Two novel types of hexokinases in the moss Physcomitrella patens

Ancient duplications, multidimensional specializations, and defense role of hexokinases in wheat

Hexokinases (HXKs), which sense and catalyze cellular sugar, play a critical role in the growth and development of various plants, including wheat, a primary source of human calories frequently attacked by fungal pathogens. However, the evolutionary dynamics and functional diversification of HXKs in wheat, particularly their roles in plant defense, remain unclear. Here, we discovered that the wheat hexokinase gene family originated through multiple ancient gene duplications across different plant lineages and has undergone comprehensive, multidimensional functional specialization in gene expression, subcellular localization, enzyme activity, and regulation of plant defense responses. Gene expression analysis suggests that two-thirds of the TaHXK genes are responsive to fungal infection. Subcellular analysis reveals that while six TaHXKs are localized in mitochondria, three TaHXKs from different phylogenetic branches are sorted into other cellular compartments. Notably, biochemical analysis shows that TaHXKs in mitochondria differ in their glucose-catalyzing activity, with TaHXK5 and TaHXK3 exhibiting the highest and lowest enzyme activity, respectively. Consistently, transient expression analysis suggests that TaHXK5 induces various plant defense responses, while TaHXK3 is defective in activating some plant defense responses. Furthermore, inactivation of the glucokinase activity of TaHXK5 compromised its function in defense activation, suggesting that mitochondrial TaHXKs display functional divergence in both enzyme activity and defense-inducing activity that are intrinsically connected. Overall, our findings reveal that the multidimensional specialization events following the ancient duplication events may have shaped the functional diversity of HXKs in wheat, shedding light on their evolutionary dynamics and potentially contributing to the improvement of wheat defense.

HXK, SnRK1, and TOR signaling in plants: Unraveling mechanisms of stress response and secondary metabolism

As sessile photoautotrophs, plants constantly encounter diverse environmental stresses. Recent research has focused on elucidating sugar and energy signaling mediated by hexokinase (HXK), sucrose non-fermenting 1-related protein kinase 1 (SnRK1), and the target of rapamycin (TOR) and assessing its intricate interplay with hormones and secondary metabolism. HXK serves as a pivotal regulator of glucose sensing and metabolism. It affects plant growth and development in response to nutrient availability. SnRK1 acts as a vital energy sensor that regulates metabolic adjustments during stress to bolster plant resilience. Moreover, TOR integrates nutrient signals to finely modulate growth and development, balancing cellular metabolism and resource allocation. Understanding the functions of HXK, SnRK1, and TOR can provide profound insights into plant adaptation mechanisms and open promising avenues for leveraging biotechnological strategies to enhance the stress tolerance and nutritional value of crops. This narrative review focuses on recent advancements in the molecular mechanisms of HXK, SnRK1, and TOR and explores their potential applications in agricultural biotechnology.

The role of hexokinases in epigenetic regulation: altered hexokinase expression and chromatin stability in yeast

BACKGROUND

Human hexokinase 2 (HK2) plays an important role in regulating Warburg effect, which metabolizes glucose to lactate acid even in the presence of ample oxygen and provides intermediate metabolites to support cancer cell proliferation and tumor growth. HK2 overexpression has been observed in various types of cancers and targeting HK2-driven Warburg effect has been suggested as a potential cancer therapeutic strategy. Given that epigenetic enzymes utilize metabolic intermediates as substrates or co-factors to carry out post-translational modification of histones and nucleic acids modifications in cells, we hypothesized that altering HK2 expression could impact the epigenome and, consequently, chromatin stability in yeast. To test this hypothesis, we established genetic models with different yeast hexokinase 2 (HXK2) expression in Saccharomyces cerevisiae yeast cells and investigated the effect of HXK2-dependent metabolism on parental nucleosome transfer, a key DNA replication-coupled epigenetic inheritance process, and chromatin stability.

RESULTS

By comparing the growth of mutant yeast cells carrying single deletion of hxk1Δ, hxk2Δ, or double-loss of hxk1Δ hxk2Δ to wild-type cells, we firstly confirmed that HXK2 is the dominant HXK in yeast cell growth. Surprisingly, manipulating HXK2 expression in yeast, whether through overexpression or deletion, had only a marginal impact on parental nucleosome assembly, but a noticeable trend with decrease chromatin instability. However, targeting yeast cells with 2-deoxy-D-glucose (2-DG), a clinical glycolysis inhibitor that has been proposed as an anti-cancer treatment, significantly increased chromatin instability.

CONCLUSION

Our findings suggest that in yeast cells lacking HXK2, alternative HXKs such as HXK1 or glucokinase 1 (GLK1) play a role in supporting glycolysis at a level that adequately maintains epigenomic stability. While our study demonstrated an increase in epigenetic instability with 2-DG treatment, the observed effect seemed to occur dependent on non-glycolytic function of Hxk2. Thus, additional research is needed to identify the molecular mechanism through which 2-DG influences chromatin stability.

The Role of Hexokinases in Epigenetic Regulation: Altered Hexokinase Expression and Chromatin Stability in Yeast

Background . Human hexokinase 2 ( HK2 ) plays an important role in regulating Warburg effect, which metabolizes glucose to lactate acid even in the presence of ample oxygen and provides intermediate metabolites to support cancer cell proliferation and tumor growth. HK2 overexpression has been observed in various types of cancers and targeting HK2 -driven Warburg effect has been suggested as a potential cancer therapeutic strategy. Given that epigenetic enzymes utilize metabolic intermediates as substrates or co-factors to carry out post-translational modification of DNA and histones in cells, we hypothesized that altering HK2 expression-mediated cellular glycolysis rates could impact the epigenome and, consequently, genome stability in yeast. To test this hypothesis, we established genetic models with different yeast hexokinase 2 ( HXK2) expression in Saccharomyces cerevisiae yeast cells and investigated the effect of HXK2 -dependent metabolism on parental nucleosome transfer, a key DNA replication-coupled epigenetic inheritance process, and chromatin stability. Results . By comparing the growth of mutant yeast cells carrying single deletion of hxk1Δ , hxk2Δ , or double-loss of hxk1Δ hxk2Δ to wild-type cells, we demonstrated that HXK2 is the dominant HXK in yeast cell growth. Surprisingly, manipulating HXK2 expression in yeast, whether through overexpression or deletion, had only a marginal impact on parental nucleosome assembly, but a noticeable trend with decrease chromatin instability. However, targeting yeast cells with 2-deoxy-D-glucose (2-DG), a HK2 inhibitor that has been proposed as an anti-cancer treatment, significantly increased chromatin instability. Conclusion . Our findings suggest that in yeast cells lacking HXK2 , alternative HXK s such as HXK1 or glucokinase 1 ( GLK1 ) play a role in supporting glycolysis at a level that adequately maintain epigenomic stability. While our study demonstrated an increase in epigenetic instability with 2-DG treatment, the observed effect seemed to occur independently of Hxk2-mediated glycolysis inhibition. Thus, additional research is needed to identify the molecular mechanism through which 2-DG influences chromatin stability.

Genome-Wide Characterization of Soybean Hexokinase Genes Reveals a Positive Role of GmHXK15 in Alkali Stress Response

Hexokinase (HXK) proteins catalyze hexose phosphorylation and are important for the sensing and signaling of sugar. In order to determine the roles played by HXKs in soybean growth and stress responsiveness, seventeen HXK genes (GmHXK1-17) were isolated and analyzed. The phylogenic analysis and subcellular location prediction showed that GmHXKs were clearly classified into type A (GmHXK1-4) and type B (GmHXK5-17). There were similar protein structures and conserved regions in GmHXKs to the HXKs of other plants. An expression analysis of the GmHXK genes in soybean organs or tissues demonstrated that GmHXK3 and GmHXK12, 15, and 16 were the dominant HXKs in all the examined tissues. In addition, salt, osmotic, and alkaline stress treatments dramatically increased the activity and transcripts of GmHXKs. There is the possibility that a type-B isoform (GmHXK15) plays a crucial role in soybean adaptation to alkali, as the expression levels of this isoform correlate well with the HXK enzyme activity. Based on an enzyme assay performed on recombinant plant HXK15 proteins expressed in Escherichia coli, we found that GmHXK15 had functional HXK activities. A further analysis indicated that GmHXK15 specifically targeted the mitochondria, and the overexpression of the GmHXK15 gene could significantly enhance the resistance of transgenic soybean to alkali stress. The present findings will serve as a basis for a further analysis of the function of the GmHXK gene family.

Hexokinase1: A glucose sensor involved in drought stress response and sugar metabolism depending on its kinase activity in strawberry

Hexokinase1 (HXK1) is a bifunctional enzyme that plays indispensable roles in plant growth, nitrogen utilization, and stress resistance. However, information on the HXK family members of strawberries and their functions in glucose sensing and metabolic regulation is scarce. In the present study, four HXKs were firstly identified in the genome of Fragaria vesca and F. pentaphylla. The conserved domains of the HXK1s were confirmed, and a site-directed mutation (S177A) was introduced into the FpHXK1. FpHXK1, which shares the highest identity with the AtHXK1 was able to restore the glucose sensitivity and developmental defects of the Arabidopsis gin2-1 mutant, but not its kinase-activity-impaired mutant (FpHXK1^S177A ). The transcription of FpHXK1 was dramatically up-regulated under PEG-simulated drought stress conditions. The inhibition of the HXK kinase activity delayed the strawberry plant's responses to drought stress. Transient overexpression of the FpHXK1 and its kinase-impaired mutant differentially affected the level of glucose, sucrose, anthocyanins, and total phenols in strawberry fruits. All these results indicated that the FpHXK1, acting as a glucose sensor, was involved in drought stress response and sugar metabolism depending on its kinase activity.

Characterization of hexokinase gene family members in Glycine max and functional analysis of GmHXK2 under salt stress

Hexokinase (HXK) is a bifunctional enzyme involved in carbohydrate metabolism and sugar signal sensing. HXK gene family has been extensively discussed in many species, while the detailed investigations of the family in Glycine max have yet to be reported. In this study, 17 GmHXK genes (GmHXKs) were identified in the G. max genome and the features of their encoded proteins, conserved domains, gene structures, and cis-acting elements were systematically characterized. The GmHXK2 gene isolated from G. max was firstly constructed into plant expression vector pMDC83 and then transformed with Agrobacterium tumefaciens into Arabidopsis thaliana. The expression of integrated protein was analyzed by Western Blotting. Subcellular localization analysis showed that the GmHXK2 was located on both vacuolar and cell membrane. Under salt stress, seedlings growth was significantly improved in Arabidopsis overexpressing GmHXK2 gene. Furthermore, physiological indicators and expression of salt stress responsive genes involved in K⁺ and Na⁺ homeostasis were significantly lower in GmHXK2-silenced soybean seedlings obtained by virus-induced gene silencing (VIGS) technique under salt stress compared with the control plants. Our study showed that GmHXK2 gene played an important role in resisting salt stress, which suggested potential value for the genetic improvement of abiotic resistant crops.

Genome-Wide Survey and Expression Analyses of Hexokinase Family in Poplar (Populus trichocarpa)

Hexokinase (HXK) family proteins exert critical roles in catalyzing hexose phosphorylation, sugar sensing, and modulation of plant growth and stress adaptation. Nevertheless, a large amount remains unknown about the molecular profile of HXK enzymes in Populus trichocarpa, a woody model tree species. A genome-wide survey of HXK-encoding genes, including phylogenies, genomic structures, exon/intron organization, chromosomal distribution, and conserved features, was conducted, identifying six putative HXK isogenes (PtHXK1-6) in the Populus genome. The evolutionary tree demonstrated that 135 homologous HXKs between 17 plant species were categorized into four major subfamilies (type A, B, C, and D), clustering one plastidic (PtHXK3) and five mitochondrial PtHXKs grouped into type A and B, respectively. The in silico deduction prompted the presence of the conserved sugar-binding core (motif 4), phosphorylation sites (motif 2 and 3), and adenosine-binding domains (motif 7). The transcriptomic sequencing (RNA-seq) and the quantitative real-time PCR (qRT-PCR) assays revealed that three isogenes (PtHXK2, 3, and 6) were abundantly expressed in leaves, stems, and roots, while others appeared to be dominantly expressed in the reproductive tissues. Under the stress exposure, PtHXK2 and 6 displayed a significant induction upon the pathogenic fungi (Fusarium solani) infection and marked promotions by glucose feeding in roots. In contrast, the PtHXK3 and 6 are ABA-responsive genes, following a dose-dependent manner. The comprehensive analyses of the genomic patterns and expression profiling provide theoretical clues and lay a foundation for unraveling the physiological and signaling roles underlying the fine-tuned PtHXKs responding to diverse stressors.

Moonlighting Proteins: The Case of the Hexokinases

Moonlighting proteins are defined as proteins with two or more functions that are unrelated and independent to each other, so that inactivation of one of them should not affect the second one and vice versa. Intriguingly, all the glycolytic enzymes are described as moonlighting proteins in some organisms. Hexokinase (HXK) is a critical enzyme in the glycolytic pathway and displays a wide range of functions in different organisms such as fungi, parasites, mammals, and plants. This review discusses HXKs moonlighting functions in depth since they have a profound impact on the responses to nutritional, environmental, and disease challenges. HXKs’ activities can be as diverse as performing metabolic activities, as a gene repressor complexing with other proteins, as protein kinase, as immune receptor and regulating processes like autophagy, programmed cell death or immune system responses. However, most of those functions are particular for some organisms while the most common moonlighting HXK function in several kingdoms is being a glucose sensor. In this review, we also analyze how different regulation mechanisms cause HXK to change its subcellular localization, oligomeric or conformational state, the response to substrate and product concentration, and its interactions with membrane, proteins, or RNA, all of which might impact the HXK moonlighting functions.

Prunus Hexokinase 3 genes alter primary C-metabolism and promote drought and salt stress tolerance in Arabidopsis transgenic plants

Hexokinases (HXKs) and fructokinases (FRKs) are the only two families of enzymes in plants that have been identified as able to phosphorylate Glucose (Glc) and Fructose (Fru). Glc can only be phosphorylated in plants by HXKs, while Fru can be phosphorylated by either HXKs or FRKs. The various subcellular localizations of HXKs in plants indicate that they are involved in diverse functions, including anther dehiscence and pollen germination, stomatal closure in response to sugar levels, stomatal aperture and reducing transpiration. Its association with modulating programmed cell death, and responses to oxidative stress and pathogen infection (abiotic and biotic stresses) also have been reported. To extend our understanding about the function of HXK-like genes in the response of Prunus rootstocks to abiotic stress, we performed a detailed bioinformatic and functional analysis of hexokinase 3-like genes (HXK3s) from two Prunus rootstock genotypes, 'M.2624' (Prunus cerasifera Ehrh × P. munsoniana W.Wight & Hedrick) and 'M.F12/1' (P. avium L.), which are tolerant and sensitive to hypoxia stress, respectively. A previous large-scale transcriptome sequencing of roots of these rootstocks, showed that this HXK3-like gene that was highly induced in the tolerant genotype under hypoxia conditions. In silico analysis of gene promoters from M.2624 and M.F12/1 genotypes revealed regulatory elements that could explain differential transcriptional profiles of HXK3 genes. Subcellular localization was determinates by both bioinformatic prediction and expression of their protein fused to the green fluorescent protein (GFP) in protoplasts and transgenic plants of Arabidopsis. Both approaches showed that they are expressed in plastids. Metabolomics analysis of Arabidopsis plants ectopically expressing Prunus HXK3 genes revealed that content of several metabolites including phosphorylated sugars (G6P), starch and some metabolites associated with the TCA cycle were affected. These transgenic Arabidopsis plants showed improved tolerance to salt and drought stress under growth chamber conditions. Our results suggest that Prunus HXK3 is a potential candidate for enhancing tolerance to salt and drought stresses in stone fruit trees and other plants.

Genome-Wide Identification and Characterization of Hexokinase Genes in Moso Bamboo (Phyllostachys edulis)

Plant hexokinases (HXKs) are a class of multifunctional proteins that not only act as the enzymes required for hexose phosphorylation but also serve as sugar sensors that repress the expression of some photosynthetic genes when internal glucose level increases and regulators of cell metabolism and some sugar-related signaling pathways independent on their catalytic actives. The HXKs have been studied in many plants; however, limited information is available on HXKs of moso bamboo (Phyllostachys edulis). In this study, we identified and characterized 12 hexokinase genes in moso bamboo. Phylogenetic analysis revealed that the moso bamboo hexokinases (PeHXKs) were classifiable into five subfamilies which represented the three types of hexokinases in plants. Gene structure and conserved motif analysis showed that the PeHXK genes contained diverse numbers of introns and exons and that the encoded proteins showed similar motif organization within each subfamily. Multiple sequence alignment revealed that the PeHXK proteins contained conserved domains, such as phosphate 1 (P1), phosphate 2 (P2), adenosine, and a sugar-binding domain. Evolutionary divergence analysis indicated that the PeHXK, OsHXK, and BdHXK families underwent negative selection and experienced a large-scale duplication event approximately 19-319 million years ago. Expression analysis of the PeHXK genes in the leaf, stem, root, and rhizome of moso bamboo seedlings indicated that the PeHXKs perform pivotal functions in the development of moso bamboo. A protein subcellular localization assay showed that PeHXK5a, PeHXK8, and PeHXK3b were predominantly localized in mitochondria, and PeHXK8 protein was also detected in the nucleus. The HXK activity of the PeHXK5a, PeHXK8, and PeHXK3b was verified by a functional complementation assay using the HXK-deficient triple-mutant yeast strain YSH7.4-3C (hxk1, hxk2, and glk1), and the results showed that the three PeHXKs had the plant HXK-specific enzyme traits. The present findings would provide a foundation for further functional analysis of the PeHXK gene family.

Biochemical properties and subcellular localization of six members of the HXK family in maize and its metabolic contribution to embryo germination

BACKGROUND

Seed germination is a crucial process in the plant life cycle when a dramatic variation of type and sugar content occurs just as the seed is hydrated. The production of hexose 6 phosphate is a key node in different pathways that are required for a successful germination. Hexokinase (HXK) is the only plant enzyme that phosphorylates glucose (Glc), so it is key to fueling several metabolic pathways depending on their substrate specificity, metabolite regulatory responses and subcellular localization. In maize, the HXK family is composed of nine genes, but only six of them (ZmHXK4-9) putatively encode catalytically active enzymes. Here, we cloned and functionally characterized putative catalytic enzymes to analyze their metabolic contribution during germination process.

RESULTS

From the six HXKs analyzed here, only ZmHXK9 has minimal hexose phosphorylating activity even though enzymatic function of all isoforms (ZmHXK4-9) was confirmed using a yeast complementation approach. The kinetic parameters of recombinant proteins showed that ZmHXK4-7 have high catalytic efficiency for Glc, fructose (Fru) and mannose (Man), ZmHXK7 has a lower Km for ATP, and together with ZmHXK8 they have lower sensitivity to inhibition by ADP, G6P and N-acetylglucosamine than ZmHXK4-6 and ZmHXK9. Additionally, we demonstrated that ZmHXK4-6 and ZmHXK9 are located in the mitochondria and their location relies on the first 30 amino acids of the N-terminal domain. Otherwise, ZmHXK7-8 are constitutively located in the cytosol. HXK activity was detected in cytosolic and mitochondrial fractions and high Glc and Fru phosphorylating activities were found in imbibed embryos.

CONCLUSIONS

Considering the biochemical characteristics, location and the expression of ZmHXK4 at onset of germination, we suggest that it is the main contributor to mitochondrial activity at early germination times, at 24 h other ZmHXKs also contribute to the total activity. While in the cytosol, ZmHXK7 could be responsible for the activity at the onset of germination, although later, ZmHXK8 also contributes to the total HXK activity. Our observations suggest that the HXKs may be redundant proteins with specific roles depending on carbon and ATP availability, metabolic needs, or sensor requirements. Further investigation is necessary to understand their specific or redundant physiological roles.

Trithorax-group protein ATX5 mediates the glucose response via impacting the HY1-ABI4 signaling module

Trithorax-group Protein ARABIDOPSIS TRITHORAX5 modulates the glucose response. Glucose is an evolutionarily conserved modulator from unicellular microorganisms to multicellular animals and plants. Extensive studies have shown that the Trithorax-group proteins (TrxGs) play essential roles in different biological processes by affecting histone modifications and chromatin structures. However, whether TrxGs function in the glucose response and how they achieve the control of target genes in response to glucose signaling in plants remain unknown. Here, we show that the Trithorax-group Protein ARABIDOPSIS TRITHORAX5 (ATX5) affects the glucose response and signaling. atx5 loss-of-function mutants display glucose-oversensitive phenotypes compared to the wild-type (WT). Genome-wide RNA-sequencing analyses have revealed that ATX5 impacts the expression of a subset of glucose signaling responsive genes. Intriguingly, we have established that ATX5 directly controls the expression of HY1 by trimethylating H3 lysine 4 of the Arabidopsis Heme Oxygenase1 (HY1) locus. Glucose signaling causes the suppression of ATX5 activity and subsequently reduces the H3K4me3 levels at the HY1 locus, thereby leading to the increased expression of ABSCISIC ACID-INSENSITIVE4 (ABI4). This result suggests that an important ATX5-HY1-ABI4 regulatory module governs the glucose response. This idea is further supported by genetic evidence showing that an atx5 hy1-100 abi4 triple mutant showed a similar glucose-insensitive phenotype as compared to that of the abi4 single mutant. Our findings show that a novel ATX5-HY1-ABI4 module controls the glucose response in Arabidopsis thaliana.

The Ability of a Charophyte Alga Hexokinase to Restore Glucose Signaling and Glucose Repression of Gene Expression in a Glucose-Insensitive Arabidopsis Hexokinase Mutant Depends on Its Catalytic Activity

Hexokinases is a family of proteins that is found in all eukaryotes. Hexokinases play key roles in the primary carbon metabolism, where they catalyze the phosphorylation of glucose and fructose, but they have also been shown to be involved in glucose signaling in both yeast and plants. We have characterized the Klebsormidium nitens KnHXK1 gene, the only hexokinase-encoding gene in this charophyte alga. The encoded protein, KnHXK1, is a type B plant hexokinase with an N-terminal membrane anchor localizing the protein to the mitochondrial membranes. We found that KnHXK1 expressed in Arabidopsis thaliana can restore the glucose sensing and glucose repression defects of the glucose-insensitive hexokinase mutant gin2-1. Interestingly, both functions require a catalytically active enzyme, since an inactive double mutant was unable to complement gin2-1. These findings differ from previous results on Arabidopsis AtHXK1 and its orthologs in rice, where catalytic and glucose sensing functions could be separated, but are consistent with recent results on the rice cytoplasmic hexokinase OsHXK7. A model with both catalytic and non-catalytic roles for hexokinases in glucose sensing and glucose repression is discussed.

Plant Hexokinases are Multifaceted Proteins

Sugars are the main carbon and energy source in cells, but they can also act as signaling molecules that affect the whole plant life cycle. Certain tissues can produce sugars and supply them to others, and this plant tissue heterogeneity makes sugar signaling a highly complex process that requires elements capable of perceiving changes in sugar concentrations among different tissues, cell compartments and developmental stages. In plants, the regulatory effects of glucose (Glc) have been the most studied to date. The first Glc sensor identified in plants was hexokinase (HXK), which is currently recognized as a dual-function protein. In addition to its catalytic activity, this enzyme can also repress the expression of some photosynthetic genes in response to high internal Glc concentrations. Additionally, the catalytic activity of HXKs has a profound impact on cell metabolism and other sugar signaling pathways that depend on phosphorylated hexoses and intermediate glycolytic products. HXKs are the only proteins that are able to phosphorylate Glc in plants, since no evidence has been provided to date concerning the existence of a glucokinase. Moreover, the intracellular localization of HXKs seems to be crucial to their activity and sensor functions. Recently, two new and surprising functions have been described for HXKs. In this review, we discuss the versatility of HXKs in regard to their catalytic and glucose sensor activities, intracellular location, protein-protein and hormone interactions, as well as how these HXK characteristics influence plant growth and development, in an effort to understand this enzyme's role in improving plant productivity.

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

Hexokinase (HXK) proteins play important roles in catalyzing hexose phosphorylation and sugar sensing and signaling. To investigate the roles of HXKs in cassava tuber root development, seven HXK genes (MeHXK1-7) were isolated and analyzed. A phylogenetic analysis revealed that the MeHXK family can be divided into five subfamilies of plant HXKs. MeHXKs were clearly divided into type A (MeHXK1) and type B (MeHXK2-7) based on their N-terminal sequences. MeHXK1-5 all had typical conserved regions and similar protein structures to the HXKs of other plants; while MeHXK6-7 lacked some of the conserved regions. An expression analysis of the MeHXK genes in cassava organs or tissues demonstrated that MeHXK2 is the dominant HXK in all the examined tissues (leaves, stems, fruits, tuber phloems, and tuber xylems). Notably, the expression of MeHXK2 and the enzymatic activity of HXK were higher at the initial and expanding tuber stages, and lower at the mature tuber stage. Furthermore, the HXK activity of MeHXK2 was identified by functional complementation of the HXK-deficient yeast strain YSH7.4-3C (hxk1, hxk2, glk1). The gene expression and enzymatic activity of MeHXK2 suggest that it might be the main enzyme for hexose phosphorylation during cassava tuber root development, which is involved in sucrose metabolism to regulate the accumulation of starch.

Dynamic and diverse sugar signaling

Sugars fuel life and exert numerous regulatory actions that are fundamental to all life forms. There are two principal mechanisms underlie sugar 'perception and signal transduction' in biological systems. Direct sensing and signaling is triggered via sugar-binding sensors with a broad range of affinity and specificity, whereas sugar-derived bioenergetic molecules and metabolites modulate signaling proteins and indirectly relay sugar signals. This review discusses the emerging sugar signals and potential sugar sensors discovered in plant systems. The findings leading to informative understanding of physiological regulation by sugars are considered and assessed. Comparative transcriptome analyses highlight the primary and dynamic sugar responses and reveal the convergent and specific regulators of key biological processes in the sugar-signaling network.

Master Regulators in Plant Glucose Signaling Networks

The daily life of photosynthetic plants revolves around sugar production, transport, storage and utilization, and the complex sugar metabolic and signaling networks integrate internal regulators and environmental cues to govern and sustain plant growth and survival. Although diverse sugar signals have emerged as pivotal regulators from embryogenesis to senescence, glucose is the most ancient and conserved regulatory signal that controls gene and protein expression, cell-cycle progression, central and secondary metabolism, as well as growth and developmental programs. Glucose signals are perceived and transduced by two principal mechanisms: direct sensing through glucose sensors and indirect sensing via a variety of energy and metabolite sensors. This review focuses on the comparative and functional analyses of three glucose-modulated master regulators in Arabidopsis thaliana, the hexokinase1 (HXK1) glucose sensor, the energy sensor kinases KIN10/KIN11 inactivated by glucose, and the glucose-activated target of rapamycin (TOR) kinase. These regulators are evolutionarily conserved, but have evolved universal and unique regulatory wiring and functions in plants and animals. They form protein complexes with multiple partners as regulators or effectors to serve distinct functions in different subcellular locales and organs, and play integrative and complementary roles from cellular signaling and metabolism to development in the plant glucose signaling networks.

Widespread dual targeting of proteins in land plants: when, where, how and why

Since the discovery of the first dual targeted protein in plants in 1995 the number of dual targeted proteins in plants has grown to over 250 proteins. Much work and investigations have focused on identifying how or what makes a protein dual targeted. Recently, more research has focused on the evolution and conservation of dual targeting of proteins in plants. This new work has demonstrated that dual targeting arose early within the evolution of plants and because it is rarely lost, once gained, it must be under some positive selection pressure. The possible reasons as why proteins are dual targeted and why it was conserved during the evolution of plants are discussed.

Mitochondrial VDAC and hexokinase together modulate plant programmed cell death

The voltage-dependent anion channel (VDAC) and mitochondrially located hexokinase have been implicated both in pathways leading to cell death on the one hand, and immortalization in tumor formation on the other. While both proteins have also been implicated in death processes in plants, their interaction has not been explored. We have examined cell death following heterologous expression of a rice VDAC in the tobacco cell line BY2 and in leaves of tobacco plants and show that it is ameliorated by co-expression of hexokinase. Hexokinase also abrogates death induced by H2O2. We conclude that the ratio of expression of the two proteins and their interaction play a major role in modulating death pathways in plants.

ATP produced by oxidative phosphorylation is channeled toward hexokinase bound to mitochondrial porin (VDAC) in beetroots (Beta vulgaris)

Mitochondrial porins or voltage-dependent anion channels (VDAC) are the main route for solute transport through outer mitochondrial membranes (OMM). In mammals, hexokinase (HK) binds to VDAC, which allows the channeling of ATP synthesized by oxidative phosphorylation toward HK. In plants, although HK has been found associated with OMM, evidence for an interaction with VDAC is scarce. Thus, in this work, we studied the physical and functional interaction between these proteins in beetroot mitochondria. To observe a physical interaction between HK and VDAC, OMM presenting HK activity were prepared from purified mitochondria. Protein complexes were solubilized from OMM with mild detergents and separated by centrifugation in glycerol gradients. Both HK activity and immunodetected VDAC were found in small (9S-13S) and large (>40S) complexes. OMM proteins were also separated according to their hydropathy by serial phase partitioning with Triton X-114. Most of HK activity was found in hydrophobic fractions where VDAC was also present. These results indicated that HK could be bound to VDAC in beetroot mitochondria. The functional interaction of HK with VDAC was demonstrated by observing the effect of apyrase on HK-catalyzed glucose phosphorylation in intact mitochondria. Apyrase, which hydrolyzes freely soluble ATP, competed efficiently with hexokinase for ATP when it was produced outside mitochondria (with PEP and pyruvate kinase), but not when it was produced inside mitochondria by oxidative phosphorylation. These results suggest that HK closely interacts with VDAC in beetroot mitochondria, and that this interaction allows the channeling of respiratory ATP toward HK through VDAC.

Hexose kinases and their role in sugar-sensing and plant development

Hexose sugars, such as glucose and fructose produced in plants, are ubiquitous in most organisms and are the origin of most of the organic matter found in nature. To be utilized, hexose sugars must first be phosphorylated. The central role of hexose-phosphorylating enzymes has attracted the attention of many researchers, leading to novel discoveries. Only two families of enzymes capable of phosphorylating glucose and fructose have been identified in plants; hexokinases (HXKs), and fructokinases (FRKs). Intensive investigations of these two families in numerous plant species have yielded a wealth of knowledge regarding the genes number, enzymatic characterization, intracellular localization, and developmental and physiological roles of several HXKs and FRKs. The emerging picture indicates that HXK and FRK enzymes found at specific intracellular locations play distinct roles in plant metabolism and development. Individual HXKs were shown for the first time to be dual-function enzymes - sensing sugar levels independent of their catalytic activity and controlling gene expression and major developmental pathways, as well as hormonal interactions. FRK, on the other hand, seems to play a central metabolic role in vascular tissues, controlling the amounts of sugars allocated for vascular development. While a clearer picture of the roles of these two types of enzymes is emerging, many questions remain unsolved, such as the specific tissues and types of cells in which these enzymes function, the roles of individual HXK and FRK genes, and how these enzymes interact with hormones in the regulation of developmental processes. It is anticipated that ongoing efforts will broaden our knowledge of these important plant enzymes and their potential uses in the modification of plant traits.

Acquisition, conservation, and loss of dual-targeted proteins in land plants

The dual-targeting ability of a variety of proteins from Physcomitrella patens, rice (Oryza sativa), and Arabidopsis (Arabidopsis thaliana) was tested to determine when dual targeting arose and to what extent it was conserved in land plants. Overall, the targeting ability of over 80 different proteins from rice and P. patens, representing 42 dual-targeted proteins in Arabidopsis, was tested. We found that dual targeting arose early in land plant evolution, as it was evident in many cases with P. patens proteins that were conserved in rice and Arabidopsis. Furthermore, we found that the acquisition of dual-targeting ability is still occurring, evident in P. patens as well as rice and Arabidopsis. The loss of dual-targeting ability appears to be rare, but does occur. Ascorbate peroxidase represents such an example. After gene duplication in rice, individual genes encode proteins that are targeted to a single organelle. Although we found that dual targeting was generally conserved, the ability to detect dual-targeted proteins differed depending on the cell types used. Furthermore, it appears that small changes in the targeting signal can result in a loss (or gain) of dual-targeting ability. Overall, examination of the targeting signals within this study did not reveal any clear patterns that would predict dual-targeting ability. The acquisition of dual-targeting ability also appears to be coordinated between proteins. Mitochondrial intermembrane space import and assembly protein40, a protein involved in oxidative folding in mitochondria and peroxisomes, provides an example where acquisition of dual targeting is accompanied by the dual targeting of substrate proteins.

Partial functional conservation of IRX10 homologs in physcomitrella patens and Arabidopsis thaliana indicates an evolutionary step contributing to vascular formation in land plants

BACKGROUND

Plant cell walls are complex multicomponent structures that have evolved to fulfil an essential function in providing strength and protection to cells. Hemicelluloses constitute a key component of the cell wall and recently a number of the genes thought to encode the enzymes required for its synthesis have been identified in Arabidopsis. The acquisition of hemicellulose synthesis capability is hypothesised to have been an important step in the evolution of higher plants.

RESULTS

Analysis of the Physcomitrella patens genome has revealed the presence of homologs for all of the Arabidopsis glycosyltransferases including IRX9, IRX10 and IRX14 required for the synthesis of the glucuronoxylan backbone. The Physcomitrella IRX10 homolog is expressed in a variety of moss tissues which were newly formed or undergoing expansion. There is a high degree of sequence conservation between the Physcomitrella IRX10 and Arabidopsis IRX10 and IRX10-L. Despite this sequence similarity, the Physcomitrella IRX10 gene is only able to partially rescue the Arabidopsis irx10 irx10-L double mutant indicating that there has been a neo- or sub-functionalisation during the evolution of higher plants. Analysis of the monosaccharide composition of stems from the partially rescued Arabidopsis plants does not show any significant change in xylose content compared to the irx10 irx10-L double mutant. Likewise, knockout mutants of the Physcomitrella IRX10 gene do not result in any visible phenotype and there is no significant change in monosaccharide composition of the cell walls.

CONCLUSIONS

The fact that the Physcomitrella IRX10 (PpGT47A) protein can partially complement an Arabidopsis irx10 irx10-L double mutant suggests that it shares some function with the Arabidopsis proteins, but the lack of a phenotype in knockout lines shows that the function is not required for growth or development under normal conditions in Physcomitrella. In contrast, the Arabidopsis irx10 and irx10 irx10-L mutants have strong phenotypes indicating an important function in growth and development. We conclude that the evolution of vascular plants has been associated with a significant change or adaptation in the function of the IRX10 gene family.

Usefulness of Physcomitrella patens for studying plant organogenesis

In this chapter, we review the main organogenesis features and associated regulation processes of the moss Physcomitrella patens (P. patens), the model plant for the Bryophytes. We highlight how the study of this descendant of the earliest plant species that colonized earth, brings useful keys to understand the mechanisms that determine and control both vascular and non vascular plants organogenesis. Despite its simple morphogenesis pattern, P. patens still requires the fine tuning of organogenesis regulators, including hormone signalling, common to the whole plant kingdom, and which study is facilitated by a high number of molecular tools, among which the powerful possibility of gene targeting/replacement. The recent discovery of moss cells reprogramming capacity completes the picture of an excellent model for studying plant organogenesis.

A reevaluation of dual-targeting of proteins to mitochondria and chloroplasts

Over 100 proteins are found in both mitochondria and chloroplasts, via a variety of processes known generally as 'dual-targeting'. Dual-targeting has attracted interest from many different research groups because of its profound implications concerning the mechanisms of protein import into these organelles and the evolution of both the protein import machinery and the targeting sequences within the imported proteins. Beyond these aspects, dual-targeting is also interesting for its implications concerning shared functions between mitochondria and chloroplasts, and especially the control of the activities of these two very different energy organelles. We discuss each of these points in the light of the latest relevant research findings and make some suggestions for where research might be most illuminating in the near future. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.