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JXB Advance Access published online on October 24, 2005
Journal of Experimental Botany, doi:10.1093/jxb/eri310
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© The Author [2005]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved.
Received April 7, 2005
Accepted September 8, 2005

RESEARCH PAPER

Harpin modulates the accumulation of salicylic acid by Arabidopsis cells via apoplastic alkalization

Andrew Clarke 1, Luis A. J. Mur 1, Robert M. Darby 1*, and Paul Kenton 1

1 Institute of Biological Sciences, University of Wales, Aberystwyth, Edward Llwyd Building, Penglais Campus, Aberystwyth SY23 3DA, Wales, UK

* To whom correspondence should be addressed.
Robert M. Darby, E-mail: rmd{at}aber.ac.uk


  Abstract

It is reported here that salicylic acid (SA) is rapidly taken up by Arabidopsis cells, and its uptake is accompanied by media alkalization and cytosolic acidification, and it is inhibited by the ionophore nigericin, suggesting that its import is linked with that of H+ and driven by a proton gradient. Such import and accumulation declined sharply within a narrow physiological pH range (pH 5.7-6.1), corresponding to a reduction in the [H+] of the media from 1.99 µmol l-1 to 0.79 µmol l-1. Following the initial uptake, SA was exported back into the media as free SA against a continued [H+]-dependent import. Since the uptake and accumulation of SA declines sharply within a narrow pH range and cell wall alkalization is an early response during incompatible plant/pathogen interactions, the bacterial elicitor harpinPss was used to investigate how SA transport may be modulated during defence responses. Harpin induced a rapid and sustained alkalization of the cell suspension media, reaching the critical pH (pH 5.9-6.1) at which SA import is inhibited at c. 60 min. Such media alkalization corresponded with a reduction in the SA associated with cells co-treated with harpin, and an inhibition of SA uptake in cells pretreated with harpin. Scavengers of ROS, or compounds which generate H2O2 or NO had little effect on the import or net export of SA, suggesting that media alkalization induced by harpin is sufficient to modulate the kinetics of SA transport.

Keywords: Apoplastic alkalization; Arabidopsis thaliana; harpin; nitric oxide; proton gradient; reactive oxygen species; salicylic acid.
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