Monosaccharide composition, chain length and linkage type influence the interactions of oligosaccharides with dry phosphatidylcholine membranes

Low molecular weight carbohydrates and abiotic stress tolerance in lentil (Lens culinaris Medikus): a review

Lentil (Lens culinaris Medikus) is a nutrient-rich, cool-season food legume that is high in protein, prebiotic carbohydrates, vitamins, and minerals. It is a staple food in many parts of the world, but crop performance is threatened by climate change, where increased temperatures and less predictable precipitation can reduce yield and nutritional quality. One mechanism that many plant species use to mitigate heat and drought stress is the production of disaccharides, oligosaccharides and sugar alcohols, collectively referred to as low molecular weight carbohydrates (LMWCs). Recent evidence indicates that lentil may also employ this mechanism - especially raffinose family oligosaccharides and sugar alcohols - and that these may be suitable targets for genomic-assisted breeding to improve crop tolerance to heat and drought stress. While the genes responsible for LMWC biosynthesis in lentil have not been fully elucidated, single nucleotide polymorphisms and putative genes underlying biosynthesis of LMWCs have been identified. Yet, more work is needed to confirm gene identity, function, and response to abiotic stress. This review i) summarizes the diverse evidence for how LMWCs are utilized to improve abiotic stress tolerance, ii) highlights current knowledge of genes that control LMWC biosynthesis in lentil, and iii) explores how LMWCs can be targeted using diverse genomic resources and markers to accelerate lentil breeding efforts for improved stress tolerance.

Metabolite-mediated adaptation of crops to drought and the acquisition of tolerance

Drought is one of the major and growing threats to agriculture productivity and food security. Metabolites are involved in the regulation of plant responses to various environmental stresses, including drought stress. The complex drought tolerance can be ascribed to several simple metabolic traits. These traits could then be used for detecting the genetic architecture of drought tolerance. Plant metabolomes show dynamic differences when drought occurs during different developmental stages or upon different levels of drought stress. Here, we reviewed the major and most recent findings regarding the metabolite-mediated plant drought response. Recent progress in the development of drought-tolerant agents is also discussed. We provide an updated schematic overview of metabolome-driven solutions for increasing crop drought tolerance and thereby addressing an impending agricultural challenge.

Identification and Alternative Splicing Profile of the Raffinose synthase Gene in Grass Species

Raffinose synthase (Rafs) is an important enzyme in the synthesis pathway of raffinose from sucrose and galactinol in higher plants and is involved in the regulation of seed development and plant responses to abiotic stresses. In this study, we analyzed the Rafs families and profiled their alternative splicing patterns at the genome-wide scale from 10 grass species representing crops and grasses. A total of 73 Rafs genes were identified from grass species such as rice, maize, foxtail millet, and switchgrass. These Rafs genes were assigned to six groups based the phylogenetic analysis. We compared the gene structures, protein domains, and expression patterns of Rafs genes, and also unraveled the alternative transcripts of them. In addition, different conserved sequences were observed at these putative splice sites among grass species. The subcellular localization of PvRafs5 suggested that the Rafs gene was expressed in the cytoplasm or cell membrane. Our findings provide comprehensive knowledge of the Rafs families in terms of genes and proteins, which will facilitate further functional characterization in grass species in response to abiotic stress.

Raffinose positively regulates maize drought tolerance by reducing leaf transpiration

Drought stress is one of the major constraints of global crop production. Raffinose, a non-reducing trisaccharide, has been considered to regulate positively the plant drought stress tolerance; however, evidence that augmenting raffinose production in leaves results in enhanced plant drought stress tolerance is lacking. The biochemical mechanism through which raffinose might act to mitigate plant drought stress remains unidentified. ZmRAFS encodes Zea mays RAFFINOSE SYNTHASE, a key enzyme that transfers galactose from the galactoside galactinol to sucrose for raffinose production. Overexpression of ZmRAFS in maize increased the RAFS protein and the raffinose content and decreased the water loss of leaves and enhanced plant drought stress tolerance. The biomass of the ZmRAFS overexpressing plants was similar to that of non-transgenic control plants when grown under optimal conditions, but was significantly greater than that of non-transgenic plants when grown under drought stress conditions. In contrast, the percentage of water loss of the detached leaves from two independent zmrafs mutant lines, incapable of synthesizing raffinose, was greater than that from null segregant controls and this phenomenon was partially rescued by supplementation of raffinose to detached zmrafs leaves. In addition, while there were differences in water loss among different maize lines, there was no difference in stomata density or aperture. Taken together, our work demonstrated that overexpression of the ZmRAFS gene in maize, in contrast to Arabidopsis, increased the raffinose content in leaves, assisted the leaf to retain water, and enhanced the plant drought stress tolerance without causing a detectable growth penalty.

Lyoprotectant Formulation and Optimization of the J-Aggregates Astaxanthin/BSA/Chitosan Nanosuspension

Astaxanthin is a carotenoid with excellent antioxidant activity. However, this small lipid-soluble molecule is insoluble in water and has low stability. Although this situation can be improved when astaxanthin is prepared as a nanosuspension, the aqueous form is still not as convenient and safe as the dry powder form for storage, transport, and use. The lyophilization process provides better protection for thermosensitive materials, but this leads to collapse and agglomeration between nanoparticles. To improve this situation, appropriate lyophilization protectants are needed to offer support between the nanoparticles, such as sugars, amino acids, and hydroxy alcohols. The purpose of this work is to screen lyophilization protectants by single-factor experiments and response surface optimization experiments and then explore the optimal ratio of compound lyophilization protectants, and finally, make excellent astaxanthin/BSA/chitosan nanosuspension (ABC-NPs) lyophilized powder. The work shows that the optimal ratio of the compounding lyophilization protectant is 0.46% oligomeric mannose, 0.44% maltose, and 0.05% sorbitol (w/v). The ABC-NPs lyophilized powder prepared under the above conditions had a re-soluble particle size of 472 nm, with a ratio of 1.32 to the particle size of the sample before lyophilization. The lyophilized powder was all in the form of a pink layer. The sample was fluffy and dissolved entirely within 10 s by shaking with water. Consequently, it is expected to solve the problem of inconvenient storage and transportation of aqueous drugs and to expand the application of nanomedicine powders and tablets.

Raffinose Family Oligosaccharides: Friend or Foe for Human and Plant Health?

Raffinose family oligosaccharides (RFOs) are widespread across the plant kingdom, and their concentrations are related to the environment, genotype, and harvest time. RFOs are known to carry out many functions in plants and humans. In this paper, we provide a comprehensive review of RFOs, including their beneficial and anti-nutritional properties. RFOs are considered anti-nutritional factors since they cause flatulence in humans and animals. Flatulence is the single most important factor that deters consumption and utilization of legumes in human and animal diets. In plants, RFOs have been reported to impart tolerance to heat, drought, cold, salinity, and disease resistance besides regulating seed germination, vigor, and longevity. In humans, RFOs have beneficial effects in the large intestine and have shown prebiotic potential by promoting the growth of beneficial bacteria reducing pathogens and putrefactive bacteria present in the colon. In addition to their prebiotic potential, RFOs have many other biological functions in humans and animals, such as anti-allergic, anti-obesity, anti-diabetic, prevention of non-alcoholic fatty liver disease, and cryoprotection. The wide-ranging applications of RFOs make them useful in food, feed, cosmetics, health, pharmaceuticals, and plant stress tolerance; therefore, we review the composition and diversity of RFOs, describe the metabolism and genetics of RFOs, evaluate their role in plant and human health, with a primary focus in grain legumes.

Raffinose synthase enhances drought tolerance through raffinose synthesis or galactinol hydrolysis in maize and Arabidopsis plants

Raffinose and its precursor galactinol accumulate in plant leaves during abiotic stress. RAFFINOSE SYNTHASE (RAFS) catalyzes raffinose formation by transferring a galactosyl group of galactinol to sucrose. However, whether RAFS contributes to plant drought tolerance and, if so, by what mechanism remains unclear. In this study, we report that expression of RAFS from maize (or corn, Zea mays) (ZmRAFS) is induced by drought, heat, cold, and salinity stresses. We found that zmrafs mutant maize plants completely lack raffinose and hyper-accumulate galactinol and are more sensitive to drought stress than the corresponding null-segregant (NS) plants. This indicated that ZmRAFS and its product raffinose contribute to plant drought tolerance. ZmRAFS overexpression in Arabidopsis enhanced drought stress tolerance by increasing myo-inositol levels via ZmRAFS-mediated galactinol hydrolysis in the leaves due to sucrose insufficiency in leaf cells and also enhanced raffinose synthesis in the seeds. Supplementation of sucrose to detached leaves converted ZmRAFS from hydrolyzing galactinol to synthesizing raffinose. Taken together, we demonstrate that ZmRAFS enhances plant drought tolerance through either raffinose synthesis or galactinol hydrolysis, depending on sucrose availability in plant cells. These results provide new avenues to improve plant drought stress tolerance through manipulation of the raffinose anabolic pathway.

ZmDREB1A Regulates RAFFINOSE SYNTHASE Controlling Raffinose Accumulation and Plant Chilling Stress Tolerance in Maize

Raffinose accumulation is positively correlated with plant chilling stress tolerance; however, the understanding of the function and regulation of raffinose metabolism under chilling stress remains in its infancy. RAFFINOSE SYNTHASE (RAFS) is the key enzyme for raffinose biosynthesis. In this study, we report that two independent maize (Zea mays) zmrafs mutant lines, in which raffinose was completely abolished, were more sensitive to chilling stress and their net photosynthetic product (total soluble sugars and starch) accumulation was significantly decreased compared with controls after chilling stress. A similar characterization of the maize dehydration responsive element (DRE)-binding protein 1A mutant (zmdreb1a) showed that ZmRAFS expression and raffinose content were significantly decreased compared with its control under chilling stress. Overexpression of maize ZmDREB1A in maize leaf protoplasts increased ZmDREB1A amounts, which consequently upregulated the expression of maize ZmRAFS and the Renilla LUCIFERASE (Rluc), which was controlled by the ZmRAFS promoter. Deletion of the single dehydration-responsive element (DRE) in the ZmRAFS promoter abolished ZmDREB1A's influence on Rluc expression, while addition of three copies of the DRE in the ZmRAFS promoter dramatically increased Rluc expression when ZmDREB1A was simultaneously overexpressed. Electrophoretic mobility shift assays and chromatin immunoprecipitation-quantitative PCR demonstrated that ZmDREB1A directly binds to the DRE motif in the promoter of ZmRAFS both in vitro and in vivo. These data demonstrate that ZmRAFS, which was directly regulated by ZmDREB1A, enhances both raffinose biosynthesis and plant chilling stress tolerance.

Does Sucrose Change Its Mechanism of Stabilization of Lipid Bilayers during Desiccation? Influences of Hydration and Concentration

Sugar-membrane interactions are believed to be responsible for cell preservation during desiccation and freezing, but the molecular mechanism by which they achieve this is still not well understood. The associated decrease of the main phase transition temperature of phospholipid bilayers is explained by two opposing views on the matter: the direct sugar-phospholipid interaction at the bilayer interface (water replacement hypothesis) and an entropy-driven phase transition with sugar molecules concentrating away from the lipid interface (hydration forces explanation). Both mechanisms are supported by experiments but molecular dynamics (MD) simulations have overwhelmingly shown the occurrence of direct sugar-phospholipid interactions. We have performed MD simulations of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) bilayers at different water and sucrose contents. The behavior of sucrose was found to depend on both the sucrose and water contents: at high sucrose concentration and at low hydration, it is best described by the hydration forces explanation model, whereas at low sucrose concentration, it is consistent with the water replacement hypothesis model. These simulations reveal that at low concentration, sucrose molecules preferentially interact directly with the membrane interface while at high concentration, they preferentially accumulate in the intermembrane solution. The transition between the two modes of interaction is revealed for the first time as being governed by the saturation of the lipid bilayer interface with sucrose molecules, and this occurs more rapidly as the level of hydration decreases.

Folding of intrinsically disordered plant LEA proteins is driven by glycerol-induced crowding and the presence of membranes

Late embryogenesis abundant (LEA) proteins are related to cellular dehydration tolerance. Most LEA proteins are predicted to have no stable secondary structure in solution, i.e., to be intrinsically disordered proteins (IDPs), but they may acquire α-helical structure upon drying. In the model plant Arabidopsis thaliana, the LEA proteins COR15A and COR15B are highly induced upon cold treatment and are necessary for the plants to attain full freezing tolerance. Freezing leads to increased intracellular crowding due to dehydration by extracellular ice crystals. In vitro, crowding by high glycerol concentrations induced partial folding of COR15 proteins. Here, we have extended these investigations to two related proteins, LEA11 and LEA25. LEA25 is much longer than LEA11 and COR15A, but shares a conserved central sequence domain with the other two proteins. We have created two truncated versions of LEA25 (2H and 4H) to elucidate the structural and functional significance of this domain. Light scattering and CD spectroscopy showed that all five proteins were largely unstructured and monomeric in dilute solution. They folded in the presence of increasing concentrations of trifluoroethanol and glycerol. Additional folding was observed in the presence of glycerol and membranes. Fourier transform infra red spectroscopy revealed an interaction of the LEA proteins with membranes in the dry state leading to a depression in the gel to liquid-crystalline phase transition temperature. Liposome stability assays revealed a cryoprotective function of the proteins. The C- and N-terminal extensions of LEA25 were important in cryoprotection, as the central domain itself (2H, 4H) only provided a low level of protection.

Functionality and prevalence of trehalose-based oligosaccharides as novel compatible solutes in ascospores of Neosartorya fischeri (Aspergillus fischeri) and other fungi

Ascospores of Neosartorya, Byssochlamys and Talaromyces can be regarded as the most stress-resistant eukaryotic cells. They can survive exposure at temperatures as high as 85°C for 100 min or more. Neosartorya fischeri ascospores are more viscous and more resistant to the combined stress of heat and desiccation than the ascospores of Talaromyces macrosporus which contain predominantly trehalose. These ascospores contain trehalose-based oligosaccharides (TOS) that are novel compatible solutes, which are accumulated to high levels. These compounds are also found in other members of the genus Neosartorya and in some other genera within the order Eurotiales that also include Byssochlamys and Talaromyces. The presence of oligosaccharides was observed in species that had a relatively high growth temperature. TOS glasses have a higher glass transition temperature (Tg ) than trehalose, and they form a stable glass with crystallizing molecules, such as mannitol. Our data indicate that TOS are important for prolonged stabilization of cells against stress. The possible unique role of these solutes in protection against dry heat conditions is discussed.

A decrease in bulk water and mannitol and accumulation of trehalose and trehalose-based oligosaccharides define a two-stage maturation process towards extreme stress resistance in ascospores of Neosartorya fischeri (Aspergillus fischeri)

Fungal propagules survive stresses better than vegetative cells. Neosartorya fischeri, an Aspergillus teleomorph, forms ascospores that survive high temperatures or drying followed by heat. Not much is known about maturation and development of extreme stress resistance in fungal cells. This study provides a novel two-step model for the acquisition of extreme stress resistance and entry into dormancy. Ascospores of 11- and 15-day-old cultures exhibited heat resistance, physiological activity, accumulation of compatible solutes and a steep increase in cytoplasmic viscosity. Electron spin resonance spectroscopy indicated that this stage is associated with the removal of bulk water and an increase of chemical stability. Older ascospores from 15- to 50-day-old cultures showed no changes in compatible solute content and cytoplasmic viscosity, but did exhibit a further increase of heat resistance and redox stability with age. This stage was also characterized by changes in the composition of the mixture of compatible solutes. Mannitol levels decreased and the relative quantities of trehalose and trehalose-based oligosaccharides increased. Dormant ascospores of N. fischeri survive in low-water habitats. After activation of the germination process, the stress resistance decreases, compatible solutes are degraded and the cellular viscosity drops. After 5 h, the hydrated cells enter the vegetative stage and redox stability has decreased notably.

Comparative study of transgenic Brachypodium distachyon expressing sucrose:fructan 6-fructosyltransferases from wheat and timothy grass with different enzymatic properties

Fructans can act as cryoprotectants and contribute to freezing tolerance in plant species, such as in members of the grass subfamily Pooideae that includes Triticeae species and forage grasses. To elucidate the relationship of freezing tolerance, carbohydrate composition and degree of polymerization (DP) of fructans, we generated transgenic plants in the model grass species Brachypodium distachyon that expressed cDNAs for sucrose:fructan 6-fructosyltransferases (6-SFTs) with different enzymatic properties: one cDNA encoded PpFT1 from timothy grass (Phleum pratense), an enzyme that produces high-DP levans; a second cDNA encoded wft1 from wheat (Triticum aestivum), an enzyme that produces low-DP levans. Transgenic lines expressing PpFT1 and wft1 showed retarded growth; this effect was particularly notable in the PpFT1 transgenic lines. When grown at 22 °C, both types of transgenic line showed little or no accumulation of fructans. However, after a cold treatment, wft1 transgenic plants accumulated fructans with DP = 3-40, whereas PpFT1 transgenic plants accumulated fructans with higher DPs (20 to the separation limit). The different compositions of the accumulated fructans in the two types of transgenic line were correlated with the differences in the enzymatic properties of the overexpressed 6-SFTs. Transgenic lines expressing PpFT1 accumulated greater amounts of mono- and disaccharides than wild type and wft1 expressing lines. Examination of leaf blades showed that after cold acclimation, PpFT1 overexpression increased tolerance to freezing; by contrast, the freezing tolerance of the wft1 expressing lines was the same as that of wild type plants. These results provide new insights into the relationship of the composition of water-soluble carbohydrates and the DP of fructans to freezing tolerance in plants.

Interactions of the amphiphiles arbutin and tryptophan with phosphatidylcholine and phosphatidylethanolamine bilayers in the dry state

BACKGROUND

Water is essential for life, but some organisms can survive complete desiccation, while many more survive partial dehydration during drying or freezing. The function of some protective molecules, such as sugars, has been extensively studied, but much less is known about the effects of amphiphiles such as flavonoids and other aromatic compounds. Amphiphiles may be largely soluble under fully hydrated conditions, but will partition into membranes upon removal of water. Little is known about the effects of amphiphiles on membrane stability and how amphiphile structure and function are related. Here, we have used two of the most intensively studied amphiphiles, tryptophan (Trp) and arbutin (Arb), along with their isolated hydrophilic moieties glycine (Gly) and glucose (Glc) to better understand structure-function relationships in amphiphile-membrane interactions in the dry state.

RESULTS

Fourier-transform infrared (FTIR) spectroscopy was used to measure gel-to-liquid crystalline phase transition temperatures (Tm) of liposomes formed from phosphatidylcholine and phosphatidylethanolamine in the presence of the different additives. In anhydrous samples, both Glc and Arb strongly depressed Tm, independent of lipid composition, while Gly had no measurable effect. Trp, on the other hand, either depressed or increased Tm, depending on lipid composition. We found no evidence for strong interactions of any of the compounds with the lipid carbonyl or choline groups, while all additives except Gly seemed to interact with the phosphate groups. In the case of Arb and Glc, this also had a strong effect on the sugar OH vibrations in the FTIR spectra. In addition, vibrations from the hydrophobic indole and phenol moieties of Trp and Arb, respectively, provided evidence for interactions with the lipid bilayers.

CONCLUSIONS

The two amphiphiles Arb and Trp interact differently with dry bilayers. The interactions of Arb are dominated by contributions of the Glc moiety, while the indole governs the effects of Trp. In addition, only Trp-membrane interactions showed a strong influence of lipid composition. Further investigations, using the large structural diversity of plant amphiphiles will help to understand how their structure determines the interaction with membranes and how that influences their biological functions, for example under freezing or dehydration conditions.

Mapping quantitative trait loci for freezing tolerance in a recombinant inbred line population of Arabidopsis thaliana accessions Tenela and C24 reveals REVEILLE1 as negative regulator of cold acclimation

The ability to increase freezing tolerance when exposed to low temperatures is a property of many plant species from temperate climates and involves a wide array of metabolic adjustments and changes in gene expression. In Arabidopsis thaliana, natural accessions show high variation in their acclimation capacity, and freezing tolerance correlates with natural habitat temperatures. To investigate the genetic basis of this variation, a recombinant inbred line population from reciprocal crosses between the accessions C24 and Tenela (Te), showing large variation in tolerance, was established. Over 250 recombinant inbred lines were genotyped for 69 single nucleotide polymorphism markers in a linkage map with 391.9 centimorgans (cM) and phenotyped for their freezing tolerance using the electrolyte leakage method that reports cell damage after a freeze-thaw cycle. Mapping of quantitative trait loci (QTL) for acclimated plants revealed three QTL regions on chromosomes 2, 4 and 5. Based on gene expression data, QTL regions were screened for genes differentially responding to low temperature in C24 and Te. Among the candidate genes, the Myb family transcription factor REVEILLE1 (At5g17300) on chromosome 5 was identified as a novel negative regulator of freezing tolerance in Arabidopsis.

Effect of physical properties on the stability of Lactobacillus bulgaricus in a freeze-dried galacto-oligosaccharides matrix

The ability of galacto-oligosaccharides (GOS) to protect Lactobacillus delbrueckii subsp. bulgaricus upon freeze drying was analyzed on the basis of their capacity to form glassy structures. Glass transition temperatures (T(g)) of a GOS matrix at various relative humidities (RH) were determined by DSC. Survival of L. bulgaricus in a glassy GOS matrix was investigated after freezing, freeze drying, equilibration at different RHs and storage at different temperatures. At 32 °C, a drastic viability loss was observed. At 20 °C, the survival was affected by the water content, having the samples stored at lower RHs, the highest survival percentages. At 4°C, no decay in the cells count was observed after 45 days of storage. The correlation between molecular mobility [as measured by Proton nuclear magnetic resonance (¹H NMR)] and loss of viability explained the efficiency of GOS as cryoprotectants. The preservation of microorganisms was improved at low molecular mobility and this condition was obtained at low water contents and low storage temperatures. These results are important in the developing of new functional foods containing pre and probiotics.

Evidence for a role of raffinose in stabilizing photosystem II during freeze-thaw cycles

A role of non-reducing sugars like sucrose and raffinose in the protection of plant cells against damage during freezing has been proposed for many species, but reports on physiological effects are conflicting. Non-aqueous fractionation of mesophyll cell compartments in Arabidopsis thaliana was used to show that sucrose and raffinose accumulate in plastids during low temperatures, pointing to a physiological role in protecting the photosynthetic apparatus. Comparing a previously described raffinose synthase (RS) mutant of A. thaliana with its corresponding wild type, accession Col-0, revealed that a lack of raffinose has no effect on electrolyte leakage from leaf cells after freeze-thaw cycles, supporting that raffinose is not essential for protecting the plasma membrane. However, in situ chlorophyll fluorescence showed that maximum quantum yield of PS II photochemistry (F (v)/F (m)) and other fluorescence parameters of cold acclimated leaves subjected to freeze-thaw cycles were significantly lower in the raffinose synthase mutant than in the corresponding wild type, indicating that raffinose is involved in stabilizing PS II of cold acclimated leaf cells against damage during freezing.

Thermotropic phase behavior and headgroup interactions of the nonbilayer lipids phosphatidylethanolamine and monogalactosyldiacylglycerol in the dry state

BACKGROUND

Although biological membranes are organized as lipid bilayers, they contain a substantial fraction of lipids that have a strong tendency to adopt a nonlamellar, most often inverted hexagonal (HII) phase. The polymorphic phase behavior of such nonbilayer lipids has been studied previously with a variety of methods in the fully hydrated state or at different degrees of dehydration. Here, we present a study of the thermotropic phase behavior of the nonbilayer lipids egg phosphatidylethanolamine (EPE) and monogalactosyldiacylglycerol (MGDG) with a focus on interactions between the lipid molecules in the interfacial and headgroup regions.

RESULTS

Liposomes were investigated in the dry state by Fourier-transform Infrared (FTIR) spectroscopy and Differential Scanning Calorimetry (DSC). Dry EPE showed a gel to liquid-crystalline phase transition below 0°C and a liquid-crystalline to HII transition at 100°C. MGDG, on the other hand, was in the liquid-crystalline phase down to -30°C and showed a nonbilayer transition at about 85°C. Mixtures (1:1 by mass) with two different phosphatidylcholines (PC) formed bilayers with no evidence for nonbilayer transitions up to 120°C. FTIR spectroscopy revealed complex interactions between the nonbilayer lipids and PC. Strong H-bonding interactions occurred between the sugar headgroup of MGDG and the phosphate, carbonyl and choline groups of PC. Similarly, the ethanolamine moiety of EPE was H-bonded to the carbonyl and choline groups of PC and probably interacted through charge pairing with the phosphate group.

CONCLUSIONS

This study provides a comprehensive characterization of dry membranes containing the two most important nonbilayer lipids (PE and MGDG) in living cells. These data will be of particular relevance for the analysis of interactions between membranes and low molecular weight solutes or soluble proteins that are presumably involved in cellular protection during anhydrobiosis.

Expression of three galactinol synthase isoforms in Coffea arabica L. and accumulation of raffinose and stachyose in response to abiotic stresses

Galactinol synthase (EC 2.4.1.123; GolS) catalyzes the first step in the synthesis of raffinose family oligosaccharides (RFOs). Their accumulation in response to abiotic stresses implies a role for RFOs in stress adaptation. In this study, the expression patterns of three isoforms of galactinol synthase (CaGolS1-2-3) from Coffea arabica were evaluated in response to water deficit, salinity and heat stress. All CaGolS isoforms were highly expressed in leaves while little to no expression were detected in flower buds, flowers, plagiotropic shoots, roots, endosperm and pericarp of mature fruits. Transcriptional analysis indicated that the genes were differentially regulated under water deficit, high salt and heat stress. CaGolS1 isoform is constitutively expressed in plants under normal growth conditions and was the most responsive during all stress treatments. CaGolS2 is unique among the three isoforms in that it was detected only under severe water deficit and salt stresses. CaGolS3 was primarily expressed under moderate and severe drought. This isoform was induced only at the third day of heat and under high salt stress. The increase in GolS transcription was not reflected into the amount of galactinol in coffee leaves, as specific glycosyltransferases most likely used galactinol to transfer galactose units to higher homologous oligosaccharides, as suggested by the increase of raffinose and stachyose during the stresses.

Frost tolerance in excised leaves of the common bugle (Ajuga reptans L.) correlates positively with the concentrations of raffinose family oligosaccharides (RFOs)

Mass increases in raffinose family oligosaccharides (RFOs, alpha1,6-galactosyl extensions of sucrose) are well documented in the generative tissues of many plants upon cold acclimation, and they (i.e. mainly the two shortest RFO members, raffinose and stachyose) have been suggested as frost stress protectants. Our focus here was on the longer RFO members as they commonly occur in the frost-hardy evergreen labiate Ajuga reptans in its natural habitat, and accumulate to their highest concentrations in winter when the plant is faced with sub-zero temperatures. We examined the effects of RFO concentration and chain length on frost tolerance using excised leaves which accumulate long-chain RFOs under both cold and warm conditions, thereby uncoupling the acclimation temperature from RFO production. We demonstrated that frost tolerance in excised A. reptans leaves correlates positively with long-chain RFO accumulation under both acclimation temperatures. After 24 d post-excision in the warm, the leaves had increased their RFO concentrations (mainly long-chain RFOs) 22-fold to 78 mg g(-1) fresh weight, and decreased their EL(50) values (temperature at which 50% leakage occurred) from -10.5 to -24.5 degrees C, suggesting a protective role for these oligosaccharides in the natural frost tolerance of A. reptans.

Fructan and its relationship to abiotic stress tolerance in plants

Numerous studies have been published that attempted to correlate fructan concentrations with freezing and drought tolerance. Studies investigating the effect of fructan on liposomes indicated that a direct interaction between membranes and fructan was possible. This new area of research began to move fructan and its association with stress beyond mere correlation by confirming that fructan has the capacity to stabilize membranes during drying by inserting at least part of the polysaccharide into the lipid headgroup region of the membrane. This helps prevent leakage when water is removed from the system either during freezing or drought. When plants were transformed with the ability to synthesize fructan, a concomitant increase in drought and/or freezing tolerance was confirmed. These experiments indicate that besides an indirect effect of supplying tissues with hexose sugars, fructan has a direct protective effect that can be demonstrated by both model systems and genetic transformation.

Dissecting the regulation of fructan metabolism in chicory (Cichorium intybus) hairy roots

Fifteen per cent of higher plants accumulate fructans. Plant development, nutritional status and stress exposure all affect fructan metabolism, and while fructan biochemistry is well understood, knowledge of its regulation has remained fragmentary. Here, we have explored chicory (Cichorium intybus) hairy root cultures (HRCs) to study the regulation of fructan metabolism in sink tissues in response to environmental cues. In standard medium (SM), HRCs did not accumulate inulin. However, upon transfer to high-carbon (C)/low-nitrogen (N) medium, expression of sucrose:sucrose 1-fructosyltransferase (1-SST) and fructan:fructan 1-fructosyltransferase (1-FFT) was strongly induced and inulin accumulated. Upon return to SM, inulin was degraded, together with a coordinate decline of 1-SST and 1-FFT expression. In HRCs, cold-induced expression of fructan 1-exohydrolases (1-FEH I and IIa) was similar to cold induction in taproots, even in the absence of accumulated inulin. For high-C/low-N induction of 1-SST and 1-FFT, and cold induction of 1-FEH I and IIa, the signaling pathways were addressed. While 1-SST and 1-FFT induction was similarly prevented by inhibitors of Ca(2+) signaling, protein kinases and phosphatases, cold induction of 1-FEH I and IIa revealed distinct signaling pathways. In summary, this study has established chicory HRCs as a convenient experimental system with which to study the regulation of fructan active enzyme (FAZY) expression in heterotrophic cells.

Cloning and functional analysis of a fructosyltransferase cDNA for synthesis of highly polymerized levans in timothy (Phleum pratense L.)

Variation in the structures of plant fructans and their degree of polymerization (DP) can be explained as the result of diverse combinations of fructosyltransferases (FTs) with different properties. Although FT genes have been isolated in a range of plant species, sucrose:fructan 6-fructosyltransferase (6-SFT) cDNAs have only been functionally characterized in a few species such as wheat. A novel FT cDNA possessing 6-SFT activity has been identified and characterized from the temperate forage grass, timothy (Phleum pratense L.). The cDNA of an FT homolog, PpFT1, was isolated from cold-acclimated timothy. A recombinant PpFT1 protein expressed in Pichia pastoris showed 6-SFT/sucrose:sucrose 1-fructosyltransferase (1-SST) activity and produced linear beta(2,6)-linked levans from sucrose with higher DPs than present in graminans formed in vitro by wheat 6-SFT (Wft1). PpFT1 and Wft1 showed remarkably different acceptor substrate specificities: PpFT1 had high affinity for 6-kestotriose to produce levans and low affinity for 1-kestotriose, whereas Wft1 preferentially used 1-kestotriose as an acceptor. The affinity of the PpFT1 recombinant enzyme for sucrose as a substrate was lower than that of the Wft1 recombinant enzyme. It is also confirmed that timothy seedlings had elevated levels of PpFT1 transcripts during the accumulation of fructans under high sucrose and cold conditions. Our results suggest that PpFT1 is a novel cDNA with unique enzymatic properties that differ from those of previously cloned plant 6-SFTs, and is involved in the synthesis of highly polymerized levans in timothy.

Environmentally friendly formulations of alachlor and atrazine: preparation, characterization, and reduced leaching

Atrazine and alachlor formulations were designed by encapsulating the herbicide molecules into phosphatidylcholine (PC) vesicles, which subsequently were adsorbed on montmorillonite. PC and montmorillonite are classified as substances of minimal toxicological risk by the U.S. EPA. PC enhanced alachlor and atrazine solubilities by 15- and 18-fold, respectively. A 6 mM PC:5 g/L clay ratio was found as optimal for PC adsorption on the clay. Active ingredient contents of the PC-clay formulations ranged up to 8.6% for atrazine and 39.5% for alachlor. Infrared spectroscopy showed hydrophobic interactions of herbicide molecules with the alkyl chains of PC, in addition to hydrophilic interactions with the PC headgroup. Release experiments in a sandy soil showed a slower rate from the PC-clay formulations than the commercial ones. Soil column experiments under moderate irrigation and bioactivity experiments indicate that a reduction in the recommended dose of alachlor and atrazine can be accomplished by using PC-clay formulations.

Freezing tolerance by vesicle-mediated fructan transport

Fructans are fructose-based polymers associated with freezing tolerance. They might act directly via membrane stabilization or indirectly by stimulating alternative cryoprotectants. Fructans and fructan biosynthetic enzymes, in general, are believed to be present in the vacuole. This paper draws particular attention to the surprising presence of fructans and fructan exohydrolase activity in the apoplast of cold-stressed plants. This observation raises questions concerning the origin of apoplastic fructans and suggests that fructans are transported to the apoplast by post-synthesis mechanisms, perhaps induced by cold. We propose a conceptual vesicle-mediated transport model for the movement of vacuolar fructans to the apoplast, where they could assist in stabilizing the plasma membrane.

Vitrification is essential for anhydrobiosis in an African chironomid, Polypedilum vanderplanki

Anhydrobiosis is an extremely dehydrated state in which organisms show no detectable metabolism but retain the ability to revive after rehydration. Thus far, two hypotheses have been proposed to explain how cells are protected during dehydration: (i) water replacement by compatible solutes and (ii) vitrification. The present study provides direct physiological and physicochemical evidence for these hypotheses in an African chironomid, Polypedilum vanderplanki, which is the largest multicellular animal capable of anhydrobiosis. Differential scanning calorimetry measurements and Fourier-transform infrared (FTIR) analyses indicated that the anhydrobiotic larvae were in a glassy state up to as high as 65 degrees C. Changing from the glassy to the rubbery state by either heating or allowing slight moisture uptake greatly decreased the survival rate of dehydrated larvae. In addition, FTIR spectra showed that sugars formed hydrogen bonds with phospholipids and that membranes remained in the liquid-crystalline state in the anhydrobiotic larvae. These results indicate that larvae of P. vanderplanki survive extreme dehydration by replacing the normal intracellular medium with a biological glass. When entering anhydrobiosis, P. vanderplanki accumulated nonreducing disaccharide trehalose that was uniformly distributed throughout the dehydrated body by FTIR microscopic mapping image. Therefore, we assume that trehalose plays important roles in water replacement and intracellular glass formation, although other compounds are surely involved in these phenomena.

Plant fructans in stress environments: emerging concepts and future prospects

Plants are sessile and sensitive organisms known to possess various regulatory mechanisms for defending themselves under stress environments. Fructans are fructose-based polymers synthesized from sucrose by fructosyltransferases (FTs). They have been increasingly recognized as protective agents against abiotic stresses. Using model membranes, numerous in vitro studies have demonstrated that fructans can stabilize membranes by direct H-bonding to the phosphate and choline groups of membrane lipids, resulting in a reduced water outflow from the dry membranes. Inulin-type fructans are flexible random-coiled structures that can adopt many conformations, allowing them to insert deeply within the membranes. The devitrification temperature (T(g)) can be adjusted by their varying molecular weights. In addition, above T(g) their low crystallization rates ensure prolonged membrane protection. Supporting, in vivo studies with transgenic plants expressing FTs showed fructan accumulation and an associated improvement in freezing and/or chilling tolerance. The water-soluble nature of fructans may allow their rapid adaptation as cryoprotectants in order to give optimal membrane protection. One of the emerging concepts for delivering vacuolar fructans to the extracellular space for protecting the plasma membrane is vesicle-mediated, tonoplast-derived exocytosis. It should, however, be noted that natural stress tolerance is a very complex process that cannot be explained by the action of a single molecule or mechanism.

Protection of liposomes against fusion during drying by oligosaccharides is not predicted by the calorimetric glass transition temperatures of the dry sugars

Sugars play an important role in the desiccation tolerance of most anhydrobiotic organisms. It has been shown in previous studies that different structural families of oligosaccharides have different efficacies to interact with phospholipid headgroups and protect membranes from solute leakage during drying. Here, we have compared three families of linear oligosaccharides (fructans (inulins), malto-oligosaccharides, manno-oligosaccharides) for their chain-length dependent protection of egg phosphatidylcholine liposomes against membrane fusion. We found increased protection with chain length up to a degree of polymerization (DP) of 5 for malto-oligosaccharides, and a decrease for inulins and manno-oligosaccharides. Differential scanning calorimetry measurements showed that for all sugars the glass transition temperature (T_g) increased with DP, although to different degrees for the different oligosaccharide families. Higher T_g values resulted in reduced membrane fusion only for malto-oligosaccharides below DP5. Contrary to expectation, for inulins, manno-oligosaccharides and malto-oligosaccharides of a DP above five, fusion increased with increasing T_g, indicating that other physical parameters are more important in determining the ability of different sugars to protect membranes against fusion during drying. Further research will be necessary to experimentally define such parameters.

Analysis of Quil A–phospholipid mixtures using drift spectroscopy

The aim of this study was to investigate molecular interactions between Quil A and phosphatidylcholine in the solid state using diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS). Analysis of the interactions was characterized on the different regions of phosphatidylcholine: hydrophobic chain, interfacial and headgroup regions. The spectra of the hydrocarbon region of phosphatidylcholine alone compared to that for the binary mixture of Quil A and phosphatidylcholine were similar. These findings suggest that Quil A did not cause conformational disorder of the fatty acyl chains of the phospholipid. In contrast, a shift in the wavenumber of the choline group and a broad band in this moiety indicate a modification of the phospholipid in the headgroup region due to interaction between Quil A and phosphatidylcholine. These results suggest possibly ionic interactions between the negatively charged glucuronic acid moiety of the Quil A molecule with the positively charged choline group. The findings could also be the result of conformational changes in the choline group because of the intercalation of sugar moieties in Quil A between the choline and phosphate groups due to hydrogen bonding. Shift of wavenumbers to lower values on the carbonyl group was observed suggesting hydrogen bonding between Quil A and phosphatidylcholine. The difference in degrees of wavenumber shift (choline>phosphate>carbonyl group) and observed broad bands indicated that Quil A preferentially interacted with phosphatidylcholine on the hydrophilic headgroup. Cholesterol influenced such interactions at relatively high concentration (60%, w/w).

Effects of cholesterol on dry bilayers: interactions between phosphatidylcholine unsaturation and glycolipid or free sugar

Cholesterol and other sterols are important components of biological membranes and are known to strongly influence the physical characteristics of lipid bilayers. Although this has been studied extensively in fully hydrated membranes, little is known about the effects of cholesterol on the stability of membranes in the dry state. Here, we present a Fourier transform infrared spectroscopy study on the effects of cholesterol on the phase behavior of dry liposomes composed of phosphatidylcholines with different degrees of fatty acid unsaturation or of mixtures of phosphatidylcholine with a plant galactolipid. In addition, we have analyzed the H-bonding of cholesterol, galactose, and a combination of the two additives to the P=O and C=O groups in dry phosphatidylcholine bilayers. The data indicate a complex balance of interactions between the different components in the dry state and a strong influence of fatty acid unsaturation on the interactions of the diacyl lipids with both cholesterol and galactose.

Fructans from oat and rye: composition and effects on membrane stability during drying

Fructans have been implicated in the abiotic stress tolerance of many plant species, including grasses and cereals. To elucidate the possibility that cereal fructans may stabilize cellular membranes during dehydration, we used liposomes as a model system and isolated fructans from oat (Avena sativa) and rye (Secale cereale). Fructans were fractionated by preparative size exclusion chromatography into five defined size classes (degree of polymerization (DP) 3 to 7) and two size classes containing high DP fructans (DP>7 short and long). They were characterized by high performance liquid chromatography (HPLC) and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). The effects of the fructans on liposome stability during drying and rehydration were assessed as the ability of the sugars to prevent leakage of a soluble marker from liposomes and liposome fusion. Both species contain highly complex mixtures of fructans, with a DP up to 17. The two DP>7 fractions from both species were unable to protect liposomes, while the fractions containing smaller fructans were protective to different degrees. Protection showed an optimum at DP 4 and the DP 3, 4, and 5 fractions from oat were more protective than all other fractions from both species. In addition, we found evidence for synergistic effects in membrane stabilization in mixtures of low DP with DP>7 fructans. The data indicate that cereal fructans have the ability to stabilize membranes under stress conditions and that there are size and species dependent differences between the fructans. In addition, mixtures of fructans, as they occur in living cells may have protective properties that differ significantly from those of the purified fractions.