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Plant and Cell Physiology Advance Access published online on August 22, 2006
Plant and Cell Physiology, doi:10.1093/pcp/pcj096
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© The Author 2006. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
Received July 3, 2006
Accepted July 25, 2006

Regular Paper

Exposure of Lemna minor to arsenite: expression levels of the components and intermediates of the ubiquitin/proteasome pathway

Cláudia Santos 1 *, Margarida Gaspar 2, Ana Caeiro 1, Cristina Branco-Price 3, Artur Teixeira 2, and Ricardo Boavida Ferreira 4

1 Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Apartado 127, 2781-901 Oeiras, Portugal
2 Departamento de Botânica e Engenharia Biológica, Instituto Superior de Agronomia, Universidade Técnica de Lisboa, 1349-017 Lisboa, Portugal
3 Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Apartado 127, 2781-901 Oeiras, Portugal; Department of Botany and Plant Sciences, University of California, Riverside, CA 92621, Riverside, USA
4 Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Apartado 127, 2781-901 Oeiras, Portugal; Departamento de Botânica e Engenharia Biológica, Instituto Superior de Agronomia, Universidade Técnica de Lisboa, 1349-017 Lisboa, Portugal

* To whom correspondence should be addressed.
Cláudia Santos, E-mail: csantos{at}itqb.unl.pt


  Abstract

In animal cells, arsenite has been reported to cause sulfhydryl depletion, generate reactive oxygen species and increase the level of large ubiquitin-protein conjugates. Plant viability tests and DNA laddering experiments have shown that Lemna minor remains viable after exposure to 50 µM NaAsO2 for periods of at least 6 h. However, protein metabolism is affected in two major ways: The synthesis of an array of stress proteins, which confer thermotolerance; an increase in the amount of large ubiquitin-protein conjugates, particularly evident after 2-3 h of stress, indicative of a role for the ubiquitin/proteasome pathway. This outcome is primarily attributed to an increased availability in protein substrates during As treatment for three main reasons: An increase in protein carbonyl content after 1-2 h of stress; moderate increments in the transcript levels of the codifying sequences for the ubiquitin pathway components chosen as markers (polyubiquitin, E1, E2 and the {beta} subunit and the ATPase subunits of the 26S proteasome); the observed increase in ubiquitin conjugates does not depend on de novo protein synthesis.

This study is the first report on the involvement of the ubiquitin/proteasome pathway in response to arsenite in plants. In addition, it addresses the simultaneous expression of selected genes encoding the various components of the pathway. The results suggest that in plants, unlike in animals, the response to a relative low level of arsenite does not induce apoptotic cell death. As a whole, the response to arsenite apparently involves a conjugation of salvage and proteolytic machineries, including Hsp synthesis and the ubiquitin/proteasome pathway.

Keywords: arsenite stress; Lemna minor; protein oxidation; thermotolerance; ubiquitin/proteasome.

The nucleotide sequences reported in this paper have been submitted to the GenBank database under the accession numbers AY683450 for the ubiquitin-activating enzyme (E1) transcript, AY683451 for the ubiquitin-conjugating enzyme (E2) transcript, AY683447 for the polyubiquitin transcript, AY683446 for the ATPase subunit of the 26S proteasome transcript and AY683448 for the beta subunit of the 26S proteasome transcript.


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