ABI4 mediates the effects of exogenous trehalose on Arabidopsis growth and starch breakdown

Arabidopsis and primary photosynthetic metabolism - more than the icing on the cake

Historically speaking, Arabidopsis was not the plant of choice for investigating photosynthesis, with physiologists and biochemists favouring other species such as Chlorella, spinach and pea. However, its inherent advantages for forward genetics rapidly led to its adoption for photosynthesis research. In the last ten years, the availability of the Arabidopsis genome sequence - still the gold-standard for plant genomes - and the rapid expansion of genetic and genomic resources have further increased its importance. Research in Arabidopsis has not only provided comprehensive information about the enzymes and other proteins involved in photosynthesis, but has also allowed transcriptional responses, protein levels and compartmentation to be analysed at a global level for the first time. Emerging technical and theoretical advances offer another leap forward in our understanding of post-translational regulation and the control of metabolism. To illustrate the impact of Arabidopsis, we provide a historical review of research in primary photosynthetic metabolism, highlighting the role of Arabidopsis in elucidation of the pathway of photorespiration and the regulation of RubisCO, as well as elucidation of the pathways of starch turnover and studies of the significance of starch for plant growth.

Sugar perception and signaling--an update

Sugars act as potent signaling molecules in plants. Several sugar sensors, including the highly studied glucose sensor HEXOKINASE1 (HXK1), have been identified or proposed. Many additional sensors likely exist, as plants respond to other sugars and sugar metabolites, such as sucrose and trehalose 6-phosphate. Sugar sensing and signaling is a highly complex process resulting in many changes in physiology and development and is integrated with other signaling pathways in plants such as those for inorganic nutrients, hormones, and different stress factors. Importantly, KIN10 and KIN11 protein kinases are central in coordinating several of the responses to sugars and stress. bZIP transcription factors were found to mediate effects of sugar signaling on gene expression and metabolite content.

Trehalose metabolism: from osmoprotection to signaling

Trehalose is a non-reducing disaccharide formed by two glucose molecules. It is widely distributed in Nature and has been isolated from certain species of bacteria, fungi, invertebrates and plants, which are capable of surviving in a dehydrated state for months or years and subsequently being revived after a few hours of being in contact with water. This disaccharide has many biotechnological applications, as its physicochemical properties allow it to be used to preserve foods, enzymes, vaccines, cells etc., in a dehydrated state at room temperature. One of the most striking findings a decade ago was the discovery of the genes involved in trehalose biosynthesis, present in a great number of organisms that do not accumulate trehalose to significant levels. In plants, this disaccharide has diverse functions and plays an essential role in various stages of development, for example in the formation of the embryo and in flowering. Trehalose also appears to be involved in the regulation of carbon metabolism and photosynthesis. Recently it has been discovered that this sugar plays an important role in plant-microorganism interactions.

The Arabidopsis ABA-INSENSITIVE (ABI) 4 factor acts as a central transcription activator of the expression of its own gene, and for the induction of ABI5 and SBE2.2 genes during sugar signaling

The transcription factor ABA INSENSITIVE 4 (ABI4), discovered nearly 10 years ago, plays a central role in a variety of functions in plants, including sugar responses. However, not until very recently has its mechanism of action begun to be elucidated. Modulating gene expression is one of the primary mechanisms of sugar regulation in plants. Nevertheless, the transcription factors involved in regulating sugar responses and their role(s) during the signal transduction cascade remain poorly defined. In this paper we analyzed the participation of ABI4, as it is one of the main transcription factors implicated in glucose signaling during early seedling development. Our studies show that ABI4 is an essential activator of its own expression during development, in ABA signaling and in sugar responses. It is also important for the glucose-mediated expression of the genes ABI5 and SBE2.2. We demonstrate that ABI4 binds directly to the promoter region of all three genes and activates their expression in vivo through at CE1-like element. Previous studies found that ABI4 also functions as a transcriptional repressor of sugar-regulated genes, therefore this transcription factor is a versatile protein with dual functions for modulating gene expression.

Extensive expression regulation and lack of heterologous enzymatic activity of the Class II trehalose metabolism proteins from Arabidopsis thaliana

Trehalose metabolism has profound effects on plant growth and metabolism, but the mechanisms involved are unclear. In Arabidopsis, 21 putative trehalose biosynthesis genes are classified in three subfamilies based on their similarity with yeast TPS1 (encoding a trehalose-6-phosphate synthase, TPS) or TPS2 (encoding a trehalose-6-phosphate phosphatase, TPP). Although TPS1 (Class I) and TPPA and TPPB (Class III) proteins have established TPS and TPP activity, respectively, the function of the Class II proteins (AtTPS5-AtTPS11) remains elusive. A complete set of promoter-beta-glucurinidase/green fluorescent protein reporters demonstrates their remarkably differential tissue-specific expression and responsiveness to carbon availability and hormones. Heterologous expression in yeast furthermore suggests that none of the encoded enzymes displays significant TPS or TPP activity, consistent with a regulatory rather than metabolic function for this remarkable class of proteins.

Validamycin A improves the response of Medicago truncatula plants to salt stress by inducing trehalose accumulation in the root nodules

In this work, the role of trehalose as an osmoprotectant against salt stress conditions was examined in root nodules of Medicago truncatula. For this purpose, we used validamycin A, a potent trehalase inhibitor, in order to induce trehalose accumulation. Validamycin A induced an increase of trehalose concentration in root nodules of M. truncatula by inhibiting trehalase activity; no effect on trehalose concentration was observed in roots and leaves. Trehalose accumulation was accompanied by a decrease in sucrose and starch content, indicating interference with carbohydrate partitioning in the plants. Under salinity conditions, sucrose accumulation appears to be induced in M. truncatula to protect nodule functioning by the inhibition of sucrose catabolism by sucrose synthase and alkaline invertase activities. However, trehalose accumulation induced by val A in nodules improved the response to salinity by increasing plant dry weight (PDW), and no effects of validamycin A on nitrogenase activity and PDW were observed in nonsalinized plants.

Trehalose Metabolites in Arabidopsis-elusive, active and central

Trehalose is an alpha, alpha-1, 1-linked glucose disaccharide. In plants, trehalose is synthesized in two steps. Firstly, trehalose-6-phosphate synthase (TPS) converts UDP-glucose and glucose-6-phosphate to trehalose-6-phosphate (T6P); secondly, T6P-phosphatase (TPP) converts T6P into trehalose and Pi. Trehalose is further cleaved into glucose by trehalase. In extracts of most plants, including Arabidopsis, levels of both trehalose and T6P are low, nearing detection limits, and this has delayed research into their function. Trehalose is transported widely in plants, but transport of T6P is not thought to occur except possibly at the subcellular level. Feeding trehalose to Arabidopsis seedlings alters carbon allocation with massive starch accumulation in cotyledons and leaves and absence of starch and growth in shoot and root apices.The Arabidopsis genome has experienced extensive radiation of genes likely encoding enzymes of T6P metabolism: 4 and 10 genes are found with homology to TPS and TPP respectively and 7 genes are found with homology to both TPS and TPP. Complementation of Saccharomyces cerevisiae mutants has shown that AtTPS1, AtTPPA and AtTPPB are functional enzymes. In contrast just a single gene encoding a protein with trehalase activity has been found. Whilst most TPS proteins appear cytosolic, strikingly, some TPPs appear targeted to chloroplasts; trehalase on the other hand is extracellular. Transporters of trehalose and T6P have yet to be described. Arabidopsis tps1 mutants are embryo lethal and results suggest that T6P is essential for several other steps in development including root growth and floral transition. Accordingly, altering T6P content has a profound effect on plant habitus and impacts metabolite profiles, sugar utilization and photosynthesis. These large effects have hindered dissection of cause and effect. In contrast, plants with large alterations in sucrose-6-phosphate concentrations are indistinguishable from wild type, suggesting very different functions for these compounds. Recently, T6P at low micromolar concentrations has been shown in vitro and in vivo to inhibit SnRK1 of the SNF1/AMPK group of protein kinases. This supports a function for T6P as a sugar signaling molecule integrating metabolism and development in plants in relation to carbon supply.Genetic engineering of Arabidopsis as well as tobacco, potato and rice with TPS or TPS/TPP protein fusions reveals that trehalose metabolism also mediates multiple abiotic stress tolerances. Trehalose applications also mediate biotic stress resistances. Both Escherichia coli and Saccharomyces cerevisiae TPS/TPP protein fusions can be used to engineer stress tolerance suggesting that metabolites rather than proteins of the trehalose pathway are key stress tolerance elicitors. Results underscore the central role of trehalose metabolites in integrating carbon metabolism and stress responses with plant development.

Molecular processes underlying the floral transition in the soybean shoot apical meristem

The transition to flowering is characterized by a shift of the shoot apical meristem (SAM) from leaf production to the initiation of a floral meristem. The flowering process is of vital importance for agriculture, but the associated events or regulatory pathways in the SAM are not well understood, especially at a system level. To address this issue, we have used a GeneChip containing 37 744 probe sets to generate a temporal profile of gene expression during the floral initiation process in the SAM of the crop legume, soybean (Glycine max). A total of 331 transcripts displayed significant changes in their expression profiles. The in silico and RT-PCR analysis on differentially regulated transcripts implies the intriguing involvement of sugar, auxin or abscisic acid (ABA) in events prior to the induction of floral homeotic transcripts. The novel involvement of ABA in the floral transition is further implicated by immunoassay, suggesting an increase in ABA levels in the SAM during this developmental transition. Furthermore, in situ localization, together with in silico data demonstrating a marked enhancement of abiotic stress-related transcripts, such as trehalose metabolism genes in SAMs, points to an overlap of abiotic stress and floral signalling pathways.

Sugar sensing and signaling

Plants, restricted by their environment, need to integrate a wide variety of stimuli with their metabolic activity, growth and development. Sugars, generated by photosynthetic carbon fixation, are central in coordinating metabolic fluxes in response to the changing environment and in providing cells and tissues with the necessary energy for continued growth and survival. A complex network of metabolic and hormone signaling pathways are intimately linked to diverse sugar responses. A combination of genetic, cellular and systems analyses have uncovered nuclear HXK1 (hexokinase1) as a pivotal and conserved glucose sensor, directly mediating transcription regulation, while the KIN10/11 energy sensor protein kinases function as master regulators of transcription networks under sugar and energy deprivation conditions. The involvement of disaccharide signals in the regulation of specific cellular processes and the potential role of cell surface receptors in mediating sugar signals add to the complexity. This chapter gives an overview of our current insight in the sugar sensing and signaling network and describes some of the molecular mechanisms involved.

The Arabidopsis GSQ5/DOG1 Cvi allele is induced by the ABA-mediated sugar signalling pathway, and enhances sugar sensitivity by stimulating ABI4 expression

ABI4 encodes an AP2 family transcription factor that is a central regulator in sugar responsive gene expression in plants. Sugar-induced ABI4 regulates plant genes essential for photosynthesis, and carbon, nitrogen and lipid metabolism. ABI4 activity is induced via the ABA-mediated sugar signalling pathway, which is initiated by the glucose sensing protein hexokinase. Natural variation in sugar sensitivity was used to identify new loci involved in sugar signalling. Five quantitative trait loci (QTLs) for glucose sensitivity (GSQ1-GSQ5) were identified in a Ler/Cvi recombinant inbred line (RIL) population. The GSQ3, GSQ4 and GSQ5 loci are positioned in regions not previously associated with known sugar-sensing genes. GSQ5 was fine mapped and cloned using a candidate-gene approach. The GSQ5 locus was shown to encode the DELAY OF GERMINATION 1 (DOG1) gene. DOG1 was previously identified as a major locus in seed dormancy control. Glucose addition induced the expression of the GSQ5/DOG1 Cvi allele, whereas the Ler and Col alleles did not respond to glucose. Positive feedback was observed between the ABA-mediated sugar signalling pathway and the GSQ5/DOG1 Cvi allele. Expression of the GSQ5/DOG1 Cvi allele requires the ABA-mediated sugar signalling pathway, of which ABI4 is an important component. In addition, sugar induction of ABI4 was promoted by the GSQ5/DOG1 Cvi allele.

The Arabidopsis sweetie mutant is affected in carbohydrate metabolism and defective in the control of growth, development and senescence

Sugars modulate many vital metabolic and developmental processes in plants, from seed germination to flowering, senescence and protection against diverse abiotic and biotic stresses. However, the exact mechanisms involved in morphogenesis, developmental signalling and stress tolerance remain largely unknown. Here we report the characterization of a novel Arabidopsis thaliana mutant, sweetie, with drastically altered morphogenesis, and a strongly modified carbohydrate metabolism leading to elevated levels of trehalose, trehalose-6-phosphate and starch. We additionally show that the disruption of SWEETIE causes significant growth and developmental alterations, such as severe dwarfism, lancet-shaped leaves, early senescence and flower sterility. Genes implicated in sugar metabolism, senescence, ethylene biosynthesis and abiotic stress were found to be upregulated in sweetie. Our physiological, biochemical, genetic and molecular data indicate that the mutation in sweetie was nuclear, single and recessive. The effects of metabolizable sugars and osmolytes on sweetie morphogenesis were distinct; in light, sweetie was hypersensitive to sucrose and glucose during vegetative growth and a partial phenotypic reversion took place in the presence of high sorbitol concentrations. However, SWEETIE encodes a protein that is unrelated to any known enzyme involved in sugar metabolism. We suggest that SWEETIE plays an important regulatory function that influences multiple metabolic, hormonal and stress-related pathways, leading to altered gene expression and pronounced changes in the accumulation of sugar, starch and ethylene.

Trehalose metabolism and signaling

Trehalose metabolism and signaling is an area of emerging significance. In less than a decade our views on the importance of trehalose metabolism and its role in plants have gone through something of a revolution. An obscure curiosity has become an indispensable regulatory system. Mutant and transgenic plants of trehalose synthesis display wide-ranging and unprecedented phenotypes for the perturbation of a metabolic pathway. Molecular physiology and genomics have provided a glimpse of trehalose biology that had not been possible with conventional techniques, largely because the products of the synthetic pathway, trehalose 6-phosphate (T6P) and trehalose, are in trace abundance and difficult to measure in most plants. A consensus is emerging that T6P plays a central role in the coordination of metabolism with development. The discovery of trehalose metabolism has been one of the most exciting developments in plant metabolism and plant science in recent years. The field is fast moving and this review highlights the most recent insights.

Transcript profiling of Zea mays roots reveals gene responses to phosphate deficiency at the plant- and species-specific levels

Maize (Zea mays) is the most widely cultivated crop around the world; however, it is commonly affected by phosphate (Pi) deficiency in many regions, particularly in acid and alkaline soils of developing countries. To cope with Pi deficiency, plants have evolved a large number of developmental and biochemical adaptations; however, for maize, the underlying molecular basis of these responses is still unknown. In this work, the transcriptional response of maize roots to Pi starvation at 1, 3, 6, and 10 d after the onset of Pi deprivation was assessed. The investigation revealed a total of 1179 Pi-responsive genes, of which 820 and 363 genes were found to be either up- or down-regulated, respectively, by 2-fold or more. Pi-responsive genes were found to be involved in various metabolic, signal transduction, and developmental gene networks. A large set of transcription factors, which may be potential targets for crop breeding, was identified. In addition, gene expression profiles and changes in specific metabolites were also correlated. The results show that several dicotyledonous plant responses to Pi starvation are conserved in maize, but that some genetic responses appear to be more specific and that Pi deficiency leads to a shift in the recycling of internal Pi in maize roots. Ultimately, this work provides a more comprehensive view of Pi-responses in a model for economically important cereals and also sets a framework to produce Pi-specific maize microarrays to study the changes in global gene expression between Pi-efficient and Pi-inefficient maize genotypes.

A bifunctional TPS-TPP enzyme from yeast confers tolerance to multiple and extreme abiotic-stress conditions in transgenic Arabidopsis

Improving stress tolerance is a major goal for agriculture. Trehalose is a key molecule involved in drought tolerance in anhydrobiotic organisms. Here we describe the construction of a chimeric translational fusion of yeast trehalose-6-phosphate synthase and trehalose-6-phosphate phosphatase. This construct was overexpressed in yeast cells displaying both TPS and TPP enzyme activities and trehalose biosynthesis capacity. In Arabidopsis thaliana, the gene fusion was overexpressed using either the 35S promoter or the stress-regulated rd29A promoter. Transgene insertion in the genome was checked by PCR and transcript expression by RT-PCR. Several independent homozygous lines were selected in the presence of kanamycin and further analyzed. Trehalose was accumulated in all these lines at low levels. No morphological or growth alterations were observed in lines overexpressing the TPS1-TPS2 construct, whereas plants overexpressing the TPS1 alone under the control of the 35S promoter had aberrant growth, color and shape. TPS1-TPS2 overexpressor lines were glucose insensitive, consistent with a suggested role of trehalose/T6P in modulating sugar sensing and carbohydrate metabolism. Moreover, TPS1-TPS2 lines displayed a significant increase in drought, freezing, salt and heat tolerance. This is the first time that trehalose accumulation in plants is shown to protect against freezing and heat stress. Therefore, these results demonstrate that engineering trehalose metabolism with a yeast TPS-TPP bifunctional enzyme confers multiple stress protection in plants, comprising a potential tool to improve stress-tolerance in crops.

Trehalose transporter 1, a facilitated and high-capacity trehalose transporter, allows exogenous trehalose uptake into cells

Trehalose is potentially a useful cryo- or anhydroprotectant molecule for cells and biomolecules such as proteins and nucleotides. A major obstacle to application is that cellular membranes are impermeable to trehalose. In this study, we isolated and characterized the functions of a facilitated trehalose transporter [trehalose transporter 1 (TRET1)] from an anhydrobiotic insect, Polypedilum vanderplanki. Tret1 cDNA encodes a 504-aa protein with 12 predicted transmembrane structures. Tret1 expression was induced by either desiccation or salinity stress. Expression was predominant in the fat body and occurred concomitantly with the accumulation of trehalose, indicating that TRET1 is involved in transporting trehalose synthesized in the fat body into the hemolymph. Functional expression of TRET1 in Xenopus oocytes showed that transport activity was stereochemically specific for trehalose and independent of extracellular pH (between 4.0 and 9.0) and electrochemical membrane potential. These results indicate that TRET1 is a trehalose-specific facilitated transporter and that the direction of transport is reversible depending on the concentration gradient of trehalose. The extraordinarily high values for apparent Km (>or=100 mM) and Vmax (>or=500 pmol/min per oocyte) for trehalose both indicate that TRET1 is a high-capacity transporter of trehalose. Furthermore, TRET1 was found to function in mammalian cells, suggesting that it confers trehalose permeability on cells, including those of vertebrates as well as insects. These characteristic features imply that TRET1 in combination with trehalose has high potential for basic and practical applications in vivo.

Improved drought tolerance without undesired side effects in transgenic plants producing trehalose

Most organisms naturally accumulating trehalose upon stress produce the sugar in a two-step process by the action of the enzymes trehalose-6-phosphate synthase (TPS) and trehalose-6-phosphate phosphatase (TPP). Transgenic plants overexpressing TPS have shown enhanced drought tolerance in spite of minute accumulation of trehalose, amounts believed to be too small to provide a protective function. However, overproduction of TPS in plants has also been found combined with pleiotropic growth aberrations. This paper describes three successful strategies to circumvent such growth defects without loosing the improved stress tolerance. First, we introduced into tobacco a double construct carrying the genes TPS1 and TPS2 (encoding TPP) from Saccharomyces cerevisiae. Both genes are regulated by an Arabidopsis RuBisCO promoter from gene AtRbcS1A giving constitutive production of both enzymes. The second strategy involved stress-induced expression by fusing the coding region of ScTPS1 downstream of the drought-inducible Arabidopsis AtRAB18 promoter. In transgenic tobacco plants harbouring genetic constructs with either ScTPS1 alone, or with ScTPS1 and ScTPS2 combined, trehalose biosynthesis was turned on only when the plants experienced stress. The third strategy involved the use of AtRbcS1A promoter together with a transit peptide in front of the coding sequence of ScTPS1, which directed the enzyme to the chloroplasts. This paper confirms that the enhanced drought tolerance depends on unknown ameliorated water retention as the initial water status is the same in control and transgenic plants and demonstrates the influence of expression of heterologous trehalose biosynthesis genes on Arabidopsis root development.

Trehalose 6-phosphate

Trehalose 6-phosphate (T6P) is a sugar signal of emerging significance. It is an essential component of the mechanisms that coordinate metabolism with plant growth adaptation and development. Its significance began to dawn when genetic modification of the trehalose pathway produced dramatic phenotypes, before the genetic proliferation of the trehalose pathway in plants was fully realised. T6P regulates sugar utilization and starch metabolism and interacts with other signalling pathways, including those mediated by plant hormones. Trehalose phosphate synthases (TPSs) and trehalose phosphate phosphatases are regulated at the gene level by sugars, nitrate, cytokinin and abscisic acid. TPSs are also regulated post-translationally. Mechanistic details of how T6P signals are emerging, but still sparse. Nevertheless, even at this stage, targeting central regulators such as T6P offers promise in crop improvement.