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Published on November 14, 2008; 10.1104/pp.108.131227

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Received October 16, 2008
Accepted November 10, 2008

Role of the Rice Hexokinases OsHXK5 and OsHXK6 as Glucose Sensors

Jung-Il Cho , Nayeon Ryoo , Joon-Seob Eom , Dae-Woo Lee , Hyun-Bi Kim , Seok-Won Jeong , Youn-Hyung Lee , Yong-Kook Kwon , Man-Ho Cho , Seong Hee Bhoo , Tae-Ryong Hahn , Youn-Il Park , Ildoo Hwang , Jen Sheen , and Jong-Seong Jeon *

Plant Metabolism Research Center & Graduate School of Biotechnology, Kyung Hee University, Yongin 446-701, Korea; Department of Biology, Chungnam National University, Daejeon 305-764, Korea; Department of Horticultural Biotechnology, Kyung Hee University, Yongin 446-701, Korea; Department of Life Sciences, Pohang University of Science and Technology, Pohang 790-784, Korea; Department of Molecular Biology, Massachusetts General Hospital, Department of Genetics, Harvard Medical School, Boston, MA 02114, USA

* Corresponding author; email: jjeon{at}khu.ac.kr.

The Arabidopsis hexokinase 1 (AtHXK1) is recognized as an important glucose sensor. However, the function of hexokinases as glucose sensors has not been clearly demonstrated in other plant species including rice. To investigate the functions of rice hexokinase isoforms, we characterized OsHXK5 ( Oryza sativa Hexokinase 5) and OsHXK6, which are evolutionarily related to AtHXK1. Transient expression analyses using green fluorescent protein (GFP) fusion constructs revealed that OsHXK5 and OsHXK6 are associated with mitochondria. Interestingly, the OsHXK5{Delta}mTP-GFP and OsHXK6{Delta}mTP-GFP fusion proteins, which lack N-terminal mitochondrial targeting peptides (mTP), were present mainly in the nucleus with a small amount of the proteins seen in the cytosol. In addition, the OsHXK5NLS-GFP and OsHXK6NLS-GFP fusion proteins harboring nuclear localization signals (NLSs) were targeted predominantly in the nucleus, suggesting that these OsHXKs retain a dual-targeting ability to mitochondria and nuclei. In transient expression assays using promoter::luciferase (LUC) fusion constructs, these two OsHXKs and their catalytically inactive alleles dramatically enhanced the glucose-dependent repression of the maize ribulose-1,6-bisphosphatase carboxylase small subunit (RbcS) and rice {alpha}-amylase (RAmy3D) genes in mesophyll protoplasts of maize and rice. Notably, the expression of OsHXK5, OsHXK6, or their mutant alleles, complemented the Arabidopsis glucose insensitive2-1 (gin2-1) mutant, thereby resulting in wild type characteristics in seedling development, glucose-dependent gene expression, and plant growth. Furthermore, transgenic rice plants overexpressing OsHXK5 or OsHXK6 exhibited hypersensitive plant growth retardation and enhanced repression of the photosynthetic gene RbcS in response to glucose treatment. These results provide evidence that rice OsHXK5 and OsHXK6 can function as glucose sensors.




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A. Karve and B. d. Moore
Function of Arabidopsis hexokinase-like1 as a negative regulator of plant growth
J. Exp. Bot., October 1, 2009; 60(14): 4137 - 4149.
[Abstract] [Full Text] [PDF]


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