title: Light-independent cell death induced by accumulation of pheophorbide a in Arabidopsis thaliana.
abstract: Tetrapyrroles are well-known photosensitizers. In plants, various intermediate molecules of tetrapyrrole metabolism have been reported to induce cell death in a light-dependent manner. In contrast to these reports, we found that pheophorbide a, a key intermediate of chlorophyll catabolism, causes cell death in complete darkness in a transgenic Arabidopsis plant, As-ACD1. In this plant, expression of mRNA for pheophorbide a oxygenase was suppressed by expression of Acd1 antisense RNA; thus, As-ACD1 accumulated an excessive amount of pheophorbide a when chlorophyll breakdown occurred. We observed that when senescence was induced by a continuous dark period, leaves of As-ACD1 plants became dehydrated. By measuring electrolyte leakage, we estimated that >50% of the leaf cells underwent cell death within a 5 d period of darkness. Light and electron microscopic observations indicated that the cellular structure had collapsed in a large population of cells. Partially covering a leaf with aluminum foil resulted in light-independent cell death in the covered region and induced bleaching in the uncovered regions. These results indicate that accumulation of pheophorbide a induces cell death under both darkness and illumination, but the mechanisms of cell death under these conditions may differ. We discuss the possible mechanism of light-independent cell death and the involvement of pheophorbide a in the signaling pathway for programmed cell death.
authors.affiliation: Institute of Low Temperatue Science, Hokkaido University, N19 W8, Kita-ku, Sapporo 060-0819, Japan.
authors.forename: Masumi
authors.initials: M
authors.lastname: Hirashima
authors.forename: Ryouichi
authors.initials: R
authors.lastname: Tanaka
authors.forename: Ayumi
authors.initials: A
authors.lastname: Tanaka
mesh_terms: D051017:Apoptosis Regulatory Proteins / Q000235:genetics / Q000378:metabolism; D017360:Arabidopsis / Q000166:cytology* / Q000235:genetics / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism; D016923:Cell Death; D002734:Chlorophyll / Q000031:analogs & derivatives* / Q000378:metabolism; D003624:Darkness; D018506:Gene Expression Regulation, Plant; D010105:Oxygenases / Q000235:genetics / Q000378:metabolism; D018515:Plant Leaves / Q000166:cytology / Q000378:metabolism; D030821:Plants, Genetically Modified / Q000166:cytology / Q000235:genetics / Q000378:metabolism; D016372:RNA, Antisense
chemical_list: D051017:Apoptosis Regulatory Proteins; D029681:Arabidopsis Proteins; D016372:RNA, Antisense; D002734:Chlorophyll; C482919:ACD1 protein, Arabidopsis; D010105:Oxygenases; C032623:pheophorbide a