Metabolism control over growth: a case for trehalose-6-phosphate in plants

Discovery of genes that positively affect biomass and stress associated traits in poplar

Woody biomass serves as a renewable resource for various industries, including pulp and paper production, construction, biofuels, and electricity generation. However, the molecular mechanisms behind biomass traits are poorly understood, which significantly curtails the speed and efficiency of their improvement. We used activation tagging to discover genes that can positively affect tree biomass-associated traits. We generated and screened under greenhouse conditions a population of 2,700 independent activation tagging lines. A total of 761 lines, which had significantly and positively affected at least one biomass-associated trait, were discovered. The tag was positioned in the genome for forty lines which were affected in multiple traits and activation of proximal genes validated for a subset. For two lines we fully recapitulated the phenotype of the original lines through overexpression. Moreover, the overexpression led to more pronounced and additional improvements, not observed in the original lines. Importantly, the overexpression of a Fasciclin-like gene (PtaFLA10) and a Patatin-like gene (PtaPAT) was found to substantially improve biomass, with a 40% increase in dry-stem weight, and enhance drought tolerance, respectively. Additionally, PtaPAT overexpression increased cellulose content, which is crucial for biofuel production. Our work shows that the activation tagging approach applied even on a non-genome saturation scale in a poplar tree can be successfully used for the discovery of genes positively modify biomass productivity. Such dominant forward genetics approaches can aid in biotechnological manipulation of woody biomass traits and help unravel the functions and mechanisms of individual genes, gene families, and regulatory modules.

Physiological and molecular responses in phosphorus-hyperaccumulating Polygonum species to high phosphorus exposure

Phosphorus (P)-hyperaccumulators for phytoextraction from P-polluted areas generally show rapid growth and accumulate large amounts of P without any toxicity symptom, which depends on a range of physiological processes and gene expression patterns that have never been explored. We investigated growth, leaf element concentrations, P fractions, photosynthetic traits, and leaf metabolome and transcriptome response in amphibious P-hyperaccumulators, Polygonum hydropiper and P. lapathifolium, to high-P exposure (5 mmol L-1), with 0.05 mmol L-1 as the control. Under high-P exposure, both species demonstrated good growth, allocating more P to metabolite P and inorganic P (Pi) accompanied by high potassium and calcium. The expression of a cluster of unigenes associated with photosynthesis was maintained or increased in P. lapathifolium, explaining the increase in net photosynthetic rate and the rapid growth under high-P exposure. Metabolites of trehalose metabolism, including trehalose 6-phosphate and trehalose, were sharply increased in both species by the high-P exposure, in line with the enhanced expression of associated unigenes, indicating that trehalose metabolic pathway was closely related to high-P tolerance. These findings elucidated the physiological and molecular responses involved in the photosynthesis and trehalose metabolism in P-hyperaccumulators to high-P exposure, and provides potential regulatory pathways to improve the P-phytoextraction capability.

Genome-Wide Identification of Trehalose-6-phosphate Synthase (TPS) Gene Family Reveals the Potential Role in Carbohydrate Metabolism in Peach

Trehalose-6-phosphate synthase (TPS) is essential for plant growth and development, linking trehalose-6-phosphate (T6P) to carbon metabolism. However, little is known about the TPS gene family in peaches and their potential roles in regulating carbohydrates in peach fruit. In this study, nine TPS genes were identified in the peach genome and named according to the homologous genes in Arabidopsis. Phylogenetic analysis showed that three subfamilies were identified, including TPSI, TPSII-1, and TPSII-2, which were also consistent with gene structure analysis. Considerable cis-elements were enriched in the promoters, including plant hormone-related elements. Tissue-specific analysis showed that these TPS genes were mainly expressed in leaves, stems, and fruit, showing different expression patterns for each gene. In addition, during fruit development, the content of trehalose-6-phosphate (T6P) was positively correlated with the expression of PpTPS7a and negatively with sucrose non-fermenting-1-related kinase 1 (SnRK1) activity. Transient overexpression and silencing of PpTPS7a in peach fruit validated its function in regulating T6P content and SnRK1 activity.

Three amino acid residues are required for the recognition of Ralstonia solanacearum RipTPS in Nicotiana tabacum

Ralstonia solanacearum causes devastating diseases in a wide range of economically important crops. It secretes a large number of virulence factors, also known as effectors, to promote its infection, and some of them are recognized when the host plant contains corresponding resistance genes. In this study we showed that a type III effector RipTPS from the avirulent R. solanacearum strain GMI1000 (RipTPSG) specifically induced cell death in Nicotiana tabacum, but not in Nicotiana benthamiana, whereas the RipTPS homolog in the virulent strain CQPS-1 (RipTPSC) induced cell death in neither N. tabacum nor N. benthamiana. These results indicated that RipTPSG is recognized in N. tabacum. Expression of RipTPSG induced upregulation of hypersensitive response (HR) -related genes in N. tabacum. The virulence of CQPS-1 was reduced when RipTPSG was genetically introduced into CQPS-1, further confirming that RipTPSG functions as an avirulence determinant. Protein sequence alignment indicated that there are only three amino acid polymorphisms between RipTPSG and RipTPSC. Site-directed mutagenesis analyses confirmed that the three amino acid residues are jointly required for the recognition of RipTPSG in N. tabacum. Expression of either RipTPSG or RipTPSC suppressed flg22-triggered reactive oxygen species (ROS) burst in N. benthamiana, suggesting that RipTPS contributes to pathogen virulence. Mutating the conserved residues in RipTPS's trehalose-phosphate synthase (TPS) domain did not block its HR induction and defense suppression activity, indicating that the TPS activity is not required for RipTPS's avirulence and virulence function.

Impaired KIN10 function restores developmental defects in the Arabidopsis trehalose 6-phosphate synthase1 (tps1) mutant

Sensing carbohydrate availability is essential for plants to coordinate their growth and development. In Arabidopsis thaliana, TREHALOSE 6-PHOSPHATE SYNTHASE 1 (TPS1) and its product, trehalose 6-phosphate (T6P), are important for the metabolic control of development. tps1 mutants are embryo-lethal and unable to flower when embryogenesis is rescued. T6P regulates development in part through inhibition of SUCROSE NON-FERMENTING1 RELATED KINASE1 (SnRK1). Here, we explored the role of SnRK1 in T6P-mediated plant growth and development using a combination of a mutant suppressor screen and genetic, cellular and transcriptomic approaches. We report nonsynonymous amino acid substitutions in the catalytic KIN10 and regulatory SNF4 subunits of SnRK1 that can restore both embryogenesis and flowering of tps1 mutant plants. The identified SNF4 point mutations disrupt the interaction with the catalytic subunit KIN10. Contrary to the common view that the two A. thaliana SnRK1 catalytic subunits act redundantly, we found that loss-of-function mutations in KIN11 are unable to restore embryogenesis and flowering, highlighting the important role of KIN10 in T6P signalling.

Mathematical Modeling Reveals That Sucrose Regulates Leaf Senescence via Dynamic Sugar Signaling Pathways

Sucrose (Suc) accumulation is one of the key indicators of leaf senescence onset, but little is known about its regulatory role. Here, we found that application of high (120-150 mM) and low levels (60 mM) of Suc to young leaf (YL) and fully expanded leaf (FEL) discs, respectively, decreased chlorophyll content and maximum photosynthetic efficiency. Electrolyte leakage and malondialdehyde levels increased at high Suc concentrations (90-120 mM in YL and 60 and 150 mM in FEL discs). In FEL discs, the senescence-associated gene NtSAG12 showed a gradual increase in expression with increased Suc application; in contrast, in YL discs, NtSAG12 was upregulated with low Suc treatment (60 mM) but downregulated at higher levels of Suc. In YL discs, trehalose-6-phosphate (T6P) accumulated at a low half-maximal effective concentration (EC50) of Suc (1.765 mM). However, T6P levels declined as trehalose 6 phosphate synthase (TPS) content decreased, resulting in the maximum velocity of sucrose non-fermenting-1-related protein kinase (SnRK) and hexokinase (HXK) occurring at higher level of Suc. We therefore speculated that senescence was induced by hexose accumulation. In FEL discs, the EC50 of T6P occurred at a low concentration of Suc (0.9488 mM); T6P levels progressively increased with higher TPS content, which inhibited SnRK activity with a dissociation constant (Kd) of 0.001475 U/g. This confirmed that the T6P-SnRK complex induced senescence in detached FEL discs.

Trehalose increases tomato drought tolerance, induces defenses, and increases resistance to bacterial wilt disease

Ralstonia solanacearum causes bacterial wilt disease, leading to severe crop losses. Xylem sap from R. solanacearum-infected tomato is enriched in the disaccharide trehalose. Water-stressed plants also accumulate trehalose, which increases drought tolerance via abscisic acid (ABA) signaling. Because R. solanacearum-infected plants suffer reduced water flow, we hypothesized that bacterial wilt physiologically mimics drought stress, which trehalose could mitigate. We found that R. solanacearum-infected plants differentially expressed drought-associated genes, including those involved in ABA and trehalose metabolism, and had more ABA in xylem sap. Consistent with this, treating tomato roots with ABA reduced both stomatal conductance and stem colonization by R. solanacearum. Treating roots with trehalose increased xylem sap ABA and reduced plant water use by lowering stomatal conductance and temporarily improving water use efficiency. Trehalose treatment also upregulated expression of salicylic acid (SA)-dependent tomato defense genes; increased xylem sap levels of SA and other antimicrobial compounds; and increased bacterial wilt resistance of SA-insensitive NahG tomato plants. Additionally, trehalose treatment increased xylem concentrations of jasmonic acid and related oxylipins. Finally, trehalose-treated plants were substantially more resistant to bacterial wilt disease. Together, these data show that exogenous trehalose reduced both water stress and bacterial wilt disease and triggered systemic disease resistance, possibly through a Damage Associated Molecular Pattern (DAMP) response pathway. This suite of responses revealed unexpected linkages between plant responses to biotic and abiotic stress and suggested that R. solanacearum-infected plants increase trehalose to improve water use efficiency and increase wilt disease resistance. The pathogen may degrade trehalose to counter these efforts. Together, these results suggest that treating tomatoes with exogenous trehalose could be a practical strategy for bacterial wilt management.

Sugars and the speed of life-Metabolic signals that determine plant growth, development and death

Plant growth and development depend on the availability of carbohydrates synthesised in photosynthesis (source activity) and utilisation of these carbohydrates for growth (sink activity). External conditions, such as temperature, nutrient availability and stress, can affect source as well as sink activity. Optimal utilisation of resources is under circadian clock control. This molecular timekeeper ensures that growth responses are adjusted to different photoperiod and temperature settings by modulating starch accumulation and degradation accordingly. For example, during the night, starch degradation is required to provide sugars for growth. Under favourable growth conditions, high sugar availability stimulates growth and development, resulting in an overall accelerated life cycle of annual plants. Key signalling components include trehalose-6-phosphate (Tre6P), which reflects sucrose availability and stimulates growth and branching when the conditions are favourable. Under sink limitation, Tre6P does, however, inhibit night-time starch degradation. Tre6P interacts with Sucrose-non-fermenting1-Related Kinase1 (SnRK1), a protein kinase that inhibits growth under starvation and stress conditions and delays development (including flowering and senescence). Tre6P inhibits SnRK1 activity, but SnRK1 increases the Tre6P to sucrose ratio under favourable conditions. Alongside Tre6P, Target of Rapamycin (TOR) stimulates processes such as protein synthesis and growth when sugar availability is high. In annual plants, an accelerated life cycle results in early leaf and plant senescence, thus shortening the lifespan. While the availability of carbohydrates in the form of sucrose and other sugars also plays an important role in seasonal life cycle events (phenology) of perennial plants, the sugar signalling pathways in perennials are less well understood.

Low-Altitude Boundary of Abies faxoniana Is More Susceptible to Long-Term Open-Top Chamber Warming in the Eastern Tibetan Plateau

With global climate change, for evaluating warming effect on subalpine forest distribution, the substantial effects of long-term warming on tree growth and soil nutrients need to be explored. In this study, we focused on different responses in the boundaries of trees and soils to warming. Using the open-top chamber (OTC), a 10-year artificial warming experiment was conducted to evaluate the impacts of warming on Abies faxoniana at three different altitudes. We determined metabolites and nutrient concentrations in needles of A. faxoniana and characterized the soil chemistries. Many kinds of sugars, amino acids, and organic acids showed higher contents at high altitude (3,500 m) compared with low altitude (2,600 m), which could have been due to the temperature differences. Warming significantly decreased needle sugar and amino acid concentrations at high altitude but increased them at low altitude. These results indicated contrasting physiological and metabolic responses of A. faxoniana to long-term warming at different altitudes. Furthermore, we found that OTC warming significantly increased the concentrations of soil extractable sodium, aluminum (Al), and manganese (Mn), while decreased potassium (K) and phosphorus (P) concentrations and pH values at low altitude rather than at middle (3,000 m) or high altitude. The soil carbon and nitrogen contents were increased only at the middle altitude. In A. faxoniana at low altitudes, more mineral nutrients iron, K, and P were demand, and a mass of Al, Mn, and zinc was accumulated under warming. Soil P limitation and heavy metals accumulation are disadvantageous for trees at low altitudes with warming. Therefore, compared with high altitudes, A. faxoniana growing at low boundary in alpine regions is expected to be more susceptible to warming.

Genetic manipulation of trehalose-6-phosphate synthase results in changes in the soluble sugar profile in transgenic sugarcane stems

Trehalose is a non-reducing disaccharide widely distributed in nature. The trehalose biosynthetic intermediate, trehalose 6-phosphate (Tre6P) is an essential regulatory and signaling molecule involved in both regulation of carbon metabolism and photosynthesis. To investigate the effect of altered trehalose synthesis on sucrose accumulation in sugarcane (Saccharum spp. hybrid), we independently overexpressed the Escherichia coli otsA (trehalose-6-phosphate synthase; TPS) and otsB (trehalose-6-phosphate phosphatase; TPP) genes and additionally partially silenced native TPS expression. In mature cane, sucrose levels in the otsA transgenic plants were lowered, whereas sucrose levels in the otsB transgenic plants were increased. Partial silencing of TPS expression in sugarcane transformed with a TPS-targeted microRNA recombinant construct was confirmed in leaf and mature internode tissue of transgenic plants. Most of the silencing transgenic lines accumulated trehalose at lower levels than the wild-type (WT) plants. The immature stalk tissue of these transgenic lines had lower levels of glucose and fructose, whereas the mature internode tissue had lower sucrose and glucose levels, when compared with the WT. Furthermore, various minor metabolites and sugars were detected in the sugarcane plants, which mostly decreased as the stalk tissue of the cane matured. The results demonstrate that manipulation of Tre6P/trehalose metabolism has the potential to modify the profile of soluble sugars accumulated in sugarcane stems.

Phosphoproteomic analysis of lettuce (Lactuca sativa L.) reveals starch and sucrose metabolism functions during bolting induced by high temperature

High temperatures induce early bolting in lettuce (Lactuca sativa L.), which decreases both quality and production. However, knowledge of the molecular mechanism underlying high temperature promotes premature bolting is lacking. In this study, we compared lettuce during the bolting period induced by high temperatures (33/25 °C, day/night) to which raised under controlled temperatures (20/13 °C, day/night) using iTRAQ-based phosphoproteomic analysis. A total of 3,814 phosphorylation sites located on 1,766 phosphopeptides from 987 phosphoproteins were identified after high-temperature treatment,among which 217 phosphoproteins significantly changed their expression abundance (116 upregulated and 101 downregulated). Most phosphoproteins for which the abundance was altered were associated with the metabolic process, with the main molecular functions were catalytic activity and transporter activity. Regarding the functional pathway, starch and sucrose metabolism was the mainly enriched signaling pathways. Hence, high temperature influenced phosphoprotein activity, especially that associated with starch and sucrose metabolism. We suspected that the lettuce shorten its growth cycle and reduce vegetative growth owing to changes in the contents of starch and soluble sugar after high temperature stress, which then led to early bolting/flowering. These findings improve our understanding of the regulatory molecular mechanisms involved in lettuce bolting.

Delineation of the Crucial Evolutionary Amino Acid Sites in Trehalose-6-Phosphate Synthase From Higher Plants

Trehalose-6-phosphate synthase (TPS) is a key enzyme in the biosynthesis of trehalose, with its direct product, trehalose-6-phosphate, playing important roles in regulating whole-plant carbohydrate allocation and utilization. Genes encoding TPS constitute a multigene family in which functional divergence appears to have occurred repeatedly. To identify the crucial evolutionary amino acid sites of TPS in higher plants, a series of bioinformatics tools were applied to investigate the phylogenetic relationships, functional divergence, positive selection, and co-evolution of TPS proteins. First, we identified 150 TPS genes from 13 higher plant species. Phylogenetic analysis placed these TPS proteins into 2 clades: clades A and B, of which clade B could be further divided into 4 subclades (B1-B4). This classification was supported by the intron-exon structures, with more introns present in clade A. Next, detection of the critical functionally divergent amino acid sites resulted in the isolation of a total of 286 sites reflecting nonredundant radical shifts in amino acid properties with a high posterior probability cutoff among subclades. In addition, positively selected sites were identified using a codon substitution model, from which 46 amino acid sites were isolated as exhibiting positive selection at a significant level. Moreover, 18 amino acid sites were highlighted both for functional divergence and positive selection; these may thus potentially represent crucial evolutionary sites in the TPS family. Further co-evolutionary analysis revealed 3 pairs of sites: 11S and 12H, 33S and 34N, and 109G and 110E as demonstrating co-evolution. Finally, the 18 crucial evolutionary amino acid sites were mapped in the 3-dimensional structure. A total of 77 sites harboring functionally and structurally important residues of TPS proteins were found by using the CLIPS-4D online tool; notably, no overlap was observed with the identified crucial evolutionary sites, providing positive evidence supporting their designation. A total of 18 sites were isolated as key amino acids by using multiple bioinformatics tools based on their concomitant functional divergence and positive selection. Almost all these key sites are located in 2 domains of this protein family where they exhibit no overlap with the structurally and functionally conserved sites. These results will provide an improved understanding of the complexity of the TPS gene family and of its function and evolution in higher plants. Moreover, this knowledge may facilitate the exploitation of these sites for protein engineering applications.

Development and characterization of a Nannochloropsis mutant with simultaneously enhanced growth and lipid production

BACKGROUND

The necessity to develop high lipid-producing microalgae is emphasized for the commercialization of microalgal biomass, which is environmentally friendly and sustainable. Nannochloropsis are one of the best industrial microalgae and have been widely studied for their lipids, including high-value polyunsaturated fatty acids (PUFAs). Many reports on the genetic and biological engineering of Nannochloropsis to improve their growth and lipid contents have been published.

RESULTS

We performed insertional mutagenesis in Nannochloropsis salina, and screened mutants with high lipid contents using fluorescence-activated cell sorting (FACS). We isolated a mutant, Mut68, which showed improved growth and a concomitant increase in lipid contents. Mut68 exhibited 53% faster growth rate and 34% higher fatty acid methyl ester (FAME) contents after incubation for 8 days, resulting in a 75% increase in FAME productivity compared to that in the wild type (WT). By sequencing the whole genome, we identified the disrupted gene in Mut68 that encoded trehalose-6-phosphate (T6P) synthase (TPS). TPS is composed of two domains: TPS domain and T6P phosphatase (TPP) domain, which catalyze the initial formation of T6P and dephosphorylation to trehalose, respectively. Mut68 was disrupted at the TPP domain in the C-terminal half, which was confirmed by metabolic analyses revealing a great reduction in the trehalose content in Mut68. Consistent with the unaffected N-terminal TPS domain, Mut68 showed moderate increase in T6P that is known for regulation of sugar metabolism, growth, and lipid biosynthesis. Interestingly, the metabolic analyses also revealed a significant increase in stress-related amino acids, including proline and glutamine, which may further contribute to the Mut68 phenotypes.

CONCLUSION

We have successfully isolated an insertional mutant showing improved growth and lipid production. Moreover, we identified the disrupted gene encoding TPS. Consistent with the disrupted TPP domain, metabolic analyses revealed a moderate increase in T6P and greatly reduced trehalose. Herein, we provide an excellent proof of concept that the selection of insertional mutations via FACS can be employed for the isolation of mutants with improved growth and lipid production. In addition, trehalose and genes encoding TPS will provide novel targets for chemical and genetic engineering, in other microalgae and organisms as well as Nannochloropsis.

Enhancing trehalose biosynthesis improves yield potential in marker-free transgenic rice under drought, saline, and sodic conditions

Edaphic factors such as salinity, sodicity, and drought adversely affect crop productivity, either alone or in combination. Despite soil sodicity being reported as an increasing problem worldwide, limited efforts have been made to address this issue. In the present study, we aimed to generate rice with tolerance to sodicity in conjunction with tolerance to salinity and drought. Using a fusion gene from E. coli coding for trehalose-6-phosphate synthase/phosphatase (TPSP) under the control of an ABA-inducible promoter, we generated marker-free, high-yielding transgenic rice (in the IR64 background) that can tolerate high pH (~9.9), high EC (~10.0 dS m-1), and severe drought (30-35% soil moisture content). The transgenic plants retained higher relative water content (RWC), chlorophyll content, K+/Na+ ratio, stomatal conductance, and photosynthetic efficiency compared to the wild-type under these stresses. Positive correlations between trehalose overproduction and high-yield parameters were observed under drought, saline, and sodic conditions. Metabolic profiling using GC-MS indicated that overproduction of trehalose in leaves differently modulated other metabolic switches, leading to significant changes in the levels of sugars, amino acids, and organic acids in transgenic plants under control and stress conditions. Our findings reveal a novel potential technological solution to tackle multiple stresses under changing climatic conditions.

Comparative Transcriptome Analysis of Gene Expression Patterns in Tomato Under Dynamic Light Conditions

Plants grown under highly variable natural light regimes differ strongly from plants grown under constant light (CL) regimes. Plant phenotype and adaptation responses are important for plant biomass and fitness. However, the underlying regulatory mechanisms are still poorly understood, particularly from a transcriptional perspective. To investigate the influence of different light regimes on tomato plants, three dynamic light (DL) regimes were designed, using a CL regime as control. Morphological, photosynthetic, and transcriptional differences after five weeks of treatment were compared. Leaf area, plant height, shoot /root weight, total chlorophyll content, photosynthetic rate, and stomatal conductance all significantly decreased in response to DL regimes. The biggest expression difference was found between the treatment with the highest light intensity at the middle of the day with a total of 1080 significantly up-/down-regulated genes. A total of 177 common differentially expressed genes were identified between DL and CL conditions. Finally, significant differences were observed in the levels of gene expression between DL and CL treatments in multiple pathways, predominantly of plant–pathogen interactions, plant hormone signal transductions, metabolites, and photosynthesis. These results expand the understanding of plant development and photosynthetic regulations under DL conditions by multiple pathways.

Functional analysis of McSnRK1 (SNF1-related protein kinase 1) in regulating Na/K homeostasis in transgenic cultured cells and roots of halophyte Mesembryanthemum crystallinum

Transgenic callus and roots of ice plant with altered SnRK1 function were established using Agrobacterium-mediated transformation. The role of McSnRK1 in controlling Na⁺ influx and Na/K ratio was demonstrated. SnRK1 kinases (SNF1-related protein kinase1) control metabolic adaptation during energy deprivation and regulate protective mechanisms against environmental stress. Yeast SNF1 activates a P-type ATPase, the Na⁺ exclusion pump, under glucose starvation. The involvement of plant SnRK1 in salt stress response is largely unknown. We previously identified a salt-induced McSnRK1 in the halophyte ice plant (Mesembryanthemum crystallinum). In the current study, the function of McSnRK1 in salt tolerance was analyzed in transgenic cultured cells and roots of ice plant. Ice plant callus constitutively expressed a high level of McSnRK1 and introducing the full-length McSnRK1 did not alter the Na/K ratio at 24 h after 200 mM NaCl treatment. However, interfering with McSnRK1 activity by introducing a truncate McSnRK1 to produce a dominant-negative form of McSnRK1 increased cellular Na⁺ accumulation and Na/K ratio. As a result, the growth of cultured cells diminished under salt treatment. Hydroponically grown ice plants with roots expressing full-length McSnRK1 had better growth and lowered Na/K ratio compared to the wild-type or vector-only plants. Roots expressing a truncate McSnRK1 had reduced growth and high Na/K ratio under 400 mM NaCl treatment. The changes in Na/K ratio in transgenic cells and whole plants demonstrated the function of SnRK1 in controlling Na⁺ flux and maintaining Na/K homeostasis under salinity. The Agrobacterium-mediated transformation system could be a versatile tool for functional analysis of genes involved in salt tolerance in the ice plant.

Oligo-carrageenan kappa increases glucose, trehalose and TOR-P and subsequently stimulates the expression of genes involved in photosynthesis, and basal and secondary metabolisms in Eucalyptus globulus

BACKGROUND

It has been previously shown that oligo-carrageenan (OC) kappa increases growth, photosynthesis and activities of enzymes involved in basal and secondary metabolisms in Eucalyptus globulus. However, it is not known whether OC kappa may induce the activation of TOR pathway and the increase in expression of genes encoding proteins involved in photosynthesis and enzymes of basal and secondary metabolisms.

RESULTS

E. globulus trees were sprayed on leaves with water (control) or with OC kappa 1 mg mL- 1, once a week, four times in total, and cultivated for 17 additional weeks (21 weeks in total). Treated trees showed a higher level of net photosynthesis than controls, beginning at week 3, a higher height, beginning at week 9, and those differences remained until week 21. In addition, treated trees showed an increase in the level of glucose beginning at week 1, trehalose at weeks 1-3, and in TOR-P level at week 1-2. On the other hand, transcripts encoding proteins involved in photosynthesis, and enzymes involved in glucose accumulation, C, N and S assimilation, and synthesis of secondary metabolites began at weeks 3-4 and with additional peaks at weeks 5-6, 8-11,13-14 and 17-19. Thus, OC kappa induced initial increases in glucose, trehalose and TOR-P levels that were followed by oscillatory increases in the level of transcripts coding for proteins involved in photosynthesis, and in basal and secondary metabolisms suggesting that initial increases in glucose, trehalose and TOR-P may trigger activation of gene expression.

CONCLUSIONS

The stimulation of growth induced by OC kappa in E. globulus trees is due, at least in part, to activation of TOR pathway and the increase in expression of genes encoding proteins involved in photosynthesis and enzymes of basal metabolism.

Transcriptomic and metabolomic analysis of ZmYUC1 mutant reveals the role of auxin during early endosperm formation in maize

Kernel size in cereal is an important agronomic trait controlled by the interaction of genetic and environmental factors. The endosperm occupies most of the kernel area; for this reason, the endosperm cells dimension, number and metabolic content strongly influence kernel properties. This paper presents the transcriptomic and metabolomic analysis of the maize defective endosperm 18 (de18) mutant, where auxin accumulation in the endosperm is impaired. This mutation, involving the ZmYuc1 gene, leads to a reduced kernel size compared to the wild-type line B37. Our results mainly indicate that IAA concentration controls sugar and protein metabolism during kernel differentiation and it is necessary for BETL formation. Furthermore, a fine tuning of different auxin conjugates is reported as the main mechanism to counteract the auxin deficit. Some candidates as master regulators of endosperm transcriptional regulation mediated by auxin are found between MYB and MADS-box gene families. A link between auxin and storage protein accumulation is highlighted, suggesting that IAA directly or indirectly, through CK or ABA, regulates the transcription of zein coding genes. This study represents a move forward with respect to the current knowledge about the role of auxin during maize endosperm differentiation thus revealing the genes that are modulated by auxin and that control agronomic traits as kernel size and metabolic composition.

Trehalose phosphate synthase 5-dependent trehalose metabolism modulates basal defense responses in Arabidopsis thaliana

Despite the recent discovery that trehalose synthesis is important for plant development and abiotic stress tolerance, the effects of trehalose on biotic stress responses remain relatively unknown. In this study, we demonstrate that TREHALOSE PHOSPHATE SYNTHASE 5 (TPS5)-dependent trehalose metabolism regulates Arabidopsis thaliana defenses against pathogens (necrotrophic Botrytis cinerea and biotrophic Pseudomonas syringae). Pathogen infection increased trehalose levels and upregulated TPS5 expression. Application of exogenous trehalose significantly improved plant defenses against B. cinerea, but increased the susceptibility of plants to P. syringae. We demonstrate that elevated trehalose biosynthesis, in transgenic plants over-expressing TPS5, also increased the susceptibility to P. syringae, but decreased the disease symptoms caused by B. cinerea. The knockout of TPS5 prevented the accumulation of trehalose and enhanced defense responses against P. syringae. Additionally, we observed that a TPS5-interacting protein (multiprotein bridging factor 1c) was required for induced expression of TPS5 during pathogen infections. Furthermore, we show that trehalose promotes P. syringae growth and disease development, via a mechanism involving suppression of the plant defense gene, Pathogenesis-Related Protein 1. These findings provide insight into the function of TPS5-dependent trehalose metabolism in plant basal defense responses.

Trehalose-6-phosphate promotes fermentation and glucose repression in Saccharomyces cerevisiae

The yeast trehalose-6-phosphate synthase (Tps1) catalyzes the formation of trehalose-6-phosphate (T6P) in trehalose synthesis. Besides, Tps1 plays a key role in carbon and energy homeostasis in this microbial cell, as shown by the well documented loss of ATP and hyper accumulation of sugar phosphates in response to glucose addition in a mutant defective in this protein. The inability of a Saccharomyces cerevisiae tps1 mutant to cope with fermentable sugars is still a matter of debate. We reexamined this question through a quantitative analysis of the capability of TPS1 homologues from different origins to complement phenotypic defects of this mutant. Our results allowed to classify this complementation in three groups. A first group enclosed TPS1 of Klyveromyces lactis with that of S. cerevisiae as their expression in Sctps1 cells fully recovered wild type metabolic patterns and fermentation capacity in response to glucose. At the opposite was the group with TPS1 homologues from the bacteria Escherichia coli and Ralstonia solanacearum, the plant Arabidopsis thaliana and the insect Drosophila melanogaster whose metabolic profiles were comparable to those of a tps1 mutant, notably with almost no accumulation of T6P, strong impairment of ATP recovery and potent reduction of fermentation capacity, albeit these homologous genes were able to rescue growth of Sctps1 on glucose. In between was a group consisting of TPS1 homologues from other yeast species and filamentous fungi characterized by 5 to 10 times lower accumulation of T6P, a weaker recovery of ATP and a 3-times lower fermentation capacity than wild type. Finally, we found that glucose repression of gluconeogenic genes was strongly dependent on T6P. Altogether, our results suggest that the TPS protein is indispensable for growth on fermentable sugars, and points to a critical role of T6P as a sensing molecule that promotes sugar fermentation and glucose repression.

The Maize Corngrass1 miRNA-Regulated Developmental Alterations Are Restored by a Bacterial ADP-Glucose Pyrophosphorylase in Transgenic Tobacco

Crop-based bioethanol has raised concerns about competition with food and feed supplies, and technologies for second- and third-generation biofuels are still under development. Alternative feedstocks could fill this gap if they can be converted to biofuels using current sugar- or starch-to-ethanol technologies. The aim of this study was to enhance carbohydrate accumulation in transgenic Nicotiana benthamiana by simultaneously expressing the maize Corngrass1 miRNA (Cg1) and E. coli ADP-glucose pyrophosphorylase (glgC), both of which have been reported to enhance carbohydrate accumulation in planta. Our findings revealed that expression of Cg1 alone increased shoot branching, delayed flowering, reduced flower organ size, and induced loss of fertility. These changes were fully restored by coexpressing Escherichia coli glgC. The transcript level of miRNA156 target SQUAMOSA promoter binding-like (SPL) transcription factors was suppressed severely in Cg1-expressing lines as compared to the wild type. Expression of glgC alone or in combination with Cg1 enhanced biomass yield and total sugar content per plant, suggesting the potential of these genes in improving economically important biofuel feedstocks. A possible mechanism of the Cg1 phenotype is discussed. However, a more detailed study including genome-wide transcriptome and metabolic analysis is needed to determine the underlying genetic elements and pathways regulating the observed developmental and metabolic changes.

New insights into the cellular mechanisms of plant growth at elevated atmospheric carbon dioxide concentrations

Rising atmospheric carbon dioxide concentration ([CO₂ ]) significantly influences plant growth, development, and biomass. Increased photosynthesis rate, together with lower stomatal conductance, has been identified as the key factors that stimulate plant growth at elevated [CO₂ ] (e[CO₂ ]). However, variations in photosynthesis and stomatal conductance alone cannot fully explain the dynamic changes in plant growth. Stimulation of photosynthesis at e[CO₂ ] is always associated with post-photosynthetic secondary metabolic processes that include carbon and nitrogen metabolism, cell cycle functions, and hormonal regulation. Most studies have focused on photosynthesis and stomatal conductance in response to e[CO₂ ], despite the emerging evidence of e[CO₂ ]'s role in moderating secondary metabolism in plants. In this review, we briefly discuss the effects of e[CO₂ ] on photosynthesis and stomatal conductance and then focus on the changes in other cellular mechanisms and growth processes at e[CO₂ ] in relation to plant growth and development. Finally, knowledge gaps in understanding plant growth responses to e[CO₂ ] have been identified with the aim of improving crop productivity under a CO₂ rich atmosphere.

Barley SIX-ROWED SPIKE3 encodes a putative Jumonji C-type H3K9me2/me3 demethylase that represses lateral spikelet fertility

The barley inflorescence (spike) comprises a multi-noded central stalk (rachis) with tri-partite clusters of uni-floretted spikelets attached alternately along its length. Relative fertility of lateral spikelets within each cluster leads to spikes with two or six rows of grain, or an intermediate morphology. Understanding the mechanisms controlling this key developmental step could provide novel solutions to enhanced grain yield. Classical genetic studies identified five major SIX-ROWED SPIKE (VRS) genes, with four now known to encode transcription factors. Here we identify and characterise the remaining major VRS gene, VRS3, as encoding a putative Jumonji C-type H3K9me2/me3 demethylase, a regulator of chromatin state. Exploring the expression network modulated by VRS3 reveals specific interactions, both with other VRS genes and genes involved in stress, hormone and sugar metabolism. We show that combining a vrs3 mutant allele with natural six-rowed alleles of VRS1 and VRS5 leads to increased lateral grain size and greater grain uniformity.

The VRS genes of barley control the fertility of the lateral spikelets on the barley inflorescence. Here, Bull et al. show that VRS3 encodes a putative Jumonji C-type histone demethylase that regulates expression of other VRS genes, and genes involved in stress, hormone and sugar metabolism.

Enzyme characteristics of pathogen-specific trehalose-6-phosphate phosphatases

Owing to the key role of trehalose in pathogenic organisms, there has recently been growing interest in trehalose metabolism for therapeutic purposes. Trehalose-6-phosphate phosphatase (TPP) is a pivotal enzyme in the most prominent biosynthesis pathway (OtsAB). Here, we compare the enzyme characteristics of recombinant TPPs from five important nematode and bacterial pathogens, including three novel members of this protein family. Analysis of the kinetics of trehalose-6-phosphate hydrolysis reveals that all five enzymes display a burst-like kinetic behaviour which is characterised by a decrease of the enzymatic rate after the pre-steady state. The observed super-stoichiometric burst amplitudes can be explained by multiple global conformational changes in members of this enzyme family during substrate processing. In the search for specific TPP inhibitors, the trapping of the complex conformational transitions in TPPs during the catalytic cycle may present a worthwhile strategy to explore.

Adaptation to dietary conditions by trehalose metabolism in Drosophila

Trehalose is a non-reducing disaccharide that serves as the main sugar component of haemolymph in insects. Trehalose hydrolysis enzyme, called trehalase, is highly conserved from bacteria to humans. However, our understanding of the physiological role of trehalase remains incomplete. Here, we analyze the phenotypes of several Trehalase (Treh) loss-of-function alleles in a comparative manner in Drosophila. The previously reported mutant phenotype of Treh affecting neuroepithelial stem cell maintenance and differentiation in the optic lobe is caused by second-site alleles in addition to Treh. We further report that the survival rate of Treh null mutants is significantly influenced by dietary conditions. Treh mutant larvae are lethal not only on a low-sugar diet but also under low-protein diet conditions. A reduction in adaptation ability under poor food conditions in Treh mutants is mainly caused by the overaccumulation of trehalose rather than the loss of Treh, because the additional loss of Tps1 mitigates the lethal effect of Treh mutants. These results demonstrate that proper trehalose metabolism plays a critical role in adaptation under various environmental conditions.

The role of Tre6P and SnRK1 in maize early kernel development and events leading to stress-induced kernel abortion

BACKGROUND

Drought stress during flowering is a major contributor to yield loss in maize. Genetic and biotechnological improvement in yield sustainability requires an understanding of the mechanisms underpinning yield loss. Sucrose starvation has been proposed as the cause for kernel abortion; however, potential targets for genetic improvement have not been identified. Field and greenhouse drought studies with maize are expensive and it can be difficult to reproduce results; therefore, an in vitro kernel culture method is presented as a proxy for drought stress occurring at the time of flowering in maize (3 days after pollination). This method is used to focus on the effects of drought on kernel metabolism, and the role of trehalose 6-phosphate (Tre6P) and the sucrose non-fermenting-1-related kinase (SnRK1) as potential regulators of this response.

RESULTS

A precipitous drop in Tre6P is observed during the first two hours after removing the kernels from the plant, and the resulting changes in transcript abundance are indicative of an activation of SnRK1, and an immediate shift from anabolism to catabolism. Once Tre6P levels are depleted to below 1 nmol∙g-1 FW in the kernel, SnRK1 remained active throughout the 96 h experiment, regardless of the presence or absence of sucrose in the medium. Recovery on sucrose enriched medium results in the restoration of sucrose synthesis and glycolysis. Biosynthetic processes including the citric acid cycle and protein and starch synthesis are inhibited by excision, and do not recover even after the re-addition of sucrose. It is also observed that excision induces the transcription of the sugar transporters SUT1 and SWEET1, the sucrose hydrolyzing enzymes CELL WALL INVERTASE 2 (INCW2) and SUCROSE SYNTHASE 1 (SUSY1), the class II TREHALOSE PHOSPHATE SYNTHASES (TPS), TREHALASE (TRE), and TREHALOSE PHOSPHATE PHOSPHATASE (ZmTPPA.3), previously shown to enhance drought tolerance (Nuccio et al., Nat Biotechnol (October 2014):1-13, 2015).

CONCLUSIONS

The impact of kernel excision from the ear triggers a cascade of events starting with the precipitous drop in Tre6P levels. It is proposed that the removal of Tre6P suppression of SnRK1 activity results in transcription of putative SnRK1 target genes, and the metabolic transition from biosynthesis to catabolism. This highlights the importance of Tre6P in the metabolic response to starvation. We also present evidence that sugars can mediate the activation of SnRK1. The precipitous drop in Tre6P corresponds to a large increase in transcription of ZmTPPA.3, indicating that this specific enzyme may be responsible for the de-phosphorylation of Tre6P. The high levels of Tre6P in the immature embryo are likely important for preventing kernel abortion.

Central Role of the Trehalose Biosynthesis Pathway in the Pathogenesis of Human Fungal Infections: Opportunities and Challenges for Therapeutic Development

Invasive fungal infections cause significant morbidity and mortality in part due to a limited antifungal drug arsenal. One therapeutic challenge faced by clinicians is the significant host toxicity associated with antifungal drugs. Another challenge is the fungistatic mechanism of action of some drugs. Consequently, the identification of fungus-specific drug targets essential for fitness in vivo remains a significant goal of medical mycology research. The trehalose biosynthetic pathway is found in a wide variety of organisms, including human-pathogenic fungi, but not in humans. Genes encoding proteins involved in trehalose biosynthesis are mechanistically linked to the metabolism, cell wall homeostasis, stress responses, and virulence of Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus. While there are a number of pathways for trehalose production across the tree of life, the TPS/TPP (trehalose-6-phosphate synthase/trehalose-6-phosphate phosphatase) pathway is the canonical pathway found in human-pathogenic fungi. Importantly, data suggest that proteins involved in trehalose biosynthesis play other critical roles in fungal metabolism and in vivo fitness that remain to be fully elucidated. By further defining the biology and functions of trehalose and its biosynthetic pathway components in pathogenic fungi, an opportunity exists to leverage this pathway as a potent antifungal drug target. The goal of this review is to cover the known roles of this important molecule and its associated biosynthesis-encoding genes in the human-pathogenic fungi studied to date and to employ these data to critically assess the opportunities and challenges facing development of this pathway as a therapeutic target.

Metabolite transport and associated sugar signalling systems underpinning source/sink interactions

Metabolite transport between organelles, cells and source and sink tissues not only enables pathway co-ordination but it also facilitates whole plant communication, particularly in the transmission of information concerning resource availability. Carbon assimilation is co-ordinated with nitrogen assimilation to ensure that the building blocks of biomass production, amino acids and carbon skeletons, are available at the required amounts and stoichiometry, with associated transport processes making certain that these essential resources are transported from their sites of synthesis to those of utilisation. Of the many possible posttranslational mechanisms that might participate in efficient co-ordination of metabolism and transport only reversible thiol-disulphide exchange mechanisms have been described in detail. Sucrose and trehalose metabolism are intertwined in the signalling hub that ensures appropriate resource allocation to drive growth and development under optimal and stress conditions, with trehalose-6-phosphate acting as an important signal for sucrose availability. The formidable suite of plant metabolite transporters provides enormous flexibility and adaptability in inter-pathway coordination and source-sink interactions. Focussing on the carbon metabolism network, we highlight the functions of different transporter families, and the important of thioredoxins in the metabolic dialogue between source and sink tissues. In addition, we address how these systems can be tailored for crop improvement.

Molecular characterization of Tps1 and Treh genes in Drosophila and their role in body water homeostasis

In insects, trehalose serves as the main sugar component of haemolymph. Trehalose is also recognized as a mediator of desiccation survival due to its proposed ability to stabilize membranes and proteins. Although the physiological role of trehalose in insects has been documented for decades, genetic evidence to support the importance of trehalose metabolism remains incomplete. We here show on the basis of genetic and biochemical evidence that the trehalose synthesis enzyme Tps1 is solely responsible for the de novo synthesis of trehalose in Drosophila. Conversely, a lack of the gene for the trehalose hydrolyzing enzyme Treh causes an accumulation of trehalose that is lethal during the pupal period, as is observed with Tps1 mutants. Lack of either Tps1 or Treh results in a significant reduction in circulating glucose, suggesting that the maintenance of glucose levels requires a continuous turnover of trehalose. Furthermore, changes in trehalose levels are positively correlated with the haemolymph water volume. In addition, both Tps1 and Treh mutant larvae exhibit a high lethality after desiccation stress. These results demonstrate that the regulation of trehalose metabolism is essential for normal development, body water homeostasis, and desiccation tolerance in Drosophila.

Interspecies and Intraspecies Analysis of Trehalose Contents and the Biosynthesis Pathway Gene Family Reveals Crucial Roles of Trehalose in Osmotic-Stress Tolerance in Cassava

Trehalose is a nonreducing α,α-1,1-disaccharide in a wide range of organisms, and has diverse biological functions that range from serving as an energy source to acting as a protective/signal sugar. However, significant amounts of trehalose have rarely been detected in higher plants, and the function of trehalose in the drought-tolerant crop cassava (Manihot esculenta Crantz) is unclear. We measured soluble sugar concentrations of nine plant species with differing levels of drought tolerance and 41 cassava varieties using high-performance liquid chromatography with evaporative light-scattering detector (HPLC-ELSD). Significantly high amounts of trehalose were identified in drought-tolerant crops cassava, Jatropha curcas, and castor bean (Ricinus communis). All cassava varieties tested contained high amounts of trehalose, although their concentrations varied from 0.23 to 1.29 mg·g(-1) fresh weight (FW), and the trehalose level was highly correlated with dehydration stress tolerance of detached leaves of the varieties. Moreover, the trehalose concentrations in cassava leaves increased 2.3-5.5 folds in response to osmotic stress simulated by 20% PEG 6000. Through database mining, 24 trehalose pathway genes, including 12 trehalose-6-phosphate synthases (TPS), 10 trehalose-6-phosphate phosphatases (TPP), and two trehalases were identified in cassava. Phylogenetic analysis indicated that there were four cassava TPS genes (MeTPS1-4) that were orthologous to the solely active TPS gene (AtTPS1 and OsTPS1) in Arabidopsis and rice, and a new TPP subfamily was identified in cassava, suggesting that the trehalose biosynthesis activities in cassava had potentially been enhanced in evolutionary history. RNA-seq analysis indicated that MeTPS1 was expressed at constitutionally high level before and after osmotic stress, while other trehalose pathway genes were either up-regulated or down-regulated, which may explain why cassava accumulated high level of trehalose under normal conditions. MeTPS1 was then transformed into tobacco (Nicotiana benthamiana). Results indicated that transgenic tobacco lines accumulated significant level of trehalose and possessed improved drought stress tolerance. In conclusion, cassava accumulated significantly high amount of trehalose under normal conditions due to multiplied trehalose biosynthesis gene families and constant expression of the active MeTPS1 gene. High levels of trehalose subsequently contributed to high drought stress tolerance.

Trehalose-6-phosphate synthesis controls yeast gluconeogenesis downstream and independent of SNF1

Trehalose-6-P (T6P), an intermediate of trehalose biosynthesis, was identified as an important regulator of yeast sugar metabolism and signaling. tps1Δ mutants, deficient in T6P synthesis (TPS), are unable to grow on rapidly fermentable medium with uncontrolled influx in glycolysis, depletion of ATP and accumulation of sugar phosphates. However, the exact molecular mechanisms involved are not fully understood. We show that SNF1 deletion restores the tps1Δ growth defect on glucose, suggesting that lack of TPS hampers inactivation of SNF1 or SNF1-regulated processes. In addition to alternative, non-fermentable carbon metabolism, SNF1 controls two major processes: respiration and gluconeogenesis. The tps1Δ defect appears to be specifically associated with deficient inhibition of gluconeogenesis, indicating more downstream effects. Consistently, Snf1 dephosphorylation and inactivation on glucose medium are not affected, as confirmed with an in vivo Snf1 activity reporter. Detailed analysis shows that gluconeogenic Pck1 and Fbp1 expression, protein levels and activity are not repressed upon glucose addition to tps1Δ cells, suggesting a link between the metabolic defect and persistent gluconeogenesis. While SNF1 is essential for induction of gluconeogenesis, T6P/TPS is required for inactivation of gluconeogenesis in the presence of glucose, downstream and independent of SNF1 activity and the Cat8 and Sip4 transcription factors.

Expression of trehalose-6-phosphate phosphatase in maize ears improves yield in well-watered and drought conditions

Maize, the highest-yielding cereal crop worldwide, is particularly susceptible to drought during its 2- to 3-week flowering period. Many genetic engineering strategies for drought tolerance impinge on plant development, reduce maximum yield potential or do not translate from laboratory conditions to the field. We overexpressed a gene encoding a rice trehalose-6-phosphate phosphatase (TPP) in developing maize ears using a floral promoter. This reduced the concentration of trehalose-6-phosphate (T6P), a sugar signal that regulates growth and development, and increased the concentration of sucrose in ear spikelets. Overexpression of TPP increased both kernel set and harvest index. Field data at several sites and over multiple seasons showed that the engineered trait improved yields from 9% to 49% under non-drought or mild drought conditions, and from 31% to 123% under more severe drought conditions, relative to yields from nontransgenic controls.

Investigation of the metabolic consequences of impregnating spinach leaves with trehalose and applying a pulsed electric field

The impregnation of leafy vegetables with cryoprotectants using a combination of vacuum impregnation (VI) and pulsed electric fields (PEF) has been proposed by our research group as a method of improving their freezing tolerance and consequently their general quality after thawing. In this study, we have investigated the metabolic consequences of the combination of these unit operations on spinach. The vacuum impregnated spinach leaves showed a drastic decrease in the porosity of the extracellular space. However, at maximum weight gain, randomly located air pockets remained, which may account for oxygen-consuming pathways in the cells being active after VI. The metabolic activity of the impregnated leaves showed a drastic increase that was further enhanced by the application of PEF to the impregnated tissue. Impregnating the leaves with trehalose by VI led to a significant accumulation of trehalose-6-phosphate (T6P), however, this was not further enhanced by PEF. It is suggested that the accumulation of T6P in the leaves may increase metabolic activity, and increase tissue resistance to abiotic stress.

Plant developmental transitions: the role of microRNAs and sugars

What determines the rate at which a multicellular organism ages is a mystery in biology. In plants the changes in morphological and physiological traits serve as markers for the developmental transitions. Mutant characterizations and genetic analyses in Arabidopsis thaliana delineate an evolutionarily conserved, microRNA156 (miR156)-guided timing mechanism that temporally regulates many aspects of biological processes during development. Recent studies now reveal that sugar metabolites, the products of photosynthesis, feed into this developmental timer by regulating miR156 levels, thereby ensuring that each developmental transition occurs under favorable conditions.

Size control in plants--lessons from leaves and flowers

To achieve optimal functionality, plant organs like leaves and petals have to grow to a certain size. Beginning with a limited number of undifferentiated cells, the final size of an organ is attained by a complex interplay of cell proliferation and subsequent cell expansion. Regulatory mechanisms that integrate intrinsic growth signals and environmental cues are required to enable optimal leaf and flower development. This review focuses on plant-specific principles of growth reaching from the cellular to the organ level. The currently known genetic pathways underlying these principles are summarized and network connections are highlighted. Putative non-cell autonomously acting mechanisms that might coordinate plant-cell growth are discussed.

Metabolic profiling of Lolium perenne shows functional integration of metabolic responses to diverse subtoxic conditions of chemical stress

Plant communities are confronted with a great variety of environmental chemical stresses. Characterization of chemical stress in higher plants has often been focused on single or closely related stressors under acute exposure, or restricted to a selective number of molecular targets. In order to understand plant functioning under chemical stress conditions close to environmental pollution conditions, the C3 grass Lolium perenne was subjected to a panel of different chemical stressors (pesticide, pesticide degradation compound, polycyclic aromatic hydrocarbon, and heavy metal) under conditions of seed-level or root-level subtoxic exposure. Physiological and metabolic profiling analysis on roots and shoots revealed that all of these subtoxic chemical stresses resulted in discrete physiological perturbations and complex metabolic shifts. These metabolic shifts involved stressor-specific effects, indicating multilevel mechanisms of action, such as the effects of glyphosate and its degradation product aminomethylphosphonic acid on quinate levels. They also involved major generic effects that linked all of the subtoxic chemical stresses with major modifications of nitrogen metabolism, especially affecting asparagine, and of photorespiration, especially affecting alanine and glycerate. Stress-related physiological effects and metabolic adjustments were shown to be integrated through a complex network of metabolic correlations converging on Asn, Leu, Ser, and glucose-6-phosphate, which could potentially be modulated by differential dynamics and interconversion of soluble sugars (sucrose, trehalose, fructose, and glucose). Underlying metabolic, regulatory, and signalling mechanisms linking these subtoxic chemical stresses with a generic impact on nitrogen metabolism and photorespiration are discussed in relation to carbohydrate and low-energy sensing.

A genetically encoded Förster resonance energy transfer sensor for monitoring in vivo trehalose-6-phosphate dynamics

Trehalose-6-phosphate is a pivotal regulator of sugar metabolism, growth, and osmotic equilibrium in bacteria, yeasts, and plants. To directly visualize the intracellular levels of intracellular trehalose-6-phosphate, we developed a series of specific Förster resonance energy transfer (FRET) sensors for in vivo microscopy. We demonstrated real-time monitoring of regulation in the trehalose pathway of Escherichia coli. In Saccharomyces cerevisiae, we could show that the concentration of free trehalose-6-phosphate during growth on glucose is in a range sufficient for inhibition of hexokinase. These findings support the hypothesis of trehalose-6-phosphate as the effector of a negative feedback system, similar to the inhibition of hexokinase by glucose-6-phosphate in mammalian cells and controlling glycolytic flux.

A Ralstonia solanacearum type III effector directs the production of the plant signal metabolite trehalose-6-phosphate

The plant pathogen Ralstonia solanacearum possesses two genes encoding a trehalose-6-phosphate synthase (TPS), an enzyme of the trehalose biosynthetic pathway. One of these genes, named ripTPS, was found to encode a protein with an additional N-terminal domain which directs its translocation into host plant cells through the type 3 secretion system. RipTPS is a conserved effector in the R. solanacearum species complex, and homologues were also detected in other bacterial plant pathogens. Functional analysis of RipTPS demonstrated that this type 3 effector synthesizes trehalose-6-phosphate and identified residues essential for this enzymatic activity. Although trehalose-6-phosphate is a key signal molecule in plants that regulates sugar status and carbon assimilation, the disruption of ripTPS did not alter the virulence of R. solanacearum on plants. However, heterologous expression assays showed that this effector specifically elicits a hypersensitive-like response on tobacco that is independent of its enzymatic activity and is triggered by the C-terminal half of the protein. Recognition of this effector by the plant immune system is suggestive of a role during the infectious process.

Ralstonia solanacearum, the causal agent of bacterial wilt disease, infects more than two hundred plant species, including economically important crops. The type III secretion system plays a major role in the pathogenicity of this bacterium, and approximately 70 effector proteins have been shown to be translocated into host plant cells. This study provides the first description of a type III effector endowed with a trehalose-6-phosphate synthase enzymatic activity and illustrates a new mechanism by which the bacteria may manipulate the plant metabolism upon infection. In recent years, trehalose-6-phosphate has emerged as an essential signal molecule in plants, connecting plant metabolism and development. The finding that a bacterial pathogen could induce the production of trehalose-6-phosphate in plant cells further highlights the importance of this metabolite in multiple aspects of the molecular physiology of plants.

The trehalose pathway in maize: conservation and gene regulation in response to the diurnal cycle and extended darkness

Energy resources in plants are managed in continuously changing environments, such as changes occurring during the day/night cycle. Shading is an environmental disruption that decreases photosynthesis, compromises energy status, and impacts on crop productivity. The trehalose pathway plays a central but not well-defined role in maintaining energy balance. Here, we characterized the maize trehalose pathway genes and deciphered the impacts of the diurnal cycle and disruption of the day/night cycle on trehalose pathway gene expression and sugar metabolism. The maize genome encodes 14 trehalose-6-phosphate synthase (TPS) genes, 11 trehalose-6-phosphate phosphatase (TPP) genes, and one trehalase gene. Transcript abundance of most of these genes was impacted by the day/night cycle and extended dark stress, as were sucrose, hexose sugars, starch, and trehalose-6-phosphate (T6P) levels. After extended darkness, T6P levels inversely followed class II TPS and sucrose non-fermenting-related protein kinase 1 (SnRK1) target gene expression. Most significantly, T6P no longer tracked sucrose levels after extended darkness. These results showed: (i) conservation of the trehalose pathway in maize; (ii) that sucrose, hexose, starch, T6P, and TPS/TPP transcripts respond to the diurnal cycle; and(iii) that extended darkness disrupts the correlation between T6P and sucrose/hexose pools and affects SnRK1 target gene expression. A model for the role of the trehalose pathway in sensing of sucrose and energy status in maize seedlings is proposed.

Over-expression of BvMTSH, a fusion gene for maltooligosyltrehalose synthase and maltooligosyltrehalose trehalohydrolase, enhances drought tolerance in transgenic rice

Plant abiotic stress tolerance has been modulated by engineering the trehalose synthesis pathway. However, many stress-tolerant plants that have been genetically engineered for the trehalose synthesis pathway also show abnormal development. The metabolic intermediate trehalose 6-phosphate has the potential to cause aberrations in growth. To avoid growth inhibition by trehalose 6-phosphate, we used a gene that encodes a bifunctional in-frame fusion (BvMTSH) of maltooligosyltrehalose synthase (BvMTS) and maltooligosyltrehalose trehalohydrolase (BvMTH) from the nonpathogenic bacterium Brevibacterium helvolum. BvMTS converts maltooligosaccharides into maltooligosyltrehalose and BvMTH releases trehalose. Transgenic rice plants that over-express BvMTSH under the control of the constitutive rice cytochrome c promoter (101MTSH) or the ABA-inducible Ai promoter (105MTSH) show enhanced drought tolerance without growth inhibition. Moreover, 101MTSH and 105MTSH showed an ABA-hyposensitive phenotype in the roots. Our results suggest that over-expression of BvMTSH enhances drought-stress tolerance without any abnormal growth and showes ABA hyposensitive phenotype in the roots.

High atomic weight, high-energy radiation (HZE) induces transcriptional responses shared with conventional stresses in addition to a core "DSB" response specific to clastogenic treatments

Plants exhibit a robust transcriptional response to gamma radiation which includes the induction of transcripts required for homologous recombination and the suppression of transcripts that promote cell cycle progression. Various DNA damaging agents induce different spectra of DNA damage as well as "collateral" damage to other cellular components and therefore are not expected to provoke identical responses by the cell. Here we study the effects of two different types of ionizing radiation (IR) treatment, HZE (1 GeV Fe(26+) high mass, high charge, and high energy relativistic particles) and gamma photons, on the transcriptome of Arabidopsis thaliana seedlings. Both types of IR induce small clusters of radicals that can result in the formation of double strand breaks (DSBs), but HZE also produces linear arrays of extremely clustered damage. We performed these experiments across a range of time points (1.5-24 h after irradiation) in both wild-type plants and in mutants defective in the DSB-sensing protein kinase ATM. The two types of IR exhibit a shared double strand break-repair-related damage response, although they differ slightly in the timing, degree, and ATM-dependence of the response. The ATM-dependent, DNA metabolism-related transcripts of the "DSB response" were also induced by other DNA damaging agents, but were not induced by conventional stresses. Both Gamma and HZE irradiation induced, at 24 h post-irradiation, ATM-dependent transcripts associated with a variety of conventional stresses; these were overrepresented for pathogen response, rather than DNA metabolism. In contrast, only HZE-irradiated plants, at 1.5 h after irradiation, exhibited an additional and very extensive transcriptional response, shared with plants experiencing "extended night." This response was not apparent in gamma-irradiated plants.

Trehalose-6-phosphate and SnRK1 kinases in plant development and signaling: the emerging picture

Carbohydrates, or sugars, regulate various aspects of plant growth through modulation of cell division and expansion. Besides playing essential roles as sources of energy for growth and as structural components of cells, carbohydrates also regulate the timing of expression of developmental programs. The disaccharide trehalose is used as an energy source, as a storage and transport molecule for glucose, and as a stress-responsive compound important for cellular protection during stress in all kingdoms. Trehalose, however, is found in very low amounts in most plants, pointing to a signaling over metabolic role for this non-reducing disaccharide. In the last decade, trehalose-6-phosphate (T6P), an intermediate in trehalose metabolism, has been shown to regulate embryonic and vegetative development, flowering time, meristem determinacy, and cell fate specification in plants. T6P acts as a global regulator of metabolism and transcription promoting plant growth and triggering developmental phase transitions in response to sugar availability. Among the T6P targets are members of the Sucrose-non-fermenting1-related kinase1 (SnRK1) family, which are sensors of energy availability and inhibit plant growth and development during metabolic stress to maintain energy homeostasis. In this review, we will discuss the opposite roles of the sugar metabolite T6P and the SnRK1 kinases in the regulation of developmental phase transitions in response to carbohydrate levels. We will focus on how these two global regulators of metabolic processes integrate environmental cues and interact with hormonal signaling pathways to modulate plant development.

The Enzyme-Like Domain of Arabidopsis Nuclear β-Amylases Is Critical for DNA Sequence Recognition and Transcriptional Activation

Plant BZR1-BAM transcription factors contain a β-amylase (BAM)-like domain, characteristic of proteins involved in starch breakdown. The enzyme-derived domains appear to be noncatalytic, but they determine the function of the two Arabidopsis thaliana BZR1-BAM isoforms (BAM7 and BAM8) during transcriptional initiation. Removal or swapping of the BAM domains demonstrates that the BAM7 BAM domain restricts DNA binding and transcriptional activation, while the BAM8 BAM domain allows both activities. Furthermore, we demonstrate that BAM7 and BAM8 interact on the protein level and cooperate during transcriptional regulation. Site-directed mutagenesis of residues in the BAM domain of BAM8 shows that its function as a transcriptional activator is independent of catalysis but requires an intact substrate binding site, suggesting it may bind a ligand. Microarray experiments with plants overexpressing truncated versions lacking the BAM domain indicate that the pseudo-enzymatic domain increases selectivity for the preferred cis-regulatory element BBRE (BZR1-BAM Responsive Element). Side specificity toward the G-box may allow crosstalk to other signaling networks. This work highlights the importance of the enzyme-derived domain of BZR1-BAMs, supporting their potential role as metabolic sensors.

Sugar signals and the control of plant growth and development

Sugars have a central regulatory function in steering plant growth. This review focuses on information presented in the past 2 years on key players in sugar-mediated plant growth regulation, with emphasis on trehalose 6-phosphate, target of rapamycin kinase, and Snf1-related kinase 1 regulatory systems. The regulation of protein synthesis by sugars is fundamental to plant growth control, and recent advances in our understanding of the regulation of translation by sugars will be discussed.

The activity of SnRK1 is increased in Phaseolus vulgaris seeds in response to a reduced nutrient supply

Phaseolus vulgaris seeds can grow and develop at the expense of the pod reserves after the fruits have been removed from the plant (Fountain etal., 1989). Because this process involves sensing the reduction of nutrients and the remobilisation of pod reserves, we investigated the effect on sucrose non-fermenting related kinase 1 (SnRK1) activity during this process. Bean fruits removed from the plant at 20 days after flowering (DAF) demonstrated active remobilisation of nutrients from the pod to the seeds. After 5 days, the pod dry weight was reduced by 50%. The process was characterized by a rapid degradation of starch, with the greatest decrease observed on day 1 after the fruits were removed. The pod nutrients were insufficient for the needs of all the seeds, and only some seeds continued their development. Those seeds exhibited a transient reduction in sucrose levels on day 1 after the fruits were removed. However, the normal level of sucrose was recovered, and the rate of starch synthesis was identical to that of a seed developed under normal conditions. Removing the fruits from the plant had no effect on the activity of SnRK1 in the pods, whereas in the seeds, the activity was increased by 35%. Simultaneously, a large reduction in seed sucrose levels was observed. The increase in SnRK1 activity was observed in both the cotyledon and embryo axes, but it was higher in the cotyledon. At 20-25 DAF, cotyledons actively accumulate storage materials. It is possible that the increase in SnRK1 activity observed in seeds developed in fruits that have been removed from the plant is part of the mechanism required for nutrient remobilisation under conditions of stress.