Truncation of Arabidopsis thaliana and Selaginella lepidophylla trehalose-6-phosphate synthase unlocks high catalytic activity and supports high trehalose levels on expression in yeast

Gene families and evolution of trehalose metabolism in plants

The genomes of Arabidopsis thaliana L., rice (Oryza sativa L.) and poplar (Populus trichocarpa Torr. & A.Gray) contain large families of genes encoding trehalose-phosphate synthase (TPS) and trehalose-phosphatase (TPP). The class I subfamily of TPS genes encodes catalytically active TPS enzymes, and is represented by only one or two genes in most species. A. thaliana is atypical in having four class I TPS genes, three of which (AtTPS2-4) encode unusual short isoforms of TPS that appear to be found only in members of the Brassicaceae family. The class II TPS genes encode TPS-like proteins with a C-terminal TPP-like domain, but there is no experimental evidence that they have any enzymatic activity and their function is unknown. Both classes of TPS gene are represented in the genomes of chlorophyte algae (Ostreococcus species) and non-flowering plants [Physcomitrella patens (Hedw.) Bruch & Schimp.(B.S.G.) and Selaginella moellendorffii (Hieron. in Engl. & Prantl.)]. This survey shows that the gene families encoding the enzymes of trehalose metabolism are very ancient, pre-dating the divergence of the streptophyte and chlorophyte lineages. It also provides a frame of reference for future studies to elucidate the function of trehalose metabolism in plants.

Multilevel genomics analysis of carbon signalling during low carbon availability: coordinating the supply and utilisation of carbon in a fluctuating environment

Plants alternate between a net surplus of carbon in the light and a net deficit at night. This is buffered by accumulating starch in the light and degrading it at night. Enough starch is accumulated to support degradation throughout the night, with a small amount remaining at the end of the 24-h diurnal cycle. This review discusses how this balance between the supply and utilisation of carbon is achieved in Arabidopsis. It is important to regulate starch turnover to avoid an acute carbon deficiency. A 2-4 h extension of the night leads to exhaustion of starch, a collapse of sugars, a switch from biosynthesis to catabolism and an acute inhibition of growth by low carbon, which is not immediately reversed when carbon becomes available again. In starchless pgm mutants, where sugars are depleted each night, this leads to a recurring inhibition of growth that is not reversed until 5-6 h into the following light period. Several lines of evidence show that starch accumulation is regulated in response to events that are initiated during periods of low carbon. Starch accumulation is decreased when small amounts of sucrose are included in the growth medium. Sets of sugar-responsive genes were identified by supplying sugars to carbon-starved seedlings, or by illuminating 5-week-old plants in the presence of 350 or 50 ppm [CO₂]. Almost all of these genes show large diurnal changes in starchless pgm mutants, which are driven by the depletion of carbon during the night. Many show significant diurnal changes in wild type plants, showing that 'anticipatory' changes in signalling pathways occur before acute carbon limitation develops. However, these diurnal changes of transcripts do not lead to immediate changes of enzyme activities. Whereas an extension of the night leads to major changes of transcripts within 4-6 h, changes in enzyme activities require several days. In pgm, enzyme activities and the levels of >150 metabolites resemble those found in wild type plants after several days in the dark. It is concluded that diurnal changes in transcript levels are integrated, over days, as changes in the levels of enzymes. We hypothesise that this facilitates an adjustment of metabolism to a mid-term shift in the conditions, while ignoring noise due to diurnal changes and day-to-day fluctuations. The rapid adjustment of starch synthesis after a period of acute carbon depletion is a consequence of the transient inhibition of growth. This leads to accumulation of sugars when carbon becomes available again, which triggers a large increase in trehalose-6-phosphate. This signal metabolite promotes thioredoxin-dependent post-translational activation of ADP glucose pyrophosphorylase. Mid-term acclimation to a decreased carbon supply may be mediated by a combination of post-translational regulation, longer-term changes in enzyme activities, and a decrease in the rate of growth.

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.

Insights on the evolution of trehalose biosynthesis

Background

The compatible solute trehalose is a non-reducing disaccharide, which accumulates upon heat, cold or osmotic stress. It was commonly accepted that trehalose is only present in extremophiles or cryptobiotic organisms. However, in recent years it has been shown that although higher plants do not accumulate trehalose at significant levels they have actively transcribed genes encoding the corresponding biosynthetic enzymes.

Results

In this study we show that trehalose biosynthesis ability is present in eubacteria, archaea, plants, fungi and animals. In bacteria there are five different biosynthetic routes, whereas in fungi, plants and animals there is only one. We present phylogenetic analyses of the trehalose-6-phosphate synthase (TPS) and trehalose-phosphatase (TPP) domains and show that there is a close evolutionary relationship between these domains in proteins from diverse organisms. In bacteria TPS and TPP genes are clustered, whereas in eukaryotes these domains are fused in a single protein.

Conclusion

We have demonstrated that trehalose biosynthesis pathways are widely distributed in nature. Interestingly, several eubacterial species have multiple pathways, while eukaryotes have only the TPS/TPP pathway. Vertebrates lack trehalose biosynthetic capacity but can catabolise it. TPS and TPP domains have evolved mainly in parallel and it is likely that they have experienced several instances of gene duplication and lateral gene transfer.

Phosphorylation and 14-3-3 binding of Arabidopsis trehalose-phosphate synthase 5 in response to 2-deoxyglucose

Trehalose-6-phosphate is a 'sugar signal' that regulates plant metabolism and development. The Arabidopsis genome encodes trehalose-6-phosphate synthase (TPS) and trehalose-6-phosphatase (TPP) enzymes. It also encodes class II proteins (TPS isoforms 5-11) that contain both TPS-like and TPP-like domains, although whether these have enzymatic activity is unknown. In this paper, we show that TPS5, 6 and 7 are phosphoproteins that bind to 14-3-3 proteins, by using 14-3-3 affinity chromatography, 14-3-3 overlay assays, and by co-immunoprecipitating TPS5 and 14-3-3 isoforms from cell extracts. GST-TPS5 bound to 14-3-3s after in vitro phosphorylation at Ser22 and Thr49 by either mammalian AMP-activated protein kinase (AMPK) or partially purified plant Snf1-related protein kinase 1 (SnRK1s). Dephosphorylation of TPS5, or mutation of either Ser22 or Thr49, abolished binding to 14-3-3s. Ser22 and Thr49 are both conserved in TPS5, 7, 9 and 10. When GST-TPS5 was expressed in human HEK293 cells, Thr49 was phosphorylated in response to 2-deoxyglucose or phenformin, stimuli that activate the AMPK via the upstream kinase LKB1. 2-deoxyglucose stimulated Thr49 phosphorylation of endogenous TPS5 in Arabidopsis cells, whereas phenformin did not. Moreover, extractable SnRK1 activity was increased in Arabidopsis cells in response to 2-deoxyglucose. The plant kinase was inactivated by dephosphorylation and reactivated by phosphorylation with human LKB1, indicating that elements of the SnRK1/AMPK pathway are conserved in Arabidopsis and human cells. We hypothesize that coordinated phosphorylation and 14-3-3 binding of nitrate reductase (NR), 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (F2KP) and class II TPS isoforms mediate responses to signals that activate SnRK1.

Trehalose-6-phosphate synthase as an intrinsic selection marker for plant transformation

Insertion of foreign DNA into plant genomes occurs randomly and with low frequency. Hence, a selectable marker is generally required to identify transgenic plants. Until now, all selection systems have been based on the use of non-plant genes, derived from microorganisms and usually conferring antibiotic or herbicide resistance. The use of microorganism-derived genes however has raised biosafety concerns. We have developed a novel selection system based on enhancing the expression of a plant-intrinsic gene and the use of a harmless selection agent. Selection takes advantage of the reduced glucose sensitivity of seedlings with enhanced expression of AtTPS1, a gene encoding trehalose-6-P synthase. As a result, transformants can be identified as developing green seedlings amongst the background of small, pale non-transformed plantlets on high glucose medium. In addition, vegetative regeneration of tobacco leaf explants is very sensitive to high external glucose. Overexpression of AtTPS1 in tobacco allows selecting glucose insensitive transgenic shoots.

Isolation and characterization of drought-related trehalose 6-phosphate-synthase gene from cultivated cotton (Gossypium hirsutum L.)

Due to the important role of cotton drought-tolerant varieties and the reported involvement in this trait of trehalose-6-phosphate-synthase, the respective gene (TPS) was isolated and characterized from cultivated cotton, Gossypium hirsutum (ZETA 2 cultivar), using a chromosome-walking technique. TPS has three exons comprising the coding region. Southern blot analysis indicated that the Gossypium genomes (A and D) contain a single copy of TPS per genome. In addition, the expression of this gene was studied in different plant tissues. Plants of the Australian cotton variety Siokra L23, known for its drought tolerance, were subjected to drought stress (using PEG 6,000 solution, for 4 h during the dark period of the day and for four consecutive days); leaves, stems and roots were collected after the end of the stress period. Total extracted RNA was examined for the presence of transcripts, in the above-mentioned tissues of stressed and well-watered plants, by reverse transcription-polymerase chain reaction (RT-PCR). The expression levels, determined semi-quantitatively, indicated that the gene was expressed in all plant tissues under both water availability conditions. However, increased expression levels of TPS were observed mainly in stressed leaves and roots compared to those of the well-watered control. This finding is in agreement with the fact that TPS participates in trehalose biosynthesis, known for its participation in stress signal transduction in higher plants.

Trehalose metabolism and glucose sensing in plants

Plants sense and respond to changes in carbon and nitrogen metabolites during development and growth according to the internal needs of their metabolism. Sugar-sensing allows plants to switch off photosynthesis when carbohydrates are abundant. These processes involve regulation of gene and protein activity to allow plants the efficient use of energy storage. Besides being a key element in carbon metabolism, glucose (Glc) has unravelled as a primary messenger in signal transduction. It has been proved that hexokinase (HXK) is a Glc sensor. An unusual disaccharide named trehalose is present in very low levels in most plants except for the desiccation-tolerant plants known as 'resurrection' plants where trehalose functions as an osmoprotectant. We have shown that overexpression of the Arabidopsis trehalose-6-phosphate synthase gene (AtTPS1) in Arabidopsis promotes trehalose and trehalose-6-phosphate (T6P) accumulation. Seedlings expressing AtTPS1 displayed a Glc-insensitive phenotype. Transgenic lines germinated normally on Glc, in contrast to wild-type seedlings showing growth retardation and absence of chlorophyll and root elongation. Gene-expression analysis in transgenic plants showed up-regulation of several genes involved in sugar signalling and metabolism. These data suggest that AtTPS1 and accordingly T6P and trehalose play an important role in the regulation of Glc sensing and signalling genes during plant development.

Dehydration-induced tps gene transcripts from an anhydrobiotic nematode contain novel spliced leaders and encode atypical GT-20 family proteins

Accumulation of the non-reducing disaccharide trehalose is associated with desiccation tolerance during anhydrobiosis in a number of invertebrates, but there is little information on trehalose biosynthetic genes in these organisms. We have identified two trehalose-6-phosphate synthase (tps) genes in the anhydrobiotic nematode Aphelenchus avenae and determined full length cDNA sequences for both; for comparison, full length tps cDNAs from the model nematode, Caenorhabditis elegans, have also been obtained. The A. avenae genes encode very similar proteins containing the catalytic domain characteristic of the GT-20 family of glycosyltransferases and are most similar to tps-2 of C. elegans; no evidence was found for a gene in A. avenae corresponding to Ce-tps-1. Analysis of A. avenae tps cDNAs revealed several features of interest, including alternative trans-splicing of spliced leader sequences in Aav-tps-1, and four different, novel SL1-related trans-spliced leaders, which were different to the canonical SL1 sequence found in all other nematodes studied. The latter observation suggests that A. avenae does not comply with the strict evolutionary conservation of SL1 sequences observed in other species. Unusual features were also noted in predicted nematode TPS proteins, which distinguish them from homologues in other higher eukaryotes (plants and insects) and in micro-organisms. Phylogenetic analysis confirmed their membership of the GT-20 glycosyltransferase family, but indicated an accelerated rate of molecular evolution. Furthermore, nematode TPS proteins possess N- and C-terminal domains, which are unrelated to those of other eukaryotes: nematode C-terminal domains, for example, do not contain trehalose-6-phosphate phosphatase-like sequences, as seen in plant and insect homologues. During onset of anhydrobiosis, both tps genes in A. avenae are upregulated, but exposure to cold or increased osmolarity also results in gene induction, although to a lesser extent. Trehalose seems likely therefore to play a role in a number of stress responses in nematodes.

The Arabidopsis trehalose-6-P synthase AtTPS1 gene is a regulator of glucose, abscisic acid, and stress signaling

In Arabidopsis (Arabidopsis thaliana), trehalose is present at almost undetectable levels, excluding its role as an osmoprotectant. Here, we report that overexpression of AtTPS1 in Arabidopsis using the 35S promoter led to a small increase in trehalose and trehalose-6-P levels. In spite of this, transgenic plants displayed a dehydration tolerance phenotype without any visible morphological alterations, except for delayed flowering. Moreover, seedlings overexpressing AtTPS1 exhibited glucose (Glc)- and abscisic acid (ABA)-insensitive phenotypes. Transgenic seedlings germinated on Glc were visibly larger with green well-expanded cotyledonary leaves and fully developed roots, in contrast with wild-type seedlings showing growth retardation and absence of photosynthetic tissue. An ABA dose-response experiment revealed a higher germination rate for transgenic plants overexpressing AtTPS1 showing insensitive germination kinetics at 2.5 mum ABA. Interestingly, germination in the presence of Glc did not trigger an increase in ABA content in plants overexpressing AtTPS1. Expression analysis by quantitative reverse transcription-PCR in transgenic plants showed up-regulation of the ABI4 and CAB1 genes. In the presence of Glc, CAB1 expression remained high, whereas ABI4, HXK1, and ApL3 levels were down-regulated in the AtTPS1-overexpressing lines. Analysis of AtTPS1 expression in HXK1-antisense or HXK1-sense transgenic lines suggests the possible involvement of AtTPS1 in the hexokinase-dependent Glc-signaling pathway. These data strongly suggest that AtTPS1 has a pivotal role in the regulation of Glc and ABA signaling during vegetative development.

Uncoupling of the glucose growth defect and the deregulation of glycolysis in Saccharomyces cerevisiae Tps1 mutants expressing trehalose-6-phosphate-insensitive hexokinase from Schizosaccharomyces pombe

In the yeast Saccharomyces cerevisiae inactivation of trehalose-6-phosphate (Tre6P) synthase (Tps1) encoded by the TPS1 gene causes a specific growth defect in the presence of glucose in the medium. The growth inhibition is associated with deregulation of the initial part of glycolysis. Sugar phosphates, especially fructose-1,6-bisphosphate (Fru1,6bisP), hyperaccumulate while the levels of ATP, Pi and downstream metabolites are rapidly depleted. This was suggested to be due to the absence of Tre6P inhibition on hexokinase. Here we show that overexpression of Tre6P (as well as glucose-6-phosphate (Glu6P))-insensitive hexokinase from Schizosaccharomyces pombe in a wild-type strain does not affect growth on glucose but still transiently enhances initial sugar phosphate accumulation. We have in addition replaced the three endogenous glucose kinases of S. cerevisiae by the Tre6P-insensitive hexokinase from S. pombe. High hexokinase activity was measured in cell extracts and growth on glucose was somewhat reduced compared to an S. cerevisiae wild-type strain but expression of the Tre6P-insensitive S. pombe hexokinase never caused the typical tps1Delta phenotype. Moreover, deletion of TPS1 in this strain expressing only the Tre6P-insensitive S. pombe hexokinase still resulted in a severe drop in growth capacity on glucose as well as sensitivity to millimolar glucose levels in the presence of excess galactose. In this case, poor growth on glucose was associated with reduced rather than enhanced glucose influx into glycolysis. Initial glucose transport was not affected. Apparently, deletion of TPS1 causes reduced activity of the S. pombe hexokinase in vivo. Our results show that Tre6P inhibition of hexokinase is not the major mechanism by which Tps1 controls the influx of glucose into glycolysis or the capacity to grow on glucose. In addition, they show that a Tre6P-insensitive hexokinase can still be controlled by Tps1 in vivo.

Trehalose metabolism: a regulatory role for trehalose-6-phosphate?

Trehalose is a disaccharide that was initially thought to be rare in plants but now appears to be ubiquitous. A recent study has established that the initial step in trehalose synthesis is essential in Arabidopsis. Evidence is emerging that the precursor of trehalose (trehalose-6-phosphate) is an important regulatory molecule. In yeast, trehalose-6-phosphate regulates sugar influx into glycolysis. In plants, trehalose-6-phosphate also appears to regulate sugar metabolism, but the underlying mechanism is unresolved and may be substantially different from that in yeast.