Arabidopsis Hexokinase-Like1 and Hexokinase1 Form a Critical Node in Mediating Plant Glucose and Ethylene Responses      

Abstract

Arabidopsis (Arabidopsis thaliana) Hexokinase-Like1 (HKL1) lacks glucose (Glc) phosphorylation activity and has been shown to act as a negative regulator of plant growth. Interestingly, the protein has a largely conserved Glc-binding domain, and protein overexpression was shown previously to promote seedling tolerance to exogenous 6% (w/v) Glc. Since these phenotypes occur independently of cellular Glc signaling activities, we have tested whether HKL1 might promote cross talk between the normal antagonists Glc and ethylene. We show that repression by 1-aminocyclopropane-1-carboxylic acid (ACC) of the Glc-dependent developmental arrest of wild-type Arabidopsis seedlings requires the HKL1 protein. We also describe an unusual root hair phenotype associated with growth on high Glc medium that occurs prominently in HKL1 overexpression lines and in glucose insensitive 2-1 (gin2-1), a null mutant of Hexokinase1 (HXK1). Seedlings of these lines produce bulbous root hairs with an enlarged base after transfer from agar plates with normal medium to plates with 6% Glc. Seedling transfer to plates with 2% Glc plus ACC mimics the high-Glc effect in the HKL1 overexpression line but not in gin2-1. A similar ACCstimulated, bulbous root hair phenotype also was observed in wild-type seedlings transferred to plates with 9% Glc. From transcript expression analyses, we found that HKL1 and HXK1 have differential roles in Glc-dependent repression of some ethylene biosynthesis genes. Since we show by coimmunoprecipitation assays that HKL1 and HXK1 can interact, these two proteins likely form a critical node in Glc signaling that mediates overlapping, but also distinct, cellular responses to Glc and ethylene treatments.

Master regulators in plant glucose signaling networks

“…This growth promotion function of HXK1 appears to be more complex, as it is integrated in each organ to maximize cell and organ size and whole plant biomass, which is proportional to available glucose signals (Moore et al, 2003; Cho et al, 2009; Hall and Sheen, unpublished). Furthermore, diverse isoforms of HXK and HXK-like ( HKL ) genes have been found in the genome of all land plants from the nonflowering moss and lycophyte to seed plants, including rice, maize, sorghum, poplar, tobacco, tomato, grape and Arabidopsis (Moore et al, 2003; Granot, 2007; Karve et al, 2008, 2010; Nilsson et al, 2011; Karve et al, 2012; Kim et al, 2013). The amplification and diversification of the HXK superfamily suggests more complex modes of plant glucose signaling and metabolism in different subcellular compartments and organs evolved to support a wide spectrum of plant growth strategies and architectures dictated by sugar availability and other essential nutrients (Moore et al, 2003; Claeyssen and Rivoal, 2007; Granot, 2007; Karve et al, 2008; Cho et al, 2009; Zhang et al, 2010; Karve et al, 2010; Nilsson et al, 2011; Karve et al, 2012; Kim et al, 2013).…”

“…Extensive genetic, molecular, cellular, and biochemical analyses have uncovered the intimate links between HXK1-mediated glucose signaling and multiple hormones, including but not limited to ethylene, abscisic acid (ABA), auxin and cytokinin (Rolland et al, 2002; Leon and Sheen, 2003; Moore et al, 2003; Yanagisawa et al, 2003; Rolland et al, 2006; Huang et al, 2008; Ramon et al, 2008; Cho et al, 2010; Karve et al, 2012; Hsu et al, 2014). In maize and Arabidopsis leaf cells, Arabidopsis HXK1 but not the yeast HXK promotes the proteasome-dependent degradation of nuclear EIN3 (ETHYLENE INSENSITIVE3) and EIL1 (EIN3-LIKE1) through the C-terminus, which is stabilized by ethylene.…”

“…To elucidate the molecular and cellular mechanisms of novel HXK1 actions in distinct glucose responses, several complementary approaches have been developed, including targeted mutagenesis and functional analyses of Arabidopsis HXK1 in gin2 , biochemical and structural analyses of glucose binding and phosphorylation activities, genetic manipulation of subcellular localization, purification and characterization of the nuclear HXK1 protein complexes, identification of HXK1 interacting proteins, and establishing HXK1-dependent transcriptome reprogramming and chromatin association (Xiao et al, 2000; Moore et al, 2003; Yanagisawa et al, 2003; Cho et al, 2006; Baena-González et al, 2007; Balasubramanian et al, 2007; Cho et al, 2009; Cho et al, 2010; Karve et al, 2012; Xiong et al, 2013). Comprehensive analyses of the most informative HXK1 mutation, S177A without detectable catalytic activity but with full glucose binding, have provided compelling evidence for the glucose sensing and signaling activities critical for diverse glucose responses uncoupled from glucose phosphorylation and metabolism (Moore et al, 2003; de Jong et al, 2014; Li and Sheen, unpublished).…”

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

“…In the absence of inorganic nutrients, this mutant also shows insensitivity to a low concentration of 2% Glc and can be complemented with catalytically inactive mutant versions of HXK1 (Cho et al, 2010). Similar to the hxk1 mutation, overexpression of hexokinase-like1 (HKL1) reduces seedling sensitivity to high Glc concentration, suggesting an antagonistic function of HXK1 and HKL1 with respect to seedling development (Karve and Moore, 2009;Karve et al, 2012).…”

Ethylene and the Regulation of Physiological and Morphological Responses to Nutrient Deficiencies

“…Auxin can stimulate ethylene production, and ethylene can affect auxin transport and accumulation Muday et al, 2012). The exact relationship of sugars and ethylene is not totally known, but hexokinases are a critical node in mediating plant Glc and ethylene responses (Karve et al, 2012). Ethylene can stimulate both ROS and NO accumulation (Shin et al, 2005;Jung et al, 2009;García et al, 2011;Steffens, 2014), whereas both ROS and NO can stimulate ethylene production (Ahlfors et al, 2009;García et al, 2011;Iqbal et al, 2013).…”