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Plant and Cell Physiology Advance Access published online on March 8, 2009
Plant and Cell Physiology, doi:10.1093/pcp/pcp035
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© The Author 2009. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Light-independent cell death induced by accumulation of pheophorbide a in Arabidopsis thaliana

Masumi Hirashima1, Ryouichi Tanaka and Ayumi Tanaka

Inst. Low Temp. Sci., Hokkaido Univ., N19 W8, Kita-ku, Sapporo 060-0819, JAPAN

Corresponding author: Prof. Ayumi Tanaka, Inst. Low Temp. Sci., Hokkaido Univ., N19 W8, Kita-ku, Sapporo 060-0819, JAPAN, TEL +81-11-706-5493, FAX +81-11-706-5493, ayumi{at}pop.lowtem.hokudai.ac.jp


  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 more than 50% of the leaf cells underwent cell death within a 5-day period of darkness. Light and electron microscopic observations indicated that the cellular structure had collapsed in a large population of cells. Partial coverage of 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 both under 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.

Keywords: Arabidopsis - senescence - pheophorbide a - cell death

1Present address, National Institute of Floricultural Science, National Agriculture and Food Research Organization, 2-1 Fujimoto, Tsukuba 305-8519, Ibaraki, Japan

(Received January 14, 2009; Accepted February 25, 2009)
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