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Mitochondria-Associated Hexokinases Play a Role in the Control of Programmed Cell Death in Nicotiana benthamiana^[W]

^a BioNanotechnology Research Center, Korea Research Institute of Bioscience and Biotechnology, Taejon 305-333, Korea
^b Division of Bioscience and Bioinformatics, Myongji University, Yongin, Kyonggi-do 449-728, Korea
^c Department of Biology, Yonsei University, Seoul 120-749, Korea
^d Faculty of Plant Biotechnology, Dong-A University, Pusan 604-714, Korea

³ To whom correspondence should be addressed. E-mail hspai{at}yonsei.ac.kr; fax 82-2-312-5657.

Recent findings suggest a pivotal role for mitochondria-associated hexokinase in the regulation of apoptosis in animal cells. In this study, virus-induced gene silencing (VIGS) of a hexokinase-encoding Hxk1 caused necrotic lesions on leaves, abnormal leaf morphology, and retarded plant growth in Nicotiana benthamiana. Hxk1 was associated with the mitochondria, and this association required the N-terminal membrane anchor. VIGS of Hxk1 reduced the cellular glucose-phosphorylating activity to 31% of control levels without changing the fructose-phosphorylating activity and did not alter hexose phosphate content severely. The affected cells showed programmed cell death (PCD) morphological markers, including nuclear condensation and DNA fragmentation. Similar to animal cell apoptosis, cytochrome c was released into the cytosol and caspase-9– and caspase-3–like proteolytic activities were strongly induced. Furthermore, based on flow cytometry, Arabidopsis thaliana plants overexpressing Arabidopsis HXK1 and HXK2, both of which are predominantly associated with mitochondria, exhibited enhanced resistance to H₂O₂- and -picolinic acid–induced PCD. Finally, the addition of recombinant Hxk1 to mitochondria-enriched fractions prevented H₂O₂/clotrimazole-induced cytochrome c release and loss of mitochondrial membrane potential. Together, these results show that hexokinase critically regulates the execution of PCD in plant cells, suggesting a link between glucose metabolism and apoptosis.

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