AOBPreview originally published online on March 13, 2006
Annals of Botany 2006 97(6):1145-1149; doi:10.1093/aob/mcl057
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The Mitochondrial Fission Regulator DRP3B Does Not Regulate Cell Death in Plants
1 Institute of Molecular and Cellular Biosciences and 2 Laboratory of Plant Molecular Genetics, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan and 3 Iwate Biotechnology Research Center, Kitakami, Iwate 024-0003, Japan
* For correspondence. E-mail mkawai{at}iam.u-tokyo.ac.jp
Received: 14 November 2005 Returned for revision: 6 January 2006 Accepted: 30 January 2006 Published electronically: 13 March 2006
| ABSTRACT |
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Background and Aims Recent reports have described dramatic alterations in mitochondrial morphology during metazoan apoptosis. A dynamin-related protein (DRP) associated with mitochondrial outer membrane fission is known to be involved in the regulation of apoptosis. This study analysed the relationship between mitochondrial fission and regulation of plant cell death.
Methods Transgenic plants were generated possessing Arabidopsis DRP3B (K56A), the dominant-negative form of Arabidopsis DRP, mitochondrial-targeted green fluorescent protein and mouse Bax.
Key Results Arabidopsis plants over-expressing DRP3B (K56A) exhibited long tubular mitochondria. In these plants, mitochondria appeared as a string-of-beads during cell death. This indicates that DRP3B (K56A) prevented mitochondrial fission during plant cell death. However, in contrast to results for mammalian cells and yeast, Bax-induced cell death was not inhibited in DRP3B (K56A)-expressing plant cells. Similarly, hydrogen peroxide-, menadione-, darkness- and salicylic acid-induced cell death was not inhibited by DRP3B (K56A) expression.
Conclusions These results indicate that the systems controlling cell death in animals and plants are not common in terms of mitochondrial fission.
Key words: Arabidopsis, Bax, cell death, DRP3B, fission, mitochondria
| INTRODUCTION |
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In healthy mammalian cells, fusion and fission events participate in regulating mitochondrial morphology (Shaw and Nunnari, 2002
It was recently demonstrated that various reactive oxygen species (ROS) and mammalian pro-apoptotic gene Bax treatments induced mitochondrial fragmentation in Arabidopsis thaliana (Yoshinaga et al., 2005a
, b
). Mitochondria showed morphological changes from a bacillus-like to a rounded shape, and their overall size halved. This indicates that mitochondrial fission occurred during plant cell death. Frank et al. (2001)
demonstrated that over-expression of a dominant-negative mutant of Drp1 inhibited Bax-induced cell death in COS-7 cells. In the present study, transgenic Arabidopsis plants were generated possessing dominant-negative DRP3B (K56A), mitochondrial-targeted green fluorescent protein (mt-GFP) and dexamethasone (DEX)-inducible Bax. Using these transgenic plants, it was shown that over-expression of DRP3B (K56A) protein could not inhibit ROS- and Bax-induced cell death in plants, unlike for mammalian cells.
| MATERIALS AND METHODS |
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Plant materials
The DEX-inducible mouse Bax transgenic line (Arabidopsis thaliana ecotype Col-0) was as described previously in Kawai-Yamada et al. (2001)
Chemicals
Hydrogen peroxide (H2O2; Wako, Osaka, Japan) was diluted with distilled water to 50 or 100 mM. DEX (Sigma, St Louis, MO, USA), menadione (MD; Sigma) and salicylic acid (SA; nacalai tesque, Kyoto, Japan) were dissolved in dimethyl sulfoxide (DMSO). During treatment, the final DMSO concentration was never higher than 0·2 %, which had no effect on Arabidopsis leaves. In order to induce cell death, 10 µM DEX, 25 or 60 µM MD, and 300 or 400 µM SA were used.
Ion leakage measurement
Three leaf discs obtained from 3-week-old Arabidopsis plants were floated on distilled water. After addition of various chemicals, leaf discs were vacuumed for 5 min. The discs were incubated at 23 °C under continuous light (60 µmol m2 s1) without shaking. Electrolyte leakage was monitored using an electrical conductivity meter (Horiba, B-173, Kyoto, Japan) (Kawai-Yamada et al., 2004
). Measured electrical conductivities of the medium were reported in µS cm1. This experiment was run in triplicate.
Microscopic observation
GFP fluorescence was examined at an excitation wavelength of 488 nm under a fluorescence microscope (DMRD, Leica, Wetzlar, Germany). In all experiments, epidermal cells of leaves were observed.
| RESULTS |
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Bax-induced cell death in transgenic Arabidopsis with the dominant-negative mutant DRP3B (K56A)
To analyse the mechanisms of plant cell death in terms of mitochondrial regulation, an Arabidopsis homologue of Drp1 (DRP3B) was investigated using a Bax-induced plant cell death system (Kawai-Yamada et al., 2001
As shown in Fig. 1A, expression of mt-GFP and dominant-negative DRP3B (K56A) protein in whole plants did not affect plant growth. Fluorescence microscopy revealed that mitochondria were longer in transgenic plants expressing mutant DRP3B (K56A) protein (Fig. 1B, DEX-).
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It was recently reported that mammalian Bax could induce plant cell death (Kawai-Yamada et al., 2001
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Mitochondrial changes in DRP3B (K56A) plants under several stresses
To confirm whether similar mitochondrial changes are induced under other conditions causing plant cell death, DRP3B (K56A) plants were subjected to several stresses. After exposure of DRP3B (K56A) leaves to stresses such as 25 µM MD, 50 mM H2O2, 400 µM SA and dark treatment (Dark), mitochondia changed from tubular to a string-of-beads form (Fig. 3A). All mitochondria aligned together (Fig. 3A, arrows). These changes were also observed when Bax was expressed in DRP3B (K56A) plants (Fig. 1B). Moreover, ion leakage was observed after dark treatment (6 d), 60 µM MD, 100 µM H2O2 and 300 µM SA treatment in DRP3B (K56A) plants. No clear distinction was observed between mt-GFP and DRP3B (K56A)/mt-GFP plants (Fig. 3B).
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| DISCUSSION |
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In mammalian cells, various key apoptotic events involve mitochondria, including the release of cytochrome c and loss of mitochondrial transmembrane potential (Green and Reed, 1998
Little is currently known about higher plant mitochondrial dynamics. However, recently, several genes involved in the control of plant mitochondrial dynamics have been identified (Logan et al., 2003
, 2004
; Sheahan et al., 2005
). Arimura and Tsutsumi (2002) revealed that DRP3B, an Arabidopsis homologue of Drp1, controlled mitochondrial fission in Arabidopsis leaves and tobacco BY-2 suspension cultures. Moreover, DRP3B (K56A) is a dominant-negative protein, and plant cells expressing DRP3B (K56A) exhibit longer but fewer mitochondria. Punctate morphology of mitochondria has been shown to be one of the indicators in the early stage of ROS stress-induced cell death (Yoshinaga et al., 2005b
). Using DRP3B dominant-negative plants, mitochondrial fission was shown to be incomplete, and mitochondria aligned in a string-of-beads form under ROS stress- and Bax-induced cell death. DRP3B (K56A) blocks fission of the outer mitochondrial membrane by reducing the affinity for GTP to DRP3B (Pitts et al., 1999
; Arimura and Tutsumi, 2002
). This indicates that mitochondrial fragmentation during plant cell death is not simply collapse, as controlled by DRP3B.
Frank et al. (2001)
demonstrated that inhibition of Drp1 by over-expression of a dominant-negative mutant blocked Bax-induced cell death in COS-7 cells. However, in the present study, cell death was not inhibited in transgenic plants over-expressing DRP3B (K56A), which differs from observations in mammals and yeast (Frank et al., 2001
; Fannjiang et al., 2004
). The function of the mitochondrial fission regulator during cell death is different between animals and plants.
| ACKNOWLEDGEMENTS |
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We thank Dr Y. Niwa and Ms Y. Takahashi for their gift of materials and help. This study was supported by a Grant-in-Aid for Scientific Research on Priority Areas (Grant No. 17051006), by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan, and CREST, JST, Japan.
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