Fructan Metabolism in Cereals: Induction in Leaves and Compartmentation in Protoplasts and Vacuoles

Generation and characterization of two new monoclonal antibodies produced by immunizing mice with plant fructans: New tools for immunolocalization of β-(2 → 1) and β-(2 → 6) fructans

Fructans are water-soluble polymers of fructose in which fructose units are linked by β-(2 → 1) and/or β-(2 → 6) linkages. In plants, they are synthesized in the vacuole but have also been reported in the apoplastic sap under abiotic stress suggesting that they are involved in plasmalemma protection and in plant-microbial interactions. However, the lack of fructan-specific antibodies currently prevents further study of their role and the associated mechanisms of action, which could be elucidated thanks to their immunolocalization. We report the production of two monoclonal antibodies (named BTM9H2 and BTM15A6) using mice immunization with antigenic compounds prepared from a mixture of plant inulins and levans conjugated to serum albumin. Their specificity towards fructans with β-(2 → 1) and/or β-(2 → 6) linkage has been demonstrated by immuno-dot blot tests on a wide range of carbohydrates. The two mAbs were used for immunocytolocalization of fructans by epifluorescence microscopy in various plant species. Fructan epitopes were specifically detected in fructan-accumulating plants, inside cells as well as on the surface of root tips, confirming both extracellular and intracellular localizations. The two mAbs provide new tools to identify the mechanism of extracellular fructan secretion and explore the roles of fructans in stress resistance and plant-microorganism interactions.

18 O enrichment of sucrose and photosynthetic and nonphotosynthetic leaf water in a C3 grass-atmospheric drivers and physiological relations

The ¹⁸ O enrichment (Δ¹⁸ O) of leaf water affects the Δ¹⁸ O of photosynthetic products such as sucrose, generating an isotopic archive of plant function and past climate. However, uncertainty remains as to whether leaf water compartmentation between photosynthetic and nonphotosynthetic tissue affects the relationship between Δ¹⁸ O of bulk leaf water (Δ¹⁸ O_LW ) and leaf sucrose (Δ¹⁸ O_Sucrose ). We grew Lolium perenne (a C₃ grass) in mesocosm-scale, replicated experiments with daytime relative humidity (50% or 75%) and CO₂ level (200, 400 or 800 μmol mol^-1 ) as factors, and determined Δ¹⁸ O_LW , Δ¹⁸ O_Sucrose and morphophysiological leaf parameters, including transpiration (E_leaf ), stomatal conductance (g_s ) and mesophyll conductance to CO₂ (g_m ). The Δ¹⁸ O of photosynthetic medium water (Δ¹⁸ O_SSW ) was estimated from Δ¹⁸ O_Sucrose and the equilibrium fractionation between water and carbonyl groups (ε_bio ). Δ¹⁸ O_SSW was well predicted by theoretical estimates of leaf water at the evaporative site (Δ¹⁸ O_e ) with adjustments that correlated with gas exchange parameters (g_s or total conductance to CO₂ ). Isotopic mass balance and published work indicated that nonphotosynthetic tissue water was a large fraction (~0.53) of bulk leaf water. Δ¹⁸ O_LW was a poor proxy for Δ¹⁸ O_Sucrose , mainly due to opposite Δ¹⁸ O responses of nonphotosynthetic tissue water (Δ¹⁸ O_non-SSW ) relative to Δ¹⁸ O_SSW , driven by atmospheric conditions.

The role of SWEET4 proteins in the post-phloem sugar transport pathway of Setaria viridis sink tissues

In the developing seeds of all higher plants, filial cells are symplastically isolated from the maternal tissue supplying photosynthate to the reproductive structure. Photoassimilates must be transported apoplastically, crossing several membrane barriers, a process facilitated by sugar transporters. Sugars Will Eventually be Exported Transporters (SWEETs) have been proposed to play a crucial role in apoplastic sugar transport during phloem unloading and the post-phloem pathway in sink tissues. Evidence for this is presented here for developing seeds of the C4 model grass Setaria viridis. Using immunolocalization, SvSWEET4 was detected in various maternal and filial tissues within the seed along the sugar transport pathway, in the vascular parenchyma of the pedicel, and in the xylem parenchyma of the stem. Expression of SvSWEET4a in Xenopus laevis oocytes indicated that it functions as a high-capacity glucose and sucrose transporter. Carbohydrate and transcriptional profiling of Setaria seed heads showed that there were some developmental shifts in hexose and sucrose content and consistent expression of SvSWEET4 homologues. Collectively, these results provide evidence for the involvement of SWEETs in the apoplastic transport pathway of sink tissues and allow a pathway for post-phloem sugar transport into the seed to be proposed.

Achieving of high-diet-fiber barley via managing fructan hydrolysis

High fructan content in the grain of cereals is an important trait in agriculture such as environmental resilience and dietary fiber food production. To understand the mechanism in determining final grain fructan content and achieve high fructan cereal, a cross breeding strategy based on fructan synthesis and hydrolysis activities was set up and have achieved barley lines with 11.8% storage fructan in the harvested grain. Our study discovered that high activity of fructan hydrolysis at later grain developmental stage leads to the low fructan content in mature seeds, simultaneously increasing fructan synthesis at early stage and decreasing fructan hydrolysis at later stage through crossing breeding is an efficient way to elevate grain diet-fiber content. A good correlation between fructan and beta glucans was also discovered with obvious interest. Field trials showed that the achieved high fructan barley produced over seven folds higher fructan content than control barley and pull carbon-flux to fructan through decreasing fructan hydrolysis without disruption starch synthesis will probably not bring yield deficiency.

Interaction between fructan metabolism and plant growth regulators

The relationship of fructan to plant growth regulators is clearly more complicated than it looks and is likely related to differences between fructan molecules in size and structure as well as localization. Fructans are a complex group of carbohydrates composed mainly of fructose units linked to a sucrose molecule. Fructans are present in plants as heterogeneous mixtures with diverse molecular structures and mass, different polymerization degrees, and linkage types between fructosyl residues. Like sucrose, they are frequently stored in leaves and other organs, acting as carbohydrate reserves. Fructans are synthesized in the cell vacuole by fructosyltransferase enzymes and catabolized by fructan exohydrolase enzymes. Several publications have shown that fructan metabolism varies with the stage of plant development and in response to the environment. Recent studies have shown a correlation between plant growth regulators (PGR), fructan metabolism, and tolerance to drought and cold. PGR are compounds that profoundly influence the growth and differentiation of plant cells, tissues, and organs. They play a fundamental role in regulating plant responses to developmental and environmental signals. In this review, we summarize the most up-to-date knowledge on the metabolism of fructans and their crosstalk with PGR signaling pathways. We identify areas that require more research to complete our understanding of the role of fructans in plants.

Diurnal Variation in Forage Nutrient Composition of Mixed Cool-Season Grass, Crabgrass, and Bermudagrass Pastures

Warm-season grasses (WSG) have been suggested as alternative low non-structural carbohydrate (NSC) pasture forages. The purpose of this study was to evaluate nutrient composition and diurnal changes in soluble carbohydrates for the warm-season annual 'Quick-N-Big' crabgrass [CRB; Digitaria sanguinalis (L.) Scop.] and the warm-season perennial 'Wrangler' bermudagrass [BER; Cynodon dactylon (L.) Pers] in comparison to mixed cool-season grass [CSG; 'Inavale' orchardgrass (Dactylis glomerata [L.]), 'Tower' tall fescue (Lolium arundinaceum [Schreb.] Darbysh.), and 'Argyle' Kentucky bluegrass (Poa pratensis [L.])]. Samples were collected at 4-h intervals over 3 d when each forage reached the boot stage of maturity. Digestible energy was greatest for CSG (2.29 ± 0.34 Mcal/kg) and lowest for BER (2.13 ± 0.34 Mcal/kg), while crude protein was lowest for CSG (16.1 ± 0.29%) and neutral detergent fiber was greatest for BER (60.0 ± 0.41; P ≤ 0.0008). Non-structural carbohydrates were greater for CSG (17.6 ± 0.26%) compared to BER (10.6 ± 0.26%) or CRB (10.9 ± 0.26%; P < 0.0001). Overall, NSC was greatest in the afternoon and evening (14.5 - 14.9 ± 0.60%) and lowest in the early morning (11.2 - 11.4 ± 0.60%; P ≤ 0.04), but diurnal variation was most pronounced in CSG vs. either WSG. Results of this study provide needed data on nutritional composition of CRB and BER and demonstrate that these grasses may serve as pasture forages for horses where NSC intake is of concern. Results also support recommendations for restricting grazing to early morning to limit NSC consumption, particularly in CSG pastures.

Genome-wide association study reveals the genetic complexity of fructan accumulation patterns in barley grain

We profiled the grain oligosaccharide content of 154 two-row spring barley genotypes and quantified 27 compounds, mainly inulin- and neoseries-type fructans, showing differential abundance. Clustering revealed two profile groups where the 'high' set contained greater amounts of sugar monomers, sucrose, and overall fructans, but lower fructosylraffinose. A genome-wide association study (GWAS) identified a significant association for the variability of two fructan types: neoseries-DP7 and inulin-DP9, which showed increased strength when applying a novel compound ratio-GWAS approach. Gene models within this region included three known fructan biosynthesis genes (fructan:fructan 1-fructosyltransferase, sucrose:sucrose 1-fructosyltransferase, and sucrose:fructan 6-fructosyltransferase). Two other genes in this region, 6(G)-fructosyltransferase and vacuolar invertase1, have not previously been linked to fructan biosynthesis and showed expression patterns distinct from those of the other three genes, including exclusive expression of 6(G)-fructosyltransferase in outer grain tissues at the storage phase. From exome capture data, several single nucleotide polymorphisms related to inulin- and neoseries-type fructan variability were identified in fructan:fructan 1-fructosyltransferase and 6(G)-fructosyltransferase genes. Co-expression analyses uncovered potential regulators of fructan biosynthesis including transcription factors. Our results provide the first scientific evidence for the distinct biosynthesis of neoseries-type fructans during barley grain maturation and reveal novel gene candidates likely to be involved in the differential biosynthesis of various types of fructan in barley.

Fructan and antioxidant metabolisms in plants of Lolium perenne under drought are modulated by exogenous nitric oxide

Drought is a major environmental factor that can trigger oxidative stress and affect plant growth and productivity. Previous studies have shown that exogenous nitric oxide (NO) can minimize oxidative stress-related damage through the modulation of antioxidant enzyme activity. Fructan accumulation also has an important role in drought tolerance, since these carbohydrates participate in osmoregulation, membrane protection and oxidant scavenging. Currently, there are few studies investigating NO-regulated fructan metabolism in response to abiotic stresses. In the present study, we sought to determine if treating plants of Lolium perenne with S-nitrosoglutathione (GSNO), a NO donor, improved drought tolerance. Two-month-old plants received water (control), GSNO and reduced glutathione (GSH) as foliar spray treatments and were then maintained under drought or well-watered conditions for 23 days. At the end of drought period, we evaluated growth, pigment content and antioxidant and fructan metabolisms. None of these conditions influenced dry mass accumulation, but the leaves of plants treated with GSNO exhibited a slight increase in pigment content under drought. GSNO treatment also induced 1-SST activity, which was associated with a 3-fold increase in fructan content. GSNO-treated plants presented higher GR activity and, consequently, increased GSH levels. L. perenne cv. AberAvon was relatively tolerant to the water stress condition employed herein, maintaining ROS homeostasis and mitigating oxidative stress, possibly due to fructan, ascorbate and glutathione pools.

Development of rubber-enriched dandelion varieties by metabolic engineering of the inulin pathway

Natural rubber (NR) is an important raw material for a large number of industrial products. The primary source of NR is the rubber tree Hevea brasiliensis, but increased worldwide demand means that alternative sustainable sources are urgently required. The Russian dandelion (Taraxacum koksaghyz Rodin) is such an alternative because large amounts of NR are produced in its root system. However, rubber biosynthesis must be improved to develop T. koksaghyz into a commercially feasible crop. In addition to NR, T. koksaghyz also produces large amounts of the reserve carbohydrate inulin, which is stored in parenchymal root cell vacuoles near the phloem, adjacent to apoplastically separated laticifers. In contrast to NR, which accumulates throughout the year even during dormancy, inulin is synthesized during the summer and is degraded from the autumn onwards when root tissues undergo a sink-to-source transition. We carried out a comprehensive analysis of inulin and NR metabolism in T. koksaghyz and its close relative T. brevicorniculatum and functionally characterized the key enzyme fructan 1-exohydrolase (1-FEH), which catalyses the degradation of inulin to fructose and sucrose. The constitutive overexpression of Tk1-FEH almost doubled the rubber content in the roots of two dandelion species without any trade-offs in terms of plant fitness. To our knowledge, this is the first study showing that energy supplied by the reserve carbohydrate inulin can be used to promote the synthesis of NR in dandelions, providing a basis for the breeding of rubber-enriched varieties for industrial rubber production.

Statistic thermodynamic analysis of fructans – Part 1: Molar mass distribution of inulin and modeling of inulin biosynthesis

The application of statistic thermodynamic principles to the biosynthesis of inulin explains the synthesizing and degrading activities of the fructan:fructan fructosyl transferase (FFT) enzyme as well as the molar mass distribution of native inulins. The direction of the FFT enzyme action is energetically forced by maximization of the mixing entropy of the polymeric distribution of inulin. The entropy maximized polymeric distribution of native inulins corresponds to the most probable distribution for linear polymers (Flory-Schulz distribution). Estimating the energetic need of the FFT enzyme from in vitro experiments and applying it to in vivo conditions provides an explanation and an accurate prediction for the molar mass found for native plant inulins in vivo.

Fluxes in central carbohydrate metabolism of source leaves in a fructan-storing C3 grass: rapid turnover and futile cycling of sucrose in continuous light under contrasted nitrogen nutrition status

This work assessed the central carbohydrate metabolism of actively photosynthesizing leaf blades of a C3 grass (Lolium perenne L.). The study used dynamic (13)C labelling of plants growing in continuous light with contrasting supplies of nitrogen ('low N' and 'high N') and mathematical analysis of the tracer data with a four-pool compartmental model to estimate rates of: (i) sucrose synthesis from current assimilation; (ii) sucrose export/use; (iii) sucrose hydrolysis (to glucose and fructose) and resynthesis; and (iv) fructan synthesis and sucrose resynthesis from fructan metabolism. The contents of sucrose, fructan, glucose, and fructose were almost constant in both treatments. Labelling demonstrated that all carbohydrate pools were turned over. This indicated a system in metabolic steady state with equal rates of synthesis and degradation/consumption of the individual pools. Fructan content was enhanced by nitrogen deficiency (55 and 26% of dry mass at low and high N, respectively). Sucrose content was lower in nitrogen-deficient leaves (2.7 versus 6.7%). Glucose and fructose contents were always low (<1.5%). Interconversions between sucrose, glucose, and fructose were rapid (with half-lives of individual pools ranging between 0.3 and 0.8 h). Futile cycling of sucrose through sucrose hydrolysis (67 and 56% of sucrose at low and high N, respectively) and fructan metabolism (19 and 20%, respectively) was substantial but seemed to have no detrimental effect on the relative growth rate and carbon-use efficiency of these plants. The main effect of nitrogen deficiency on carbohydrate metabolism was to increase the half-life of the fructan pool from 27 to 62 h and to effectively double its size.

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.

Day-length effects on carbon stores for respiration of perennial ryegrass

• The mechanism controlling the use of stored carbon in respiration is poorly understood. Here, we explore if the reliance on stores as respiratory substrate depends on day length. • Lolium perenne (perennial ryegrass) was grown in continuous light (275 μmol photons m(-2) s(-1) ) or in a 16 : 8 h day : night regime (425 μmol m(-2) s(-1) during the photoperiod), with the same daily photosynthetic photon flux density (PPFD). Plants in stands were labelled with (13)CO(2) : (12)CO(2) for various time intervals. The rates and isotopic signatures of shoot- and root-respired CO(2) were measured after labelling, and water-soluble carbohydrates were determined in biomass. The tracer kinetics in respired CO(2) was analysed with compartmental models to infer the sizes, half-lives and contributions of respiratory substrate pools. • Stores were the main source for respiration in both treatments (c. 60% of all respired carbon). But, continuous light slowed the turnover (+270%) and increased the size (+160%) of the store relative to the 16 : 8 h day : night regime. This effect corresponded with a greatly elevated fructan content. Yet, day length had no effect on sizes and half-lives of other pools serving respiration. • We suggest that the residence time of respiratory carbon was strongly influenced by partitioning of carbon to fructan stores.

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.

Ryegrass leaf fructan synthesis is oxygen dependent and abolished by endomembrane inhibitors

Valid models are the foundation of systems biology. However, even well-established models may warrant reassessment. A testable feature of the currently accepted vacuolar model for fructan biosynthesis is its independence from metabolic energy at substrate level. The effects of limiting energy provision on fructan biosynthesis in grass leaves were determined. It was found that, in darkness in air, the rate of fructan accumulation was reduced to half relative to a light control. In darkness under anoxia the process was immediately abolished. In the light, the leaf sucrose concentration remained high, but in darkness +/- O(2), 40% of this sucrose was rapidly degraded. The constant rate of dark-aerobic fructan accumulation was independent of the decrease in sucrose concentration. Constant rates of aerobic fructan synthesis were independent of marked changes in extractable polymerase rates. In the dark under anoxia, fructan accumulation was abolished but leaves maintained > or = 80% of the extractable polymerase. Extractable polymerase rates cannot explain the rates of fructan accumulation observed in vivo, if the process is vacuolar. It was shown that the results were inconsistent with a vacuolar site for fructan synthesis. Six inhibitors of endomembrane function were shown to abolish fructan synthesis in vivo.

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.

Purification, cloning and functional characterization of a fructan 6-exohydrolase from wheat (Triticum aestivum L.)

Fructans, beta2-1 and/or beta2-6 linked polymers of fructose, are produced by fructosyltransferases (FTs) from sucrose. They are important storage carbohydrates in many plants. Fructan reserves, widely distributed in plants, are believed to be mobilized via fructan exohydrolases (FEHs). The purification, cloning, and functional characterization of a 6-FEH from wheat (Triticum aestivum L.) are reported here. It is the first FEH shown to hydrolyse exclusively beta2-6 bonds found in a fructan-producing plant. The enzyme was purified to homogeneity using ammonium sulphate precipitation, ConA affinity-, ion exchange-, and size exclusion chromatography and yielded a single band of 70 kDa following SDS-PAGE. Sequence information obtained by mass spectrometry of in-gel trypsin digests demonstrated the presence of a single protein. Moreover, these unique peptide sequences, together with some ESTs coding for them, could be used in a RT-PCR based strategy to clone a 1.7 kb cDNA. Functionality tests of the cDNA performed after heterologous expression in the yeast Pichia pastoris showed--as did the native enzyme from wheat--a very high activity of the produced protein against bacterial levan, 6-kestose, and phlein whilst sucrose and inulin were not used as substrates. Therefore the enzyme is a genuine 6-FEH. In contrast to most FEHs from fructan-accumulating plants, this FEH is not inhibited by sucrose. The relative abundance of 6-FEH transcripts in various tissues of wheat was investigated using quantitative RT-PCR.

Molecular genetics of fructan metabolism in perennial ryegrass

Fructans are the main storage carbohydrates of temperate grasses, sustaining regrowth immediately after defoliation, as well as contributing to the nutritive value of feed. Fructan metabolism is based on the substrate sucrose and involves fructosyltransferases (FTs) for biosynthesis and fructan exohydrolases (FEHs) for degradation. Sucrose is also utilized by invertases (INVs), which hydrolyse it into its constituent monosaccharides for use in metabolism. The isolation, molecular characterization, functional analysis, and phylogenetic relationships of genes encoding FTs, FEHs, and INVs from temperate grasses are reviewed, with an emphasis on perennial ryegrass (Lolium perenne L.). The roles these enzymes play in fructan accumulation and remobilization, and future biotechnological applications in molecular plant breeding are discussed.

Production of tailor-made fructans in sugar beet by expression of onion fructosyltransferase genes

The consumption of fructans as a low caloric food ingredient or dietary fibre is rapidly increasing due to health benefits. Presently, the most important fructan source is chicory, but these fructans have a simple linear structure and are prone to degradation. Additional sources of high-quality tailor-made fructans would provide novel opportunities for their use as food ingredients. Sugar beet is a highly productive crop that does not normally synthesize fructans. We have introduced specific onion fructosyltransferases into sugar beet. This resulted in an efficient conversion of sucrose into complex, onion-type fructans, without the loss of storage carbohydrate content.

Distinct regulation of sucrose: sucrose-1-fructosyltransferase (1-SST) and sucrose: fructan-6-fructosyltransferase (6-SFT), the key enzymes of fructan synthesis in barley leaves: 1-SST as the pacemaker

•  Previously we have cloned sucrose: fructan-6-fructosyltransferase (6-SFT) from barley (Hordeum vulgare) and proposed that synthesis of fructans in grasses depends on the concerted action of two main enzymes: sucrose: sucrose-1-fructosyltransferase (1-SST), as in other fructan producing plants, and 6-SFT, found only in grasses. •  Here we report the cloning of barley 1-SST, verifying the activity of the encoded protein by expression in Pichia pastoris. As expected, the barley 1-SST is homologous to invertases and fructosyltransferases, and in particular to barley 6-SFT. •  The gene expression pattern of 1-SST and 6-SFT, along with the corresponding enzyme activities and fructan levels, were investigated in excised barley leaves subjected to a light-dark regime known to sequentially induce fructan accumulation and mobilization. The turnover of transcripts and enzyme activities of 1-SST and 6-SFT was compared, using appropriate inhibitors. •  We found the 1-SST transcripts and enzymatic activity respond quickly, being subject to a rapid turnover. By contrast, the 6-SFT transcripts and enzymatic activity were found to be much more stable. The much higher responsiveness of 1-SST to regulatory processes, as compared with 6-SFT, clearly indicates that 1-SST plays the role of the pacemaker enzyme of fructan synthesis in barley leaves.

The enclosed and exposed part of the peduncle of wheat (Triticum aestivum) - spatial separation of fructan storage

•  Although fructan accumulation is reported in photosynthetically active organs, the long-term storage of fructan mainly occurs in more heterotrophic tissues. Significant amounts of fructan are stored in the internodes during grain filling of wheat (Triticum aestivum). The uppermost internode (peduncle) of wheat consists of a lower unexposed (i.e. enclosed by the flag leaf sheath and thus heterotrophic part, P_l ) and an upper exposed autotrophic part (P_u ). •  Diurnal and long-term changes of fructan and sucrose (the precursor of fructan synthesis) contents were studied in P_l and P_u of potted wheat plants. •  At mid grain-filling the sucrose concentration in P_u increased almost threefold during the light period and decreased in the following night. Diurnal changes in sucrose concentration were much less expressed in P_l . Fructan concentration was significantly higher in P_l than in P_u and did not change during the light period. •  In another experiment, field grown wheat plants were sampled at regular intervals between 5 d before anthesis and grain maturity. At the time of maximum fructan content, 88% of the fructans in the total peduncle were stored in the heterotrophic P_l . Within P_l , fructan accumulation started in the older segments. The reason for the sharp separation of fructan storage between P_l and P_u remains unclear.

Fructan biosynthesis in transgenic plants

Data from plants transformed to accumulate fructan are assessed in the context of natural concentrations of reserve carbohydrates and natural fluxes of carbon in primary metabolism: Transgenic fructan accumulation is universally reported as an instantaneous endpoint concentration. In exceptional cases, concentrations of 60-160 mg g(-1) fresh mass were reported and compare favourably with naturally occurring maximal starch and fructan content in leaves and storage organs. Generally, values were less than 20 mg g(-1) for plants transformed with bacterial genes and <9 mg g(-1) for plant-plant transformants. Superficially, the results indicate a marked modification of carbon partitioning. However, transgenic fructan accumulation was generally constitutive and involved accumulation over time-scales of weeks or months. When calculated as a function of accumulation period, fluxes into the transgenic product were low, in the range 0.00002-0.03 nkat g(-1). By comparison with an estimated minimum daily carbohydrate flux in leaves for a natural fructan-accumulating plant in field conditions (37 nkat g(-1)), transgenic fructan accumulation was only 0.00005-0.08% of primary carbohydrate flux and does not indicate radical modification of carbon partitioning, but rather, a quantitatively minor leakage into transgenic fructan. Possible mechanisms for this low fructan accumulation in the transformants are considered and include: (i) rare codon usage in bacterial genes compared with eukaryotes, (ii) low transgene mRNA concentrations caused by low expression and/or high turnover, (iii) resultant low expression of enzyme protein, (iv) resultant low total enzyme activity, (v) inappropriate kinetic properties of the gene products with respect to substrate concentrations in the host, (vi) in situ product hydrolysis, and (vii) levan toxicity. Transformants expressing bacterial fructan synthesis exhibited a number of aberrant phenotypes such as stunting, leaf bleaching, necrosis, reduced tuber number and mass, tuber cortex discoloration, reduction in starch accumulation, and chloroplast agglutination. In severe cases of developmental aberration, potato tubers were replaced by florets. Possible mechanisms to explain these aberrations are discussed. In most instances, the attempted subcellular targeting of the transgene product was not demonstrated. Where localization was attempted, the transgene product generally mis-localized, for example, to the cell perimeter or to the endomembrane system, instead of the intended target, the vacuole. Fructosyltransferases exhibited different product specificities in planta than in vitro, expression in planta generally favouring the formation of larger fructan oligomers and polymers. This implies a direct influence of the intracellular environment on the capacity for polymerization of fructosyltransferases and may have implications for the mechanism of natural fructan polymerization in vivo.

Purification and characterization of the raffinose oligosaccharide chain elongation enzyme, galactan : galactan galactosyltransferase (GGT), from Ajuga reptans leaves

Galactan: galactan galactosyltransferase (GGT), an enzyme involved in the biosynthesis of the long-chain raffinose family of oligosaccharides (RFOs) in Ajuga reptans, catalyses the transfer of an alpha-galactosyl residue from one molecule of RFO to another one resulting in the next higher RFO oligomer. This novel galactinol (alpha-galactosyl-myo-inositol)-independent alpha-galactosyltransferase is responsible for the accumulation of long-chain RFOs in vivo. Warm treatment (20 degrees C) of excised leaves resulted in a 34-fold increase of RFO concentration and a 200-fold increase of GGT activity after 28 days. Cold treatment (10 degrees C/3 degrees C day/night) resulted in a 26- and 130-fold increase, respectively. These data support the role of GGT as a key enzyme in the synthesis and accumulation of long-chain RFOs. GGT was purified from leaves in a 4-step procedure which involved fractionated precipitation with ammonium sulphate as well as lectin affinity, anion exchange, and size-exclusion chromatography and resulted in a 200-fold purification. Purified GGT had an isoelectric point of 4.7, a pH optimum around 5, and its transferase reaction displayed saturable concentration dependence for both raffinose (Km = 42 mM) and stachyose (Km = 58 mM). GGT is a glycoprotein with a 10% glycan portion. The native molecular mass was 212 kDa as determined by size-exclusion chromatography. Purified GGT showed one single active band after native PAGE or IEF separation, respectively, which separated into three bands on SDS-PAGE at 48 kDa, 66 kDa, and 60 kDa. The amino acid sequence of four tryptic peptides obtained from the major 48-kDa band showed a high homology to plant alpha-galactosidase (EC 3.2.1.22) sequences. GGT differed, however, in its substrate specificity from alpha-galactosidases; it neither hydrolysed nor transferred alpha-galactosyl-groups from melibiose, galactinol, UDP-galactose, manninotriose, and manninotetrose. Galactinol, sucrose, and galactose inhibited the GGT reaction considerably at 10-50 mM.

NO3 – AFFECTS CARBOHYDRATE LOSSES FROM WHEAT ROOTS

The influence of NO3 – on carbohydrate (C) losses from the roots of 21-day-old wheat seedlings was studied under light and N supplies ranging from deficient to excessive (0–8 mM NO3 –). C loss is not influenced by the quantity of internal soluble carbohydrates (SC), but is affected by the nitrate status of the seedlings. In low illuminance, the NO3 – loss is significantly higher than that in high illuminance, whereas C loss is significantly higher in high illuminance than in low illuminance, in spite of there being little difference between the seedlings of both light treatments in the SC concentration in the roots at 0 h, suggesting the existence of a negative correlation between NO3 – and C efflux and a close relationship between the C efflux system and NO3 – assimilation. Low NO3 – and light reduced the C loss, which was decreased to a greater extent by low NO3 – than by low light, indicating that C loss was more dependent on NO3 – than on C export. The high decline in C loss, irrespective of whether there is an increase in NO3 – loss (i.e. in low light) or a decrease in NO3 – loss (i.e. at low nitrate), may indicate that the two types of losses involve different mechanisms.

Regulation of sucrose metabolism in higher plants: localization and regulation of activity of key enzymes

Sucrose (Suc) plays a central role in plant growth and development. It is a major end product of photosynthesis and functions as a primary transport sugar and in some cases as a direct or indirect regulator of gene expression. Research during the last 2 decades has identified the pathways involved and which enzymes contribute to the control of flux. Availability of metabolites for Suc synthesis and 'demand' for products of sucrose degradation are important factors, but this review specifically focuses on the biosynthetic enzyme sucrose-phosphate synthase (SPS), and the degradative enzymes, sucrose synthase (SuSy), and the invertases. Recent progress has included the cloning of genes encoding these enzymes and the elucidation of posttranslational regulatory mechanisms. Protein phosphorylation is emerging as an important mechanism controlling SPS activity in response to various environmental and endogenous signals. In terms of Suc degradation, invertase-catalyzed hydrolysis generally has been associated with cell expansion, whereas SuSy-catalyzed metabolism has been linked with biosynthetic processes (e.g., cell wall or storage products). Recent results indicate that SuSy may be localized in multiple cellular compartments: (1) as a soluble enzyme in the cytosol (as traditionally assumed); (2) associated with the plasma membrane; and (3) associated with the actin cytoskeleton. Phosphorylation of SuSy has been shown to occur and may be one of the factors controlling localization of the enzyme. The purpose of this review is to summarize some of the recent developments relating to regulation of activity and localization of key enzymes involved in sucrose metabolism in plants.

Effect of storage on fructan and fructooligosaccharide of onion (Allium cepa L.)

The purpose of this study was a comparative examination of the fructan and fructooligosaccharide (FOS) content of different varieties of onions (Allium cepa L. cv. Sturon, Hysam, Durco, Grano de Oro, and Caribo) and the changes produced during their commercial storage. In fresh onions, the Grano de Oro variety presented a remarkably different behavior, showing low contents of total fructans and FOS and high levels of reducing sugars. In the other varieties, Sturon, Hysam, Durco, and Caribo, fructans were the main carbohydrates, the lowest polymerized FOS being the major oligomer. Storage period caused in these varieties important increased levels of free fructose attributed to fructan hydrolysis. Maleic hydrazide treatment had no significant effect in avoiding the hydrolysis of fructans during storage conditions for the Sturon variety. Varieties with >16% dry matter or 15% soluble solids contents could be stored for 6 months at 0 degrees C and 60-65% relative humidity.

Transport processes of solutes across the vacuolar membrane of higher plants

The central vacuole is the largest compartment of a mature plant cell and may occupy more than 80% of the total cell volume. However, recent results indicate that beside the large central vacuole, several small vacuoles may exist in a plant cell. These vacuoles often belong to different classes and can be distinguished either by their contents in soluble proteins or by different types of a major vacuolar membrane protein, the aquaporins. Two vacuolar proton pumps, an ATPase and a PPase energize vacuolar uptake of most solutes. The electrochemical gradient generated by these pumps can be utilized to accumulate cations by a proton antiport mechanism or anions due to the membrane potential difference. Uptake can be catalyzed by channels or by transporters. Growing evidence shows that for most ions more than one transporter/channel exist at the vacuolar membrane. Furthermore, plant secondary products may be accumulated by proton antiport mechanisms. The transport of some solutes such as sucrose is energized in some plants but occurs by facilitated diffusion in others. A new class of transporters has been discovered recently: the ABC type transporters are directly energized by MgATP and do not depend on the electrochemical force. Their substrates are organic anions formed by conjugation, e.g. to glutathione. In this review we discuss the different transport processes occurring at the vacuolar membrane and focus on some new results obtained in this field.

Cloning, developmental, and tissue-specific expression of sucrose:sucrose 1-fructosyl transferase from Taraxacum officinale. Fructan localization in roots

Sucrose:sucrose 1-fructosyl transferase (1-SST) is the key enzyme initiating fructan synthesis in Asteraceae. Using reverse transcriptase-PCR, we isolated the cDNA for 1-SST from Taraxacum officinale. The cDNA-derived amino acid sequence showed very high homology to other Asteracean 1-SSTs (Cichorium intybus 86%, Cynara scolymus 82%, Helianthus tuberosus 80%), but homology to 1-SST from Allium cepa (46%) and Aspergillus foetidus (18%) was much lower. Fructan concentrations, 1-SST activities, 1-SST protein, and mRNA concentrations were compared in different organs during vegetative and generative development of T. officinale plants. Expression of 1-SST was abundant in young roots but very low in leaves. 1-SST was also expressed at the flowering stages in roots, stalks, and receptacles. A good correlation was found between northern and western blots showing transcriptional regulation of 1-SST. At the pre-flowering stage, 1-SST mRNA concentrations and 1-SST activities were higher in the root phloem than in the xylem, resulting in the higher fructan concentrations in the phloem. Fructan localization studies indicated that fructan is preferentially stored in phloem parenchyma cells in the vicinity of the secondary sieve tube elements. However, inulin-like crystals occasionally appeared in xylem vessels.

Cloning of sucrose:sucrose 1-fructosyltransferase from onion and synthesis of structurally defined fructan molecules from sucrose

Sucrose (Suc):Suc 1-fructosyltransferase (1-SST) is the key enzyme in plant fructan biosynthesis, since it catalyzes de novo fructan synthesis from Suc. We have cloned 1-SST from onion (Allium cepa) by screening a cDNA library using acid invertase from tulip (Tulipa gesneriana) as a probe. Expression assays in tobacco (Nicotiana plumbaginifolia) protoplasts showed the formation of 1-kestose from Suc. In addition, an onion acid invertase clone was isolated from the same cDNA library. Protein extracts of tobacco protoplasts transformed with this clone showed extensive Suc-hydrolyzing activity. Conditions that induced fructan accumulation in onion leaves also induced 1-SST mRNA accumulation, whereas the acid invertase mRNA level decreased. Structurally different fructan molecules could be produced from Suc by a combined incubation of protein extract of protoplasts transformed with 1-SST and protein extract of protoplasts transformed with either the onion fructan:fructan 6G-fructosyltransferase or the barley Suc:fructan 6-fructosyltransferase.

Cloning of the fructan biosynthesis pathway of Jerusalem artichoke

To study the regulation of fructan synthesis in plants, we isolated two full-size cDNA clones encoding the two enzymes responsible for fructan biosynthesis in Jerusalem artichoke (Helianthus tuberosus): 1-sucrose:sucrose fructosyl transferase (1-SST) and 1-fructan:fructan fructosyl transferase (1-FFT). Both enzymes have recently been purified to homogeneity from Jerusalem artichoke tubers (Koops and Jonker (1994) J.Exp.Bot.45, 1623-1631; Koops and Jonker (1996) Plant Physiol. 110, 1167-1175) and their amino acid sequences have been partially determined. Using RT-PCR and primers based on these sequences, specific fragments of the genes were amplified from tubers of Jerusalem artichoke. These fragments were used as probes to isolate the cDNAs encoding 1-SST and 1-FFT from a tuber-specific lambdal ZAP library. The deduced amino acid sequences of both cDNAs perfectly matched the sequences of the corresponding purified proteins. At the amino acid level, the cDNA sequences showed 61% homology to each other and 59% homology to tomato vacuolar invertase. Based on characteristics of the deduced amino acid sequence, the first 150 bp of both genes encode a putative vacuolar targeting signal. Southern blot hybridization revealed that both 1-SST and 1-FFT are likely to be encoded by single-copy genes. Expression studies based on RNA blot analysis showed organ-specific and developmental expression of both genes in growing tubers. Lower expression was detected in flowers and in stem. In other organs, including leaf, roots and dormant tubers, no expression could be detected. In tubers, the spatial and developmental expression correlates with the accumulation of fructans. Using the 1-sst and 1-fft cDNAs, chimeric genes were constructed driven by the CaMV 35S promoter. Analysis of transgenic petunia plants carrying these constructs showed that both cDNAs encode functional fructosyltransferase enzymes. Plants transformed with the 35S-1-sst construct accumulated the oligofructans 1-kestose (GF2), 1,1-nystose (GF3) and 1,1,1-fructosylnystose (GF4). Plants transformed with the 35S-1-fft construct did not accumulate fructans, probably because of the absence of suitable substrates for 1-FFT, i.e. fructans with a degree of polymerization > or = 3 (GF2, GF3, etc.). Nevertheless, protein extracts from these transgenic plants were able to convert GF3, when added as a substrate into fructans with a higher degree of polymerization. Progeny of crosses between a 35S-1-sst-containing plant and a 35S-1-fft-containing plant, showed accumulation of high-molecular-weight fructans in old, senescent leaves. Based on the comparison of the predicted amino acid sequences of 1-sst and 1-fft with those of other plant fructosyl transferase genes, we postulate that both plant fructan genes have evolved from plant invertase genes.

Fructan synthesis in transgenic tobacco and chicory plants expressing barley sucrose: fructan 6-fructosyltransferase

We have recently cloned a cDNA encoding sucrose:fructan 6-fructosyltransferase (6-SFT), a key enzyme of fructan synthesis forming the beta-2,6 linkages typical of the grass fructans, graminans and phleins [Sprenger et al. (1995) Proc. Natl. Acad. Sci. USA 92, 11652-11656]. Here we report functional expression of 6-SFT from barley in transgenic tobacco and chicory. Transformants of tobacco, a plant naturally unable to form fructans, synthesized the trisaccharide kestose and a series of unbranched fructans of the phlein type (beta-2,6 linkages). Transformants of chicory, a plant naturally producing only unbranched fructans of the inulin type (beta-2,1 linkages), synthesized in addition branched fructans of the graminan type, particularly the tetrasaccharide bifurcose which is also a main fructan in barley leaves.

Fructan biosynthesis in excised leaves of wheat (Triticum aestivum L.): a comparison of de novo synthesis in vivo and in vitro

Carbohydrate accumulation by excised, continuously illuminated leaves of wheat (Triticum aestivum L.) was followed over a 24 h period. At 0 h, the tissue contained no detectable fructan. In the initial 6 h, only sucrose was accumulated. After 6 h the de novo synthesis of fructans was induced. Fructans accumulated in the sequence 1-kestose, bifurcose, nystose, oligofructans of apparent degree of polymerization (DP) up to 9 and finally, 6-kestose, which was first detected after 22 h. A cell-free protein extract from leaves illuminated for 24 h catalyzed the de novo synthesis of fructan from sucrose. The properties of this fructan synthetic activity (FSA) were characterized. The FSA was stable, exhibiting < 20% loss of activity when stored at 5 °C or 25 °C for 6 h. The FSA exhibited an apparent K_m,suc of 114 mM, and an apparent pH optimum at 5.5. The in vitro synthesis of fructan of DP > 3 was not inhibited by sucrose even at 1000 mM. Pyridoxal-hydrochloride at 20 mM did not enhance rates of enzymatic fructan synthesis or significantly inhibit the release of free fructose in the optimized enzymatic reaction. The rate of oligofructan synthesis in the optimized reaction approximated to rates of accumulation in the leaf (1.35 mg g h^-1 and 1.18 mg g h^-1 respectively). The sequence of oligofructan synthesis in vitro was the same as that observed in the leaf, with the exception that 6-kestose was synthesized early in the time course, in parallel with 1-kestose and bifurcose. Fructans of apparent DP ≤ 8 were detected after 10 h of incubation. When incubated with bifurcose as sole substrate, the cell-free preparation liberated free monosaccharides, without the accumulation of trisaccharide or sucrose as intermediates. The results are discussed with reference to current, conflicting models for the biosynthesis of fructan in cereals.

The influence of altered gravity on carbohydrate metabolism in excised wheat leaves

We developed a system to study the influence of altered gravity on carbohydrate metabolism in excised wheat leaves by means of clinorotation. The use of excised leaves in our clinostat studies offered a number of advantages over the use of whole plants, most important of which were minimization of exogenous mechanical stress and a greater amount of carbohydrate accumulation during the time of treatment. We found that horizontal clinorotation of excised wheat leaves resulted in significant reductions in the accumulation of fructose, sucrose, starch and fructan relative to control, vertically clinorotated leaves. Photosynthesis, dark respiration and the extractable activities of ADP glucose pyrophosphorylase (EC 2.7.7.27), sucrose phosphate synthase (EC 2.4.4.14), sucrose sucrose fructosyltransferase (EC 2.4.1.99), and fructan hydrolase (EC 3.2.1.80) were unchanged due to altered gravity treatment.

Fructan exohydrolase from grasses

In grasses, fructan reserves are mobilized from vegetative plant parts during seasonal growth, after defoliation during grazing and from stems during seed filling. Well-illuminated leaves show a diurnal pattern of fructan accumulation during the light and mobilization during the dark. In expanding leaves, fructans are accumulated in cells of the elongation zone and when mobilized are considered to contribute assimilate for synthetic processes. Even in leaves which do not contain high fructan concentrations, high rates of fructan turnover occur. The process of fructan mobilization appears to be regulated in relation to ontogenic events, demand for assimilate during growth and in response to environmental stress. Hydrolysis of fructans in bacteria is catalyzed by both endo- and exohydrolases. However, in higher plants only fructan exohydrolases (FEH) (EC 3.2.1.80) have been reported. FEH has been extracted from only a limited number of grass species. The pH optimum of FEH activities varies between pH 45-5-5, the temperature optimum ranges from 25-40 °C and FEH is considered to be entirely localized in vacuoles. Estimates of the K_m for FEH assayed using high molecular weight fructan substrates vary widely and should be considered carefully because most substrates are ill-defined. Many studies indicate that crude and partially-purified FEH activity is highest when assayed using a fructan substrate extracted from the species that was the source of the enzyme activity. Inulin extracted from members of the Asteraceae is generally less readily hydrolyzed and levans from bacteria are relatively poor substrates for FEH from grasses. Glycosidic-linkage-specific hydrolysis has been demonstrated for an FEH activity extracted from barley. This FEH activity hydrolyzed β-2,1-glycosidic linkages more rapidly than β-2,6-linkages. Most other studies are less conclusive because ill-defined fructan substrates were used. Two isoforms of FEH are reported in leaves of Lolium spp., but the roles of isoforms and their kinetic characteristics are not known. FEH activity in different tissues may be regulated by metabolic concentrations, sucrose (5-10 mw) being a strong inhibitor in vitro of FEH from some species. Results of experiments with Dactylis glomerata indicate control of expression of FEH activity at the gene level. In stem bases, FEH activity increased after defoliation. The increase was abolished by applications of inhibitors of protein synthesis and was apparently repressed by application of various sugars. Although the rates of fructan hydrolysis measured in vitro are sufficient to explain the in vivo rates of fructan hydrolysis, it is yet to be shown whether fructan hydrolysis in vivo is due to the activity of FEH exclusively, or FEH and invertase-like activities. The overriding conclusion is that the various studies of FEH from grasses present a confusing and incomplete picture of the function, activity and kinetics of this enzyme. This is due in part to the lack of defined, commercially-available substrates. The chromatographic techniques available to most laboratories do not permit purification of sufficient quantities of high molecular weight fructans of specific degree of polymerization, or fructan oligosaccharides with glycosidic linkages which differ from that of the inulin series for enzyme characterization. It is recommended that a few well-defined oligosaccharides be adopted as substrate standards for future research.

Fructan content of wheat seedlings (Triticum aestivum L.) under hypoxia and following re-aeration

We studied fructan accumulation in wheat seedlings (Triticum aestivum L. cv. Alcedo) caused by oxygen shortage around the root. Plants were cultivated in either nitrogen-flushed or aerated nutrient solution. In response to the nitrogen treatment there was an accumulation of soluble carbohydrates in shoots as well as in roots. The main contribution was due to fructans. The content of fructo-oligosaccharides had been increased five-fold in the roots and seven-fold in the shoots. This is incompatible with the assumption that higher substrate consumption follows enhanced fermentation under oxygen shortage. Re-aeration of the nitrogen-flushed nutrient solution resulted in enhanced consumption of stored carbohydrates, mainly of fructans, accompanied by high growth rates of the roots. The possibility of utilizing fructans quickly if oxygen is available is a possible advantage for plants adapted to hypoxia. Particularly however, it is suggested that the low energy requirements for sucrose: sucrose fructosyl transferase, allowing fructan synthesis even under oxygen shortage, may improve the fitness of plants to survive temporary hypoxia in the rhizosphere.

Fructan biosynthesis in excised leaves of Lolium temulentum L.: V. Enzymatic de novo synthesis of large fructans from sucrose

Leaves of Lolium temulentum L. were induced to accumulate fructan by excision and continuous illumination. Enzyme extracts were prepared and protein was concentrated by precipitation with ammonium sulphate at 100% saturation. The preparation was incubated at a protein concentration equivalent to 3-4 g f.wt of tissue cm^-3 with sucrose at an initial concentration of 400 mol^-3 . The preparation catalyzed the synthesis of large fructans of apparent degree of polymerization (DP) ⩽20, based upon comparison with the chromatographic mobilities of oligoinulins on thin-layer chromatography (TLC). The products were formed at a rate approximating to rates of fructan accumulation in leaf tissue. The overall pattern of in vitro products, when separated on TLC, resembled that of the native leaf fructans though the relative abundances of some of the products differed from the in vivo pattern. The reaction did not catalyze the formation of the high molecular weight fructans (M_r > 3 kDa; DP > 20) characteristic of this tissue. Equivalent protein preparations derived from uninduced leaves exhibited negligible rates of fructan synthesis. Feeding cycloheximide to leaves prevented fructan synthesis both by the tissue and by protein extracts. Cycloheximide did not directly inhibit in vitro fructan synthesis, suggesting that its in vivo effect was at the translational level. The characteristics of the in vitro reaction are compared with the properties of in vivo fructan synthesis and are discussed with respect to its possible physiological relevance.

Storage of sugars and mannitol in petioles of celery leaves

Mannitol and sucrose are produced photosynthetically in the mesophyll and translocated in the phloem of celery leaves. The thick and fleshy petioles of these leaves act as reversible carbohydrate sinks, storing mainly mannitol, glucose and fructose but very little sucrose (less than 3.3 mg ml^-1 sap) in their parenchyma. The concentrations of soluble carbohydrates and the activities of enzymes of sucrose cleavage were asayed in the petiole parenchyma during leaf development. Mannitol (about 20 mg ml^-1 sap), glucose and fructose (about 10 to 15 mg ml^-1 sap each), changed little with development. Soluble acid invertase accounted for over 95 % of the total enzymic sucrose cleavage measured in vitro. Its activity was closely related to development with a sharp decrease during leaf maturation. With protoplasts from petiole parenchyma and vacuoles isolated from them it was demonstrated that 28% of the total soluble acid invertase of the tissue was in the apoplast and 72 % in the vacuole. Alkaline invertase and sucrose synthase were totally cytosolic. Glucose and fructose, the main storage sugars of these protoplasts, were vacuolar in location, whereas mannitol was both cytosolic (19%) and vacuolar (81%).