Hexokinases HXKs are major regulatory
Hexokinases (HXKs) are major regulatory enzymes in sugar metabolism and in sugar sensing in plants (Claeyssen and Rivoal, 2007, Granot et al., 2013, Sheen, 2014, Aguilera-Alvarado and Sánchez-Nieto, 2017). HXK can phosphorylate both glucose and fructose to respective hexose-6-phosphates as a first step of glycolysis. Moreover, mitochondria-associated HXKs (mtHXKs) have a key role in the control of cell death and senescence. HXK isoenzymes are integral component of permeability transition (PT) pore through their interaction with voltage-dependent anion KW 3902 mg (VDAC). mtHXK proteins can bind to VDAC, thereby they inhibit the opening of PT pore and cytochrome c release from the intermembrane space, which can prevent the induction of cell death. The loss of the integrity of the inner mitochondrial membrane can cause mitochondrial dysfunction such as ROS production and ATP depletion. Increasing glucose phosphorylation activity by mtHXKs may reduce cell death owing to the inhibition of the opening of mitochondrial PT pore and a more efficient glucose metabolism due to the better access to ATP (Sarowar et al., 2008, Sun et al., 2008, Camacho-Pereira et al., 2009, Godbole et al., 2013). In contrast to active mtHXKs, the loss of mtHXK activity increases ROS production, cyt c release and cell death induction in tomato leaf tissues. However, this important function of mtHXKs in the initiation phase of dark-induced senescence has not been investigated at various leaf positions of intact plants. In addition, chloroplastic HXKs also play a crucial role in the regulation of ROS levels (Bolouri-Moghaddam et al., 2010). Thus, the early changes in the expression and activity of various HXKs can be significant, because this is the first step before alterations in mitochondrial and photosynthetic functions (Zhang and Xing, 2008, Breeze et al., 2011, Liebsch and Keech, 2016). Moreover, it was observed earlier that the expression of photosynthesis-associated nuclear genes (e.g. Chl a/b binding protein, small subunit of ribulose-1,5-bisphosphate carboxylase oxygenase) was repressed by high glucose concentrations. These changes showed correlation with the increased expression of HXKs and with the rate of glucose-induced leaf yellowing (Xiao et al., 2000, Moore et al., 2003). However, low sugar levels can also induce leaf yellowing and senescence, but the role of mtHXKs is not clear in this process (Van Doorn, 2008). In tomato, four HXK genes (SlHXK1–4) were identified (Menu et al., 2001, Dai et al., 2002, Kandel-Kfir et al., 2006), which were expressed in various organs, including leaves and fruits (Damari-Weissler et al., 2006). Based on the use of GFP fusion protein it was shown that tomato SlHXK1, 2 and 3 are associated with mitochondrial envelope membrane and SlHXK4 is localized to plastids (Kandel-Kfir et al., 2006). SlHXK4 belongs to A type HXKs, having a 30 amino acid-long chloroplast transit peptide on its N-terminal, while SlHXK1, 2 and 3 are members of B type HXKs. They share a common, hydrophobic region at the N-terminal, which anchors these proteins to the mitochondrial outer membrane. Based on the analysis of the upstream promoter sequences of tomato HXK genes, the most abundant cis-regulatory element (CRE) was GT1CONSENSUS with 45 duplications, which plays a role in the regulation of many light-dependent genes (Poór et al., 2015). However, there are differences between HXK activities in sink and source leaves. In fully developed, photosynthetically active source leaves HXKs and fructokinases function mainly during dark period, when starch and sucrose degradation yields free glucose and fructose monomers. In sink tissues, however, HXKs and fructokinases might be required during both dark and light periods (Granot et al., 2013). These results suggest that HXKs are regulated by light and function differently in light or dark periods in plant metabolism, development and stress responses in the leaves of different maturity levels.