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Plant and Cell Physiology Advance Access originally published online on May 11, 2009
Plant and Cell Physiology 2009 50(6):1156-1170; doi:10.1093/pcp/pcp067
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© The Author 2009. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved.
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A Mutant Strain Arabidopsis thaliana that Lacks Vacuolar Membrane Zinc Transporter MTP1 Revealed the Latent Tolerance to Excessive Zinc

Miki Kawachi1, Yoshihiro Kobae1, Haruki Mori1, Rie Tomioka1, Youngsook Lee2 and Masayoshi Maeshima1,*

1Laboratory of Cell Dynamics, Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, 464-8601 Japan
2Division of Molecular Life Sciences, Pohang University of Science and Technology, Pohang, 790-784 Korea

*Corresponding author: E-mail, maeshima{at}agr.nagoya-u.ac.jp; Fax, +81-52-789-4096.


  Abstract

A mutant line of Arabidopsis thaliana that lacks a vacuolar membrane Zn2+/H+ antiporter MTP1 is sensitive to zinc. We examined the physiological changes in this loss-of-function mutant under high-Zn conditions to gain an understanding of the mechanism of adaptation to Zn stress. When grown in excessive Zn and observed using energy-dispersive X-ray analysis, wild-type roots were found to accumulate Zn in vacuolar-like organelles but mutant roots did not. The Zn content of mutant roots, determined by chemical analysis, was one-third that of wild-type roots grown in high-Zn medium. Severe inhibition of root growth was observed in mtp1-1 seedlings in 500 µM ZnSO4. Suppression of cell division and elonga-tion by excessive Zn was reversible and the cells resumed growth in normal medium. In mutant roots, a marked formation of reactive oxygen species (ROS) appeared in the meristematic zone, where the MTP1 gene was highly expressed. Zn treatment enhanced the expression of several genes involved in Zn tolerance: namely, the plasma membrane Zn2+-export ATPase, HMA4, and plasma and vacuolar membrane proton pumps. CuZn-superoxide dismutases, involved in the detoxification of ROS, were also induced. The expression of plasma membrane Zn-uptake transporter, ZIP1, was suppressed. The up- or down-regulation of these genes might confer the resistance to Zn toxicity. These results indicate an essential role of MTP1 in detoxification of excessive Zn and provide novel information on the latent adaptation mechanism to Zn stress, which is hidden by MTP1.

Keywords: Arabidopsis thaliana - Metal tolerance - MTP - Zinc transporter - Vacuole.

Abbreviations: AHA, Arabidopsis thaliana plasma mem-brane H+-ATPase; EDX, energy dispersive X-ray; HMA, heavy metal transporter; ICP-AES, inductively coupled plasma atomic-emission spectroscopy; GUS, β-glucuronidase; MTP, metal tolerant protein; ROS, reactive oxygen species; TEM, transmission electron microscopy; V-ATPase, vacuolar mem-brane H+-ATPase; VHA-a, subunit a of V-ATPase; V-PPase, vacuolar H+-pyrophosphatase; MES, 2-(N-morpholino) ethanesulfonate.

(Received April 16, 2009; Accepted May 4, 2009)
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