Role of rice cytosolic hexokinase OsHXK7 in sugar signaling and metabolism

Abstract

We characterized the function of the rice cytosolic hexokinase OsHXK7 (Oryza sativa Hexokinase7), which is highly upregulated when seeds germinate under O2 -deficient conditions. According to transient expression assays that used the promoter:luciferase fusion construct, OsHXK7 enhanced the glucose (Glc)-dependent repression of a rice α-amylase gene (RAmy3D) in the mesophyll protoplasts of maize, but its catalytically inactive mutant alleles did not. Consistently, the expression of OsHXK7, but not its catalytically inactive alleles, complemented the Arabidopsis glucose insensitive2-1 (gin2-1) mutant, thereby resulting in the wild type characteristics of Glc-dependent repression, seedling development, and plant growth. Interestingly, OsHXK7-mediated Glc-dependent repression was abolished in the O2 -deficient mesophyll protoplasts of maize. This result provides compelling evidence that OsHXK7 functions in sugar signaling via a glycolysis-dependent manner under normal conditions, but its signaling role is suppressed when O2 is deficient. The germination of two null OsHXK7 mutants, oshxk7-1 and oshxk7-2, was affected by O2 deficiency, but overexpression enhanced germination in rice. This result suggests the distinct role that OsHXK7 plays in sugar metabolism and efficient germination by enforcing glycolysis-mediated fermentation in O2 -deficient rice.

Cytological and transcriptome analyses reveal abrupt gene expression for meiosis and saccharide metabolisms that associated with pollen abortion in autotetraploid rice

“…Sucrose synthase 3 ( OsSUS3 ) was down-regulated at the meiotic stage and up-regulated at the single microspore stage, and OsSUS2 was up-regulated at the single microspore stage. In addition, hexokinase genes have a great influence on the plant sugar signalling pathway (Cho et al 2006 , 2009 ; Kim et al 2016 ). The hexokinase 10 ( OsHXK10 ) plays a vital role in pollen germination, anther dehiscence, and hence grain filling in rice (Xu et al 2008 ).…”

Transitioning to the Next Phase: The Role of Sugar Signaling throughout the Plant Life Cycle

“…These findings, together with the function of SnRK1 for germination under starvation (Lu et al, 2007), are consistent with starvation signals being required for starch mobilization during anaerobic germination and an antagonistic function of T6P and SnRK1. Coleoptile length during germination at low oxygen was decreased in rice hexokinase (HXK7) mutants but increased in overexpressors (Kim et al, 2016). The authors suggest that HXK7 fuels anaerobic fermentation and that its catalytic function rather than its role in sugar signaling is responsible for anaerobic germination tolerance.…”

Structure, Expression, and Functional Analysis of the Hexokinase Gene Family in Cassava

“…Recently, Cheng et al (2011) identified that OsHXK1, 7, and 8 in O. sativa are located in the cytoplasm and nucleus, since they lack the plastidic transit peptide and the membrane anchor domain; thus, these enzymes are classified as type-C HXKs [10]. The variety and multiple subcellular localizations of HXKs indicate that these proteins may have functions in various physiological processes of plants, such as anther dehiscence, pollen germination [11], stomatal closure [12], germination [13], programmed cell death [14], and responses to abiotic or biotic stresses [15]. It has been identified that HXKs have important roles in sugar metabolism and sugar signaling [7,16].…”