Integrated Transcriptomics and Metabolomics Analysis of the Fructan Metabolism Response to Low-Temperature Stress in Garlic

As the main reserve carbohydrate in garlic, fructan contributes to garlic's yield and quality formation. Numerous studies have shown that plant fructan metabolism induces a stress response to adverse environments. However, the transcriptional regulation mechanism of garlic fructan in low-temperature environments is still unknown. In this study, the fructan metabolism of garlic seedlings under low-temperature stress was revealed by transcriptome and metabolome approaches. With the extension of stress time, the number of differentially expressed genes and metabolites increased. Using weighted gene co-expression network analysis (WGCNA), three key enzyme genes related to fructan metabolism were screened (a total of 12 transcripts): sucrose: sucrose 1-fructosyltransferase (1-SST) gene; fructan: fructan 6G fructosyltransferase (6G-FFT) gene; and fructan 1-exohydrolase (1-FEH) gene. Finally, two hub genes were obtained, namely Cluster-4573.161559 (6G-FFT) and Cluster-4573.153574 (1-FEH). The correlation network and metabolic heat map analysis between fructan genes and carbohydrate metabolites indicate that the expression of key enzyme genes in fructan metabolism plays a positive promoting role in the fructan response to low temperatures in garlic. The number of genes associated with the key enzyme of fructan metabolism in trehalose 6-phosphate was the highest, and the accumulation of trehalose 6-phosphate content may mainly depend on the key enzyme genes of fructan metabolism rather than the enzyme genes in its own synthesis pathway. This study not only obtained the key genes of fructan metabolism in garlic seedlings responding to low temperatures but also preliminarily analyzed its regulatory mechanism, providing an important theoretical basis for further elucidating the cold resistance mechanism of garlic fructan metabolism.

High-Throughput Extraction and Enzymatic Determination of Sugars and Fructans in Fructan-Accumulating Plants

Fructans are carbohydrates present in more than 15% of flowering plants. They represent the major pool of carbohydrates in some species, especially when facing cold or drought. However, the functions of fructans with high or low degrees of polymerization (DP), their diurnal use, and the regulation of their synthesis and degradation in response to stresses still remain unclear. Here we present an enzymatic protocol adapted to 96-well microplates that simultaneously allows the determination of fructans and glucose, fructose, and sucrose. Moreover, the protocol allows to estimate the average DP of the fructans in the samples. The protocol is based on the enzymatic degradation of fructans into glucose and fructose and their subsequent conversion into gluconate 6-phosphate concomitant with the formation of NADH in the presence of ATP.

Diurnal patterns of growth and transient reserves of sink and source tissues are affected by cold nights in barley

Barley is described to mostly use sucrose for night carbon requirements. To understand how the transient carbon is accumulated and utilized in response to cold, barley plants were grown in a combination of cold days and/or nights. Both daytime and night cold reduced growth. Sucrose was the main carbohydrate supplying growth at night, representing 50-60% of the carbon consumed. Under warm days and nights, starch was the second contributor with 26% and malate the third with 15%. Under cold nights, the contribution of starch was severely reduced, due to an inhibition of its synthesis, including under warm days, and malate was the second contributor to C requirements with 24-28% of the total amount of carbon consumed. We propose that malate plays a critical role as an alternative carbon source to sucrose and starch in barley. Hexoses, malate, and sucrose mobilization and starch accumulation were affected in barley elf3 clock mutants, suggesting a clock regulation of their metabolism, without affecting growth and photosynthesis however. Altogether, our data suggest that the mobilization of sucrose and malate and/or barley growth machinery are sensitive to cold.

6-SFT, a Protein from Leymus mollis, Positively Regulates Salinity Tolerance and Enhances Fructan Levels in Arabidopsis thaliana

Fructans play vital roles in abiotic stress tolerance in plants. In this study, we isolated the sucrose:6-fructosyltransferase gene, which is involved in the synthesis of fructans, from Leymus mollis by rapid amplification of cDNA ends. The Lm-6-SFT gene was introduced into Arabidopsis thaliana cv. Columbia by Agrobacterium-mediated transformation. The transgenic plants were evaluated under salt stress conditions. The results showed that the expression of Lm-6-SFT was significantly induced by light, abscisic acid (ABA), salicylic acid (SA), and salt treatment in L. mollis plants. Overexpression of Lm-6-SFT in Arabidopsis promoted seed germination and primary root growth during the early vegetative growth stage under salt stress. We also found that the transgenic plants expressing Lm-6-SFT had increased proline and fructan levels. β-Glucuronidase staining and promoter analysis indicated that the promoter of Lm-6-SFT was regulated by light, ABA, and salt stress. Quantitative PCR suggested that overexpression of Lm-6-SFT could improve salt tolerance by interacting with the expression of some salt stress tolerance genes. Thus, we demonstrated that the Lm-6-SFT gene is a candidate gene that potentially confers salt stress tolerance to plants. Our study will aid the elucidation of the regulatory mechanism of 6-SFT genes in herb plants.

Structural Modifications of Fructans in Aloe barbadensis Miller (Aloe Vera) Grown under Water Stress

Aloe barbadensis Miller (Aloe vera) has a Crassulaceae acid metabolism which grants the plant great tolerance to water restrictions. Carbohydrates such as acemannans and fructans are among the molecules responsible for tolerating water deficit in other plant species. Nevertheless, fructans, which are prebiotic compounds, have not been described nor studied in Aloe vera, whose leaf gel is known to possess beneficial pharmaceutical, nutritional and cosmetic properties. As Aloe vera is frequently cultivated in semi-arid conditions, like those found in northern Chile, we investigated the effect of water deficit on fructan composition and structure. For this, plants were subjected to different irrigation regimes of 100%, 75%, 50% and 25% field capacity (FC). There was a significant increase in the total sugars, soluble sugars and oligo and polyfructans in plants subjected to water deficit, compared to the control condition (100% FC) in both leaf tips and bases. The amounts of fructans were also greater in the bases compared to the leaf tips in all water treatments. Fructans also increase in degree of polymerization with increasing water deficit. Glycosidic linkage analyses by GC-MS, led to the conclusion that there are structural differences between the fructans present in the leaves of control plants with respect to plants irrigated with 50% and 25% FC. Therefore, in non-stressed plants, the inulin, neo-inulin and neo-levan type of fructans predominate, while in the most stressful conditions for the plant, Aloe vera also synthesizes fructans with a more branched structure, the neofructans. To our knowledge, the synthesis and the protective role of neo-fructans under extreme water deficit has not been previously reported.

A sucrose:fructan-6-fructosyltransferase (6-SFT) gene from Psathyrostachys huashanica confers abiotic stress tolerance in tobacco

Fructans are accessible carbohydrate reserves in various plant species, which possess many physiological functions including anti-oxidation, stabilizing subcellular structures, and osmotic adjustment. In addition, fructans may play important roles in stress tolerance in plant species. In this study, we isolated a Psathyrostachys huashanica (2n=2x=14, NsNs) sucrose:fructan-6-fructosyltransferase (Ph-6-SFT) using homologous cloning and genomic walking. Sequencing and gene structure analysis showed that Ph-6-SFT contains four exons and three introns, with a transcript of 2207 bp. Sequence analysis indicated that the coding sequence of Ph-6-SFT is 1851 bp long and it encodes 616 amino acids, where the structure shares high similarity with 6-SFTs from other plants. Furthermore, Ph-6-SFT was transferred into tobacco (Nicotiana tabacum L.) cv. W38 via Agrobacterium-mediated transformation. Compared with the wild-type plants, the transgenic tobacco plants exhibited a much higher tolerance of drought, cold, and high salinity. In all conditions, physiological studies showed that the tolerance of transgenic plants was associated with the accumulation of carbohydrate and proline, but reductions in malondialdehyde. Our results suggest that the 6-SFT gene from P. huashanica enhanced stress tolerance in tobacco plants and it may be applied as a genetic tool for improving stress tolerance in other crops.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2009-2010

This review is the sixth update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2010. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, arrays and fragmentation are covered in the first part of the review and applications to various structural typed constitutes the remainder. The main groups of compound that are discussed in this section are oligo and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals. Many of these applications are presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions and applications to chemical synthesis.

Comparative analyses reveal potential uses of Brachypodium distachyon as a model for cold stress responses in temperate grasses

BACKGROUND

Little is known about the potential of Brachypodium distachyon as a model for low temperature stress responses in Pooideae. The ice recrystallization inhibition protein (IRIP) genes, fructosyltransferase (FST) genes, and many C-repeat binding factor (CBF) genes are Pooideae specific and important in low temperature responses. Here we used comparative analyses to study conservation and evolution of these gene families in B. distachyon to better understand its potential as a model species for agriculturally important temperate grasses.

RESULTS

Brachypodium distachyon contains cold responsive IRIP genes which have evolved through Brachypodium specific gene family expansions. A large cold responsive CBF3 subfamily was identified in B. distachyon, while CBF4 homologs are absent from the genome. No B. distachyon FST gene homologs encode typical core Pooideae FST-motifs and low temperature induced fructan accumulation was dramatically different in B. distachyon compared to core Pooideae species.

CONCLUSIONS

We conclude that B. distachyon can serve as an interesting model for specific molecular mechanisms involved in low temperature responses in core Pooideae species. However, the evolutionary history of key genes involved in low temperature responses has been different in Brachypodium and core Pooideae species. These differences limit the use of B. distachyon as a model for holistic studies relevant for agricultural core Pooideae species.

TaMYB13 is a transcriptional activator of fructosyltransferase genes involved in β-2,6-linked fructan synthesis in wheat

Fructans are soluble fructosyl-oligosaccharides deposited in many cool-season grass species as a carbon reserve; they are synthesised by fructosyltransferases. In wheat and barley fructans can accumulate in mature stems at a very high level and serve as an important carbon source for grain filling. Fructan synthesis in temperate cereals is regulated by sucrose level and developmental signals, and functions as a metabolic adjustment for carbon balance between carbon supply and sink demand. In this study the expression levels of a highly homologous group of Triticum aestivumMYB genes (TaMYB13-1, TaMYB13-2 and TaMYB13-3) were found to be positively correlated with the mRNA levels of sucrose:sucrose 1-fructosyltransferase (1-SST) and sucrose:fructan 6-fructosyltransferase (6-SFT) in wheat stems among recombinant inbred lines with a wide range of fructan concentrations through Affymetrix array expression analysis. This expression correction extended to expression profiles during stem development. TaMYB13 contains an R2R3-type MYB domain. In vitro random DNA-binding site selection followed by base substitution mutagenesis revealed that TaMYB13 bound to a (A/G/T)TT(A/T/C)GGT core sequence, which was present in the promoters of wheat Ta1-SST and Ta6-SFT genes as well as a barley Hv6-SFT gene. Transactivation analysis showed that TaMYB13 was a transcriptional activator and could markedly enhance the expression of 1-SST and 6-SFT promoter-driven reporter genes in wheat. Elimination of TaMYB13-binding sites in Ta6-SFT and Ta1-SST promoters markedly reduced TaMYB13-mediated reporter gene transactivation. These data suggest that TaMYB13 and its orthologues are positive regulators for controlling the expression of major fructosyltransferases involved in the fructan synthetic pathway in temperate cereals.

Functional characterization of a sucrose:fructan 6-fructosyltransferase of the cold-resistant grass Bromus pictus by heterelogous expression in Pichia pastoris and Nicotiana tabacum and its involvement in freezing tolerance

We have previously reported the molecular characterization of a putative sucrose:fructan 6-fructosyltransferase (6-SFT) of Bromus pictus, a graminean species from Patagonia, tolerant to cold and drought. Here, this enzyme was functionally characterized by heterologous expression in Pichia pastoris and Nicotiana tabacum. Recombinant P. pastoris Bp6-SFT showed comparable characteristics to barley 6-SFT and an evident fructosyltransferase activity synthesizing bifurcose from sucrose and 1-kestotriose. Transgenic tobacco plants expressing Bp6-SFT, showed fructosyltransferase activity and fructan accumulation in leaves. Bp6-SFT plants exposed to freezing conditions showed a significantly lower electrolyte leakage in leaves compared to control plants, indicating less membrane damage. Concomitantly these transgenic plants resumed growth more rapidly than control ones. These results indicate that Bp6-SFT transgenic tobacco plants that accumulate fructan showed enhanced freezing tolerance compared to control plants.

Fructan accumulation and transcription of candidate genes during cold acclimation in three varieties of Poa pratensis

Poa pratensis, a type species for the grass family (Poaceae), is an important cool season grass that accumulates fructans as a polysaccharide reserve. We studied fructan contents and expression of candidate fructan metabolism genes during cold acclimation in three varieties of P. pratensis adapted to different environments: Northern Norway, Denmark, and the Netherlands. Fructan content increased significantly during cold acclimation and varieties showed significant differences in the level of fructan accumulation. cDNA sequences of putative fructosyltransferase (FT), fructan exohydrolase (FEH), and cold acclimation protein (CAP) genes were identified and cloned. In agreement with a function in fructan biosynthesis, transcription of a putative sucrose:fructan 6-fructosyltransferase (Pp6-SFT) gene was induced during cold acclimation and fructan accumulation in all three P. pratensis varieties. Transcription of putative PpFEH and PpCAP genes was also induced by cold acclimation; however, transcription of these two genes was several-fold higher in the variety from Norway compared to the other two varieties. The results presented here suggest that Pp6-SFT is involved in fructan biosynthesis in P. pratensis. FEHs have previously been suggested to be involved in fructan biosynthesis and freezing tolerance, and induced expression of PpFEH during fructan accumulation could also suggest a role in fructan biosynthesis. However, based on the different PpFEH transcription rates among varieties and similar expression of PpFEH and PpCAP, we suggest that PpFEH is more likely to be involved in mediating freezing tolerance, e.g., by regulating the cell osmotic potential through fructan degradation.

Unexpected presence of graminan- and levan-type fructans in the evergreen frost-hardy eudicot Pachysandra terminalis (Buxaceae): purification, cloning, and functional analysis of a 6-SST/6-SFT enzyme

About 15% of flowering plants accumulate fructans. Inulin-type fructans with β(2,1) fructosyl linkages typically accumulate in the core eudicot families (e.g. Asteraceae), while levan-type fructans with β(2,6) linkages and branched, graminan-type fructans with mixed linkages predominate in monocot families. Here, we describe the unexpected finding that graminan- and levan-type fructans, as typically occurring in wheat (Triticum aestivum) and barley (Hordeum vulgare), also accumulate in Pachysandra terminalis, an evergreen, frost-hardy basal eudicot species. Part of the complex graminan- and levan-type fructans as accumulating in vivo can be produced in vitro by a sucrose:fructan 6-fructosyltransferase (6-SFT) enzyme with inherent sucrose:sucrose 1-fructosyltransferase (1-SST) and fructan 6-exohydrolase side activities. This enzyme produces a series of cereal-like graminan- and levan-type fructans from sucrose as a single substrate. The 6-SST/6-SFT enzyme was fully purified by classic column chromatography. In-gel trypsin digestion led to reverse transcription-polymerase chain reaction-based cDNA cloning. The functionality of the 6-SST/6-SFT cDNA was demonstrated after heterologous expression in Pichia pastoris. Both the recombinant and native enzymes showed rather similar substrate specificity characteristics, including peculiar temperature-dependent inherent 1-SST and fructan 6-exohydrolase side activities. The finding that cereal-type fructans accumulate in a basal eudicot species further confirms the polyphyletic origin of fructan biosynthesis in nature. Our data suggest that the fructan syndrome in P. terminalis can be considered as a recent evolutionary event. Putative connections between abiotic stress and fructans are discussed.

Molecular mechanisms underlying frost tolerance in perennial grasses adapted to cold climates

We review recent progress in understanding cold and freezing stress responses in forage grass species, notably Lolium and Festuca species. The chromosomal positions of important frost tolerance and winter survival QTLs on Festuca and Lolium chromosomes 4 and 5 are most likely orthologs of QTLs on Triticeae chromosome 5 which correspond to a cluster of CBF-genes and the major vernalization gene. Gene expression and protein accumulation analyses after cold acclimation shed light on general responses to cold stress. These responses involve modulation of transcription levels of genes encoding proteins involved in cell signalling, cellular transport and proteins associated with the cell membrane. Also, abundance levels of proteins directly involved in photosynthesis were found to be different between genotypes of differing frost tolerance levels, stressing the importance of the link between the function of the photosynthetic apparatus under cold stress and frost tolerance levels. The significance of the ability to undergo photosynthetic acclimation and avoid photoinhibition is also evident from numerous studies in forage grasses. Other interesting candidate mechanisms for freezing tolerance in forage grasses are molecular responses to cold stress which have evolved after the divergence of temperate grasses. This includes metabolic machinery for synthesis of fructans and novel ice-binding proteins.

Phylogenetic analyses reveal the shady history of C4 grasses

Grasslands cover more than 20% of the Earth's terrestrial surface, and their rise to dominance is one of the most dramatic events of biome evolution in Earth history. Grasses possess two main photosynthetic pathways: the C(3) pathway that is typical of most plants and a specialized C(4) pathway that minimizes photorespiration and thus increases photosynthetic performance in high-temperature and/or low-CO(2) environments. C(4) grasses dominate tropical and subtropical grasslands and savannas, and C(3) grasses dominate the world's cooler temperate grassland regions. This striking pattern has been attributed to C(4) physiology, with the implication that the evolution of the pathway enabled C(4) grasses to persist in warmer climates than their C(3) relatives. We combined geospatial and molecular sequence data from two public archives to produce a 1,230-taxon phylogeny of the grasses with accompanying climate data for all species, extracted from more than 1.1 million herbarium specimens. Here we show that grasses are ancestrally a warm-adapted clade and that C(4) evolution was not correlated with shifts between temperate and tropical biomes. Instead, 18 of 20 inferred C(4) origins were correlated with marked reductions in mean annual precipitation. These changes are consistent with a shift out of tropical forest environments and into tropical woodland/savanna systems. We conclude that C(4) evolution in grasses coincided largely with migration out of the understory and into open-canopy environments. Furthermore, we argue that the evolution of cold tolerance in certain C(3) lineages is an overlooked innovation that has profoundly influenced the patterning of grassland communities across the globe.

Cloning and functional characterization of a fructan 1-exohydrolase (1-FEH) in the cold tolerant Patagonian species Bromus pictus

Fructans are fructose polymers synthesized in a wide range of species such as bacteria, fungi and plants. Fructans are synthesized by fructosyltransferases (FTs) and depolymerized by fructan exohydrolases (FEHs). Bromus pictus is a graminean decaploid species from the Patagonian region of Argentina, which accumulates large amounts of fructans even at temperate temperatures. The first gene isolated from B. pictus fructan metabolism was a putative sucrose:fructan 6-fructosyltransferase (6-SFT). Here, a complete cDNA of the first fructan exohydrolase (FEH) from B. pictus (Bp1-FEHa) was isolated using RT-PCR strategies. The Bp1-FEHa encoding gene is present as a single copy in B. pictus genome. Functional characterization in Pichia pastoris confirmed Bp1-FEHa is a fructan exohydrolase with predominant activity towards beta-(2-1) linkages. Its expression was analyzed in different leaf sections, showing the highest expression levels in the second section of the sheath and the tip of the blade. Bp1-FEHa expression was studied along with FEH and FT activities and fructan accumulation profile in response to chilling conditions during a 7-day time course experiment. Bp1-FEHa expression and FEH activity followed a similar pattern in response to low temperatures, especially in basal sections of the sheaths. In these sections the FEH and FT activities were particularly high and they were significantly correlated to fructan accumulation profile, along with cold treatment.