Role of the rice hexokinases OsHXK5 and OsHXK6 as glucose sensors.
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.
Cho
Jung-Il
JI
Plant Metabolism Research Center and Graduate School of Biotechnology, Department of Horticultural Biotechnology, Kyung Hee University, Yongin 446-701, Korea.
Ryoo
Nayeon
N
Eom
Joon-Seob
JS
Lee
Dae-Woo
DW
Kim
Hyun-Bi
HB
Jeong
Seok-Won
SW
Lee
Youn-Hyung
YH
Kwon
Yong-Kook
YK
Cho
Man-Ho
MH
Bhoo
Seong Hee
SH
Hahn
Tae-Ryong
TR
Park
Youn-Il
YI
Hwang
Ildoo
I
Sheen
Jen
J
Jeon
Jong-Seong
JS
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
D029681:Arabidopsis Proteins; D010940:Plant Proteins; D012097:Repressor Proteins; D049452:Green Fluorescent Proteins; D006593:Hexokinase; D005947:Glucose