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JXB Advance Access published online on September 12, 2005
Journal of Experimental Botany, doi:10.1093/jxb/eri284
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© The Author [2005]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved.
Received May 10, 2005
Accepted August 5, 2005

RESEARCH PAPER

Mechanisms of the light-dependent induction of cell death in tobacco plants with delayed senescence

Astrid Wingler 1*, Emily Brownhill 1, and Nathalie Pourtau 1

1 Department of Biology, University College London, Gower Street, London WC1E 6BT, UK

* To whom correspondence should be addressed.
Astrid Wingler, E-mail: a.wingler{at}ucl.ac.uk


  Abstract

The relationship between leaf senescence and cell death was investigated using tobacco with delayed senescence due to auto-regulated production of cytokinin (SAG12-IPT). Although leaf senescence ultimately results in cell death, the results show that senescence and cell death can be uncoupled: in nutrient-deficient, but not in fertilized SAG12-IPT plants, necrotic lesions were detected in old, but otherwise green leaves. By contrast, wild-type leaves of the same age were yellow, but not necrotic. Chlorophyll fluorescence analysis revealed an over-reduction of the electron transport chain in old SAG12-IPT leaves, in combination with characteristic spatial patterns of minimum fluorescence (F0), quantum efficiency of open photosystem II centres (Fv/Fm) and non-photochemical quenching (NPQ), as determined by fluorescence imaging. The same patterns of F0, Fv/Fm, and NPQ were induced by incubation of leaf discs from nutrient-deficient SAG12-IPT plants under illumination, but not in the dark, indicating that light-dependent reactions were responsible for the cell death. RT-PCR analysis showed that the pathogenesis-related (PR) genes PR-1b and PR-Q were strongly induced in old SAG12-IPT tobacco leaves with necrotic lesions. In addition, the ethylene-synthesis gene ACO was induced before lesions became visible in SAG12-IPT. It is proposed that over-reduction of the electron transport chain in combination with decreased electron consumption due to nutrient-deficiency led to oxidative stress, which, mediated by ethylene formation, can induce PR gene expression and hypersensitive cell death. Probably as a consequence of inefficient nutrient mobilization, flower development was prematurely aborted and reproduction thereby impaired in nutrient-deficient SAG12-IPT plants.

Keywords: Apoptosis; cytokinin; defence; hypersensitive response; oxidative stress; programmed cell death; senescence; tobacco.
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