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Published on July 9, 2008; 10.1104/pp.108.122770

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Received May 10, 2008
Accepted July 1, 2008

Mobilization of Rubisco and stromal-localized fluorescent proteins of chloroplasts to the vacuole by an ATG gene-dependent autophagic process

Hiroyuki Ishida *, Kohki Yoshimoto , Masanori Izumi , Daniel Reisen , Yuichi Yano , Amane Makino , Yoshinori Ohsumi , Maureen R. Hanson , and Tadahiko Mae

Department of Applied Plant Science, Graduate School of Agricultural Sciences, Tohoku University, Tsutsumidori-Amamiyamachi, Aoba-ku, Sendai 981-8555, Japan; Department of Cell Biology, National Institute for Basic Biology, Myodaiji-cho, Okazaki 444-8585, Japan; Department of Molecular Biology and Genetics, Cornell University, Ithaca, NY 14853, USA

* Corresponding author; email: hiroyuki{at}biochem.tohoku.ac.jp.

During senescence and at times of stress, plants can mobilize needed nitrogen from chloroplasts from leaves to other organs. Much of the total leaf nitrogen is allocated to the most abundant plant protein, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). While bulk degradation of the cytosol and organelles in plants occurs by autophagy, the role of autophagy in the degradation of chloroplast proteins is still unclear. We have visualized the fate of Rubisco, stromal-targeted green fluorescent protein (GFP) and DsRed, and GFP-labeled Rubisco in order to investigate the involvement of autophagy in the mobilization of stromal proteins to the vacuole. Using immuno-electron microscopy (IEM), we previously demonstrated that Rubisco is released from the chloroplast into Rubisco-containing bodies (RCBs) in naturally senescent leaves (Chiba et al., 2003, Plant Cell Physiol 44: 914-921). When leaves of transgenic Arabidopsis (Arabidopsis thaliana) plants expressing stromal-targeted fluorescent proteins were incubated with concanamycin A to inhibit vacuolar H+-ATPase activity, spherical bodies exhibiting GFP or DsRed fluorescence without chlorophyll fluorescence were observed in the vacuolar lumen. Double-labeled IEM with anti-Rubisco and anti-GFP antibodies confirmed that the fluorescent bodies correspond to RCBs. RCBs could also be visualized using GFP-labeled Rubisco directly. RCBs were not observed in leaves of a T-DNA insertion mutant in ATG5, one of the essential genes for autophagy. Stromal-targeted DsRed and GFP-ATG8 fusion proteins were observed together in autophagic bodies in the vacuole. We conclude that Rubisco and stromal-targeted fluorescent proteins can be mobilized to the vacuole through an ATG gene-dependent autophagic process without prior chloroplast destruction.






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