Plant Physiology Preview
Published on February 20, 2008; 10.1104/pp.108.115782
OPEN ACCESS ARTICLE
Received January 2, 2008
Accepted February 11, 2008
Examining the Specific Contributions of Individual Arabidopsis Metallothioneins to Copper Distribution and Metal Tolerance
Woei-Jiun Guo , Metha Meetam , and Peter B. Goldsbrough *
Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907, USA
* Corresponding author; email: goldsbrough{at}purdue.edu.
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 MTs (MT1a, MT2a, MT2b, MT3, MT4a, and MT4b) were expressed in the Cu- and Zn-sensitive yeast mutants,  cup1 and zrc1cot1, 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 Cd 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 Cd toxicity.
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