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DEVELOPMENT AND HORMONE ACTION

Autophagy Plays a Role in Chloroplast Degradation during Senescence in Individually Darkened Leaves^1,[W],[OA]

Department of Applied Plant Science, Graduate School of Agricultural Science, Tohoku University, Tsutsumidori-Amamiyamachi, Aoba-ku, Sendai 981–8555, Japan (S.W., H.I., M.I., T.M., A.M.); RIKEN Plant Science Center, Suehiro-cho 1–7–22, Tsurumi-ku, Yokohama 230–0045, Japan (K.Y.); and Department of Cell Biology, National Institute for Basic Biology, Myodaiji-cho, Okazaki 444–8585, Japan (Y.O.)

Chloroplasts contain approximately 80% of total leaf nitrogen and represent a major source of recycled nitrogen during leaf senescence. While bulk degradation of the cytosol and organelles in plants is mediated by autophagy, its role in chloroplast catabolism is largely unknown. We investigated the effects of autophagy disruption on the number and size of chloroplasts during senescence. When leaves were individually darkened, senescence was promoted similarly in both wild-type Arabidopsis (Arabidopsis thaliana) and in an autophagy-defective mutant, atg4a4b-1. The number and size of chloroplasts decreased in darkened leaves of wild type, while the number remained constant and the size decrease was suppressed in atg4a4b-1. When leaves of transgenic plants expressing stroma-targeted DsRed were individually darkened, a large accumulation of fluorescence in the vacuolar lumen was observed. Chloroplasts exhibiting chlorophyll fluorescence, as well as Rubisco-containing bodies, were also observed in the vacuole. No accumulation of stroma-targeted DsRed, chloroplasts, or Rubisco-containing bodies was observed in the vacuoles of the autophagy-defective mutant. We have succeeded in demonstrating chloroplast autophagy in living cells and provide direct evidence of chloroplast transportation into the vacuole.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Hiroyuki Ishida (hiroyuki{at}biochem.tohoku.ac.jp).

^[W] The online version of this article contains Web-only data.

^[OA] Open access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.130013

^* Corresponding author; e-mail hiroyuki{at}biochem.tohoku.ac.jp.

Received September 19, 2008; accepted December 9, 2008; published December 12, 2008.

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