title: Mechanisms of the light-dependent induction of cell death in tobacco plants with delayed senescence.
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 (F(v)/F(m)) and non-photochemical quenching (NPQ), as determined by fluorescence imaging. The same patterns of F0, F(v)/F(m), 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.
authors.affiliation: Department of Biology, University College London, Gower Street, London WC1E 6BT, UK. a.wingler@ucl.ac.uk
authors.forename: Astrid
authors.initials: A
authors.lastname: Wingler
authors.forename: Emily
authors.initials: E
authors.lastname: Brownhill
authors.forename: Nathalie
authors.initials: N
authors.lastname: Pourtau
mesh_terms: D016923:Cell Death / Q000528:radiation effects; D002734:Chlorophyll / Q000378:metabolism; D003583:Cytokinins / Q000096:biosynthesis / Q000378:metabolism; D004579:Electron Transport; D035264:Flowers / Q000502:physiology / Q000528:radiation effects; D005453:Fluorescence; D018506:Gene Expression Regulation, Plant / Q000528:radiation effects; D017343:Genes, Plant / Q000235:genetics; D008027:Light; D010788:Photosynthesis / Q000502:physiology / Q000528:radiation effects; D045332:Photosystem II Protein Complex / Q000378:metabolism; D018515:Plant Leaves / Q000166:cytology / Q000502:physiology / Q000528:radiation effects; D010940:Plant Proteins / Q000235:genetics; D030821:Plants, Genetically Modified; D012098:Reproduction / Q000502:physiology / Q000528:radiation effects; D013997:Time Factors; D014026:Nicotiana / Q000166:cytology* / Q000235:genetics / Q000502:physiology / Q000528:radiation effects*
chemical_list: D003583:Cytokinins; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D002734:Chlorophyll