title: Role of the rice hexokinases OsHXK5 and OsHXK6 as glucose sensors.
abstract: The Arabidopsis (Arabidopsis thaliana) hexokinase 1 (AtHXK1) is recognized as an important glucose (Glc) sensor. However, the function of hexokinases as Glc sensors has not been clearly demonstrated in other plant species, including rice (Oryza sativa). To investigate the functions of rice hexokinase isoforms, we characterized OsHXK5 and OsHXK6, which are evolutionarily related to AtHXK1. Transient expression analyses using GFP fusion constructs revealed that OsHXK5 and OsHXK6 are associated with mitochondria. Interestingly, the OsHXK5DeltamTP-GFP and OsHXK6DeltamTP-GFP fusion proteins, which lack N-terminal mitochondrial targeting peptides, 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 were targeted predominantly in the nucleus, suggesting that these OsHXKs retain a dual-targeting ability to mitochondria and nuclei. In transient expression assays using promoterluciferase fusion constructs, these two OsHXKs and their catalytically inactive alleles dramatically enhanced the Glc-dependent repression of the maize (Zea mays) Rubisco small subunit (RbcS) and rice alpha-amylase genes in mesophyll protoplasts of maize and rice. Notably, the expression of OsHXK5, OsHXK6, or their mutant alleles complemented the Arabidopsis glucose insensitive2-1 mutant, thereby resulting in wild-type characteristics in seedling development, Glc-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 Glc treatment. These results provide evidence that rice OsHXK5 and OsHXK6 can function as Glc sensors.
authors.affiliation: Plant Metabolism Research Center and Graduate School of Biotechnology, Department of Horticultural Biotechnology, Kyung Hee University, Yongin 446-701, Korea.
authors.forename: Jung-Il
authors.initials: JI
authors.lastname: Cho
authors.forename: Nayeon
authors.initials: N
authors.lastname: Ryoo
authors.forename: Joon-Seob
authors.initials: JS
authors.lastname: Eom
authors.forename: Dae-Woo
authors.initials: DW
authors.lastname: Lee
authors.forename: Hyun-Bi
authors.initials: HB
authors.lastname: Kim
authors.forename: Seok-Won
authors.initials: SW
authors.lastname: Jeong
authors.forename: Youn-Hyung
authors.initials: YH
authors.lastname: Lee
authors.forename: Yong-Kook
authors.initials: YK
authors.lastname: Kwon
authors.forename: Man-Ho
authors.initials: MH
authors.lastname: Cho
authors.forename: Seong Hee
authors.initials: SH
authors.lastname: Bhoo
authors.forename: Tae-Ryong
authors.initials: TR
authors.lastname: Hahn
authors.forename: Youn-Il
authors.initials: YI
authors.lastname: Park
authors.forename: Ildoo
authors.initials: I
authors.lastname: Hwang
authors.forename: Jen
authors.initials: J
authors.lastname: Sheen
authors.forename: Jong-Seong
authors.initials: JS
authors.lastname: Jeon
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000201:enzymology; D029681:Arabidopsis Proteins / Q000378:metabolism; D015374:Biosensing Techniques; D017796:Caulimovirus / Q000201:enzymology / Q000235:genetics; D017930:Genes, Reporter; D005947:Glucose / Q000378:metabolism; D049452:Green Fluorescent Proteins / Q000235:genetics; D006593:Hexokinase / Q000378:metabolism*; D008969:Molecular Sequence Data; D012275:Oryza / Q000201:enzymology*; D010940:Plant Proteins / Q000378:metabolism*; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; D010940:Plant Proteins; D012097:Repressor Proteins; D049452:Green Fluorescent Proteins; D006593:Hexokinase; D005947:Glucose