b-1 mutant restored Cu accumulation in roots. The mt1a-2 mt2b-1 mutant had normal metal tolerance. However, when MT deficiency was combined with phytochelatin deficiency, growth of the mt1a-2 mt2b-1 cad1-3 triple mutant was more sensitive to Cu and cadmium compared to the cad1-3 mutant. Together these results provide direct evidence for functional contributions of MTs to plant metal homeostasis. MT1a, in particular, plays a role in Cu homeostasis in the roots under elevated Cu. Moreover, MTs and phytochelatins function cooperatively to protect plants from Cu and cadmium toxicity.

² Present address: Institute of Plant and Microbial Biology, Academia Sinica, Taipei 115, Taiwan.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Peter B. Goldsbrough (goldsbrough{at}purdue.edu).

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.115782

^* Corresponding author; e-mail goldsbrough{at}purdue.edu.

Received January 2, 2008; accepted February 11, 2008; published February 20, 2008.