title: Accumulation of chlorophyll catabolites photosensitizes the hypersensitive response elicited by Pseudomonas syringae in Arabidopsis.
abstract: • The staygreen (SGR) gene encodes a chloroplast-targeted protein which promotes chlorophyll degradation via disruption of light-harvesting complexes (LHCs). • Over-expression of SGR in Arabidopsis (SGR-OX) in a Columbia-0 (Col-0) background caused spontaneous necrotic flecking. To relate this to the hypersensitive response (HR), Col-0, SGR-OX and RNAi SGR (SGRi) lines were challenged with Pseudomonas syringae pv tomato (Pst) encoding the avirulence gene avrRpm1. Increased and decreased SGR expression, respectively, accelerated and suppressed the kinetics of HR-cell death. In Col-0, SGR transcript increased at 6  h after inoculation (hai) when tissue electrolyte leakage indicated the initiation of cell death. • Excitation of the chlorophyll catabolite pheophorbide (Pheide) leads to the formation of toxic singlet oxygen ((1)O(2)). Pheide was first detected at 6  hai with Pst avrRpm1 and was linked to (1)O(2) generation and correlated with reduced Pheide a oxygenase (PaO) protein concentrations. The maximum quantum efficiency of photosystem II (F(v)/F(m)), quantum yield of electron transfer at photosystem II (φPSII), and photochemical quenching (qP) decreased at 6  hai in Col-0 but not in SGRi. Disruption of photosynthetic electron flow will cause light-dependent H(2)O(2) generation at 6  hai. • We conclude that disruption of LHCs, possibly influenced by SGR, and absence of PaO produce phototoxic chlorophyll catabolites and oxidative stress leading to the HR.
authors: Institute of Biological, Environmental and Rural Sciences, Aberystwyth University, Ceredigion SY23 3DA, UK. lum@aber.ac.uk
authors: Luis A J
authors: LA
authors: Mur
authors: Sylvain
authors: S
authors: Aubry
authors: Madhav
authors: M
authors: Mondhe
authors: Alison
authors: A
authors: Kingston-Smith
authors: Joe
authors: J
authors: Gallagher
authors: Emma
authors: E
authors: Timms-Taravella
authors: Caron
authors: C
authors: James
authors: Istvan
authors: I
authors: Papp
authors: Stefan
authors: S
authors: Hörtensteiner
authors: Howard
authors: H
authors: Thomas
authors: Helen
authors: H
authors: Ougham
mesh_terms: D017360:Arabidopsis / Q000201:enzymology / Q000276:immunology / Q000378:metabolism* / Q000382:microbiology*; D029681:Arabidopsis Proteins / Q000378:metabolism; D016923:Cell Death / Q000528:radiation effects; D002734:Chlorophyll / Q000378:metabolism*; D008027:Light; D045342:Light-Harvesting Protein Complexes / Q000378:metabolism; D008954:Models, Biological; D010084:Oxidation-Reduction / Q000528:radiation effects; D010105:Oxygenases / Q000378:metabolism; D010788:Photosynthesis / Q000528:radiation effects; D044224:Pseudomonas syringae / Q000502:physiology* / Q000528:radiation effects; D017382:Reactive Oxygen Species / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; D045342:Light-Harvesting Protein Complexes; D017382:Reactive Oxygen Species; D002734:Chlorophyll; D010105:Oxygenases; C112915:pheide a oxygenase