Hexokinase as a sugar sensor in higher plants

A comprehensive assessment of photosynthetic acclimation to shade in C4 grass (Cynodon dactylon (L.) Pers.)

BACKGROUND

Light deficit in shaded environment critically impacts the growth and development of turf plants. Despite this fact, past research has predominantly concentrated on shade avoidance rather than shade tolerance. To address this, our study examined the photosynthetic adjustments of Bermudagrass when exposed to varying intensities of shade to gain an integrative understanding of the shade response of C4 turfgrass.

RESULTS

We observed alterations in photosynthetic pigment-proteins, electron transport and its associated carbon and nitrogen assimilation, along with ROS-scavenging enzyme activity in shaded conditions. Mild shade enriched Chl b and LHC transcripts, while severe shade promoted Chl a, carotenoids and photosynthetic electron transfer beyond QA- (ET0/RC, φE0, Ψ0). The study also highlighted differential effects of shade on leaf and root components. For example, Soluble sugar content varied between leaves and roots as shade diminished SPS, SUT1 but upregulated BAM. Furthermore, we observed that shading decreased the transcriptional level of genes involving in nitrogen assimilation (e.g. NR) and SOD, POD, CAT enzyme activities in leaves, even though it increased in roots.

CONCLUSIONS

As shade intensity increased, considerable changes were noted in light energy conversion and photosynthetic metabolism processes along the electron transport chain axis. Our study thus provides valuable theoretical groundwork for understanding how C4 grass acclimates to shade tolerance.

Comprehensive genomic identification of cotton starch synthase genes reveals that GhSS9 regulates drought tolerance

Introduction

Starch metabolism is involved in the stress response. Starch synthase (SS) is the key enzyme in plant starch synthesis, which plays an indispensable role in the conversion of pyrophosphoric acid to starch. However, the SS gene family in cotton has not been comprehensively identified and systematically analyzed.

Result

In our study, a total of 76 SS genes were identified from four cotton genomes and divided into five subfamilies through phylogenetic analysis. Genetic structure analysis proved that SS genes from the same subfamily had similar genetic structure and conserved sequences. A cis-element analysis of the SS gene promoter showed that it mainly contains light response elements, plant hormone response elements, and abiotic stress elements, which indicated that the SS gene played key roles not only in starch synthesis but also in abiotic stress response. Furthermore, we also conducted a gene interaction network for SS proteins. Silencing GhSS9 expression decreased the resistance of cotton to drought stress. These findings suggested that SS genes could be related to drought stress in cotton, which provided theoretical support for further research on the regulation mechanism of SS genes on abiotic starch synthesis and sugar levels.

Guard-Cell Hexokinase Increases Water-Use Efficiency Under Normal and Drought Conditions

Water is a limiting resource for many land plants. Most of the water taken up by plants is lost to the atmosphere through the stomata, which are adjustable pores on the leaf surface that allow for gas exchange between the plant and the atmosphere. Modulating stomatal activity might be an effective way to reduce plants' water consumption and enhance their productivity under normal, as well as water-limiting conditions. Our recent discovery of stomatal regulation by sugars that is mediated by guard-cell hexokinase (HXK), a sugar-sensing enzyme, has raised the possibility that HXK might be used to increase plant water-use efficiency (WUE; i.e., carbon gain per unit of water). We show here that transgenic tomato and Arabidopsis plants with increased expression of HXK in their guard cells (GCHXK plants) exhibit reduced transpiration and higher WUE without any negative effects on growth under normal conditions, as well as drought avoidance and improved photosynthesis and growth under limited-water conditions. Our results demonstrate that exclusive expression of HXK in guard cells is an effective tool for improving WUE, and plant performance under drought.

Characterization of Glycolytic Pathway Genes Using RNA-Seq in Developing Kernels of Eucommia ulmoides

Eucommia ulmoides Oliver, the only member of the Eucommiaceae family, is a rare and valuable tree used to produce a highly valued traditional Chinese medicine and contains α-linolenic acid (ALA) up to 60% of the total fatty acids in the kernels (embryos). Glycolysis provides both cellular energy and the intermediates for other biosynthetic processes. However, nothing was known about the molecular basis of the glycolytic pathway in E. ulmoides kernels. The purposes of this study were to identify novel genes of E. ulmoides related to glycolytic metabolism and to analyze the expression patterns of selected genes in the kernels. Transcriptome sequencing based on the Illumina platform generated 96,469 unigenes in four cDNA libraries constructed using RNAs from 70 and 160 days after flowering kernels of both low- and high-ALA varieties. We identified and characterized the digital expression of 120 unigenes coding for 24 protein families involved in kernel glycolytic pathway. The expression levels of glycolytic genes were generally higher in younger kernels than in more mature kernels. Importantly, several unigenes from kernels of the high-ALA variety were expressed more than those from the low-ALA variety. The expression of 10 unigenes encoding key enzymes in the glycolytic pathway was validated by qPCR using RNAs from six kernel stages of each variety. The qPCR data were well consistent with their digital expression in transcriptomic analyses. This study identified a comprehensive set of genes for glycolytic metabolism and suggests that several glycolytic genes may play key roles in ALA accumulation in the kernels of E. ulmoides.

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.

The Mysterious Rescue of adg1-1/tpt-2 - an Arabidopsis thaliana Double Mutant Impaired in Acclimation to High Light - by Exogenously Supplied Sugars

An Arabidopsis thaliana double mutant (adg1-1/tpt-2) defective in the day- and night-path of photoassimilate export from the chloroplast due to a knockout in the triose phosphate/phosphate translocator (TPT; tpt-2) and a lack of starch [mutation in ADP glucose pyrophosphorylase (AGPase); adg1-1] exhibits severe growth retardation, a decrease in the photosynthetic capacity, and a high chlorophyll fluorescence (HCF) phenotype under high light conditions. These phenotypes could be rescued when the plants were grown on sucrose (Suc) or glucose (Glc). Here we address the question whether Glc-sensing hexokinase1 (HXK1) defective in the Glc insensitive 2 (gin2-1) mutant is involved in the sugar-dependent rescue of adg1-1/tpt-2. Triple mutants defective in the TPT, AGPase, and HXK1 (adg1-1/tpt-2/gin2-1) were established as homozygous lines and grown together with Col-0 and Landsberg erecta (Ler) wild-type plants, gin2-1, the adg1-1/tpt-2 double mutant, and the adg1-1/tpt-2/gpt2-1 triple mutant [additionally defective in the glucose 6-phosphate/phosphate translocator 2 (GPT2)] on agar in the presence or absence of 50 mM of each Glc, Suc, or fructose (Fru). The growth phenotype of the double mutant and both triple mutants could be rescued to a similar extent only by Glc and Suc, but not by Fru. All three sugars were capable of rescuing the HCF and photosynthesis phenotype, irrespectively of the presence or absence of HXK1. Quantitative RT-PCR analyses of sugar-responsive genes revealed that plastidial HXK (pHXK) was up-regulated in adg1-1/tpt-2 plants grown on sugars, but showed no response in adg1-1/tpt-2/gin2-1. It appears likely that soluble sugars are directly taken up by the chloroplasts and enter further metabolism, which consumes ATP and NADPH from the photosynthetic light reaction and thereby rescues the photosynthesis phenotype of the double mutant. The implication of sugar turnover and probably signaling inside the chloroplasts for the concept of retrograde signaling is discussed.

Two novel types of hexokinases in the moss Physcomitrella patens

BACKGROUND

Hexokinase catalyzes the phosphorylation of glucose and fructose, but it is also involved in sugar sensing in both fungi and plants. We have previously described two types of hexokinases in the moss Physcomitrella. Type A, exemplified by PpHxk1, the major hexokinase in Physcomitrella, is a soluble protein that localizes to the chloroplast stroma. Type B, exemplified by PpHxk2, has an N-terminal membrane anchor. Both types are found also in vascular plants, and localize to the chloroplast stroma and mitochondrial membranes, respectively.

RESULTS

We have now characterized all 11 hexokinase encoding genes in Physcomitrella. Based on their N-terminal sequences and intracellular localizations, three of the encoded proteins are type A hexokinases and four are type B hexokinases. One of the type B hexokinases has a splice variant without a membrane anchor, that localizes to the cytosol and the nucleus. However, we also found two new types of hexokinases with no obvious orthologs in vascular plants. Type C, encoded by a single gene, has neither transit peptide nor membrane anchor, and is found in the cytosol and in the nucleus. Type D hexokinases, encoded by three genes, have membrane anchors and localize to mitochondrial membranes, but their sequences differ from those of the type B hexokinases. Interestingly, all moss hexokinases are more similar to each other in overall sequence than to hexokinases from other plants, even though characteristic sequence motifs such as the membrane anchor of the type B hexokinases are highly conserved between moss and vascular plants, indicating a common origin for hexokinases of the same type.

CONCLUSIONS

We conclude that the hexokinase gene family is more diverse in Physcomitrella, encoding two additional types of hexokinases that are absent in vascular plants. In particular, the presence of a cytosolic and nuclear hexokinase (type C) sets Physcomitrella apart from vascular plants, and instead resembles yeast, where all hexokinases localize to the cytosol. The fact that all moss hexokinases are more similar to each other than to hexokinases from vascular plants, even though both type A and type B hexokinases are present in all plants, further suggests that the hexokinase gene family in Physcomitrella has undergone concerted evolution.

Carbon and nitrogen nutrient balance signaling in plants

Cellular carbon (C) and nitrogen (N) metabolism must be tightly coordinated to sustain optimal growth and development for plants and other cellular organisms. Furthermore, C/N balance is also critical for the ecosystem response to elevated atmospheric CO(2). Despite numerous physiological and molecular studies in C/N balance or ratio response, very few genes have been shown to play important roles in C/N balance signaling. During recent five years, exciting progress was made through genetic and genomic studies. Several DNA microarray studies have shown that more than half of the transcriptome is regulated by C, N and the C-N combination. Three genetic studies involving distinct bioassays have demonstrated that a putative nitrate transporter (NTR2.1), a putative glutamate receptor (GLR1.1) and a putative methyltransferase (OSU1) have important functions in the C/N balance response. OSU1 is identical to QUA2/TSD2 which has been implicated to act in cell wall biogenesis, indicating a link between cell wall property and the C/N balance signaling. Given that many investigations are only focused on C alone or N alone, the C/N balance bioassays and gene expression patterns are discussed to assist phenotypic characterization of C/N balance signaling. Further, re-examination of those previously reported sugar or nitrogen responsive genes in C/N balance response may be necessary to dissect the C/N signaling pathways. In addition, key components involved in C-N interactions in bacterial, yeast and animal systems and whether they are functionally conserved in plants are discussed. These rapid advances have provided the first important step towards the construction of the complex yet elegant C/N balance signaling networks in plants.

Soluble sugars--metabolism, sensing and abiotic stress: a complex network in the life of plants

Plants are autotrophic and photosynthetic organisms that both produce and consume sugars. Soluble sugars are highly sensitive to environmental stresses, which act on the supply of carbohydrates from source organs to sink ones. Sucrose and hexoses both play dual functions in gene regulation as exemplified by the upregulation of growth-related genes and downregulation of stress-related genes. Although coordinately regulated by sugars, these growth- and stress-related genes are upregulated or downregulated through HXK-dependent and/or HXK-independent pathways. Sucrose-non-fermenting-1- (SNF1-) related protein pathway, analogue to the protein kinase (SNF-) yeast-signalling pathway, seems also involved in sugar sensing and transduction in plants. However, even if plants share with yeast some elements involved in sugar sensing, several aspects of sugar perception are likely to be peculiar to higher plants. In this paper, we have reviewed recent evidences how plants sense and respond to environmental factors through sugar-sensing mechanisms. However, we think that forward and reverse genetic analysis in combination with expression profiling must be continued to uncover many signalling components, and a full biochemical characterization of the signalling complexes will be required to determine specificity and cross-talk in abiotic stress signalling pathways.

Actin-based cellular framework for glucose signaling by Arabidopsis hexokinase1

Glucose functions in plants both as a metabolic resource as well as a hormone that regulates expression of many genes. Arabidopsis hexokinase1 (HXK1) is the best understood plant glucose sensor/transducer, yet we are only now appreciating the cellular complexity of its signaling functions. We have recently shown that one of the earliest detectable responses to plant glucose treatments are extensive alterations of cellular F-actin. Interestingly, AtHXK1 is predominantly located on mitochondria, yet also can interact with actin. A normal functioning actin cytoskeleton is required for HXK1 to act as an effector in glucose signaling assays. We have suggested that HXK1 might alter F-actin dynamics and thereby influence the formation and/or stabilization of cytoskeleton-bound polysomes. In this Addendum, we have extended our initial observations on the subcellular targeting of HXK1 and its interaction with F-actin. We then further consider the cellular context in which HXK1 might regulate gene expression.

Transgenic approaches to crop improvement

Transgenic crops are now grown commercially on several million hectares, principally in North America. To date, the predominant crops are maize (corn), soybean, cotton, and potatoes. In addition, there have been field trials of transgenics from at least 52 species including all the major field crops, vegetables, and several herbaceous and woody species. This review summarizes recent data relating to such trials, particularly in terms of the trends away from simple, single gene traits such as herbicide and insect resistance towards more complex agronomic traits such as growth rate and increased photosynthetic efficiency. Much of the recent information is derived from inspection of patent databases, a useful source of information on commercial priorities. The review also discusses the time scale for the introduction of these transgenes into breeding populations and their eventual release as new varieties.