Transport and Metabolism of Sucrose versus Hexoses in Relation to Growth in Etiolated Pea Stem

Sucrose, supplied to detached pea (Pisum sativum L. var Alaska) epicotyls through cut bases, supported better growth of apical tissue than supplied glucose and/or fructose. The hexoses were converted mainly to sucrose in basal regions of the epicotyl but some moved as such through the epicotyl and accumulated at the apex (plumule) at a rate faster than sucrose. A greater proportion of the carbon derived from supplied hexoses than from sucrose was used for synthesis of ethanol-insoluble products throughout the epicotyl. By use of asymmetrically labeled sucrose, it was shown that neither hexose moiety was used preferentially for the synthesis of metabolites. Supplied sucrose moved as such only up to the region of cell elongation where it was hydrolyzed and completely equilibrated before moving into more apical regions. The results indicate that better growth with supplied sucrose than hexose could not have resulted from differential effects on cell division, more rapid uptake or transport of sucrose, enhanced wall synthesis, or cleavage by sucrose synthase. It is concluded that transported sucrose versus hexoses must undergo or evoke different reactions which affect growth in the region of cell elongation.     

Carbohydrate utilisation by cell suspension cultures of Phaseolus vulgaris

Uptake of sugar by Phaseolus vulgaris cell suspension cultures from a sucrose supplemented medium is predominantly in the hexose form. This is due to a rapid cleavage of the sucrose by an apoplastic acid invertase activity and an apparent very low demand for and uptake of carbon from the medium prior to induction of cell growth and division. Glucose is preferentially taken up, leading to an accumulation of fructose in the medium. However, when the glucose is depleted the cells do take up the fructose at a rate similar to that of glucose. When glucose or fructose is supplied individually to cell cultures, both are utilised very efficiently with growth slightly better on the fructose medium. Hexose uptake is largely an active process with diffusion uptake even at the highest concentrations (> 50 m M ) contributing less than 30%. The hexose uptake system of the cells has a greater affinity for glucose (K_m= 240 µ M ) than for fructose (K_m= 960 µ M ) but the maximum uptake (V_max) is similar. The major difference in the kinetic properties of hexose uptake is that glucose is a strong inhibitor of fructose uptake, while fructose has little effect on glucose uptake. The differences in the kinetic properties of the uptake system for the two hexoses can largely explain the observed pattern of hexose utilisation when both glucose and fructose are present in the medium.     

Uptake and Metabolism of Sugars by Suspension Cultured Catharanthus roseus Cells

The uptake and metabolism of sugars by suspension-cultured Catharanthusroseus cells were investigated. Substantially all the sucrosein the culture medium was hydrolyzed to glucose and fructosebefore being taken up by the cells. The activity of invertasebound to cell walls, determined in situ, was high at the earlystage of culture. Glucose was more easily taken up by the cellsthan was fructose. Tracer experiments using [U-¹⁴C]glucose and[U-¹⁴C]fructose indicated that glucose is a better precursorfor respiration than fructose, while fructose is preferentiallyutilized for the synthesis of sucrose, especially in the earlyphase of cell growth. Possible metabolic routes of sugar insuspension-cultured Catharanthus roseus cells are discussedin the context of these results. Catharanthus roseus, Madagascar periwinkle, suspension culture, sucrose, glucose, fructose, metabolism, glycolysis     

Unsaponifiable Lipid Biosynthesis in the Seedling of Euphorbia lathyris L.: A Bypass via Alcoholic Fermentation

[1-¹⁴C]-ethanol supplied to the cotyledons of 9-d-old Euphorbialathyris seedlings was rapidly incorporated into unsaponifiablelipids, particularly into sterols, latex triterpenols and intothe triterpene ketones of the epicuticular wax. The [¹⁴C]-triterpenoidproduction from ethanol was hardly affected by sucrose in theexternal medium when sucrose uptake rates were low, but whenthe uptake rate was higher the [¹⁴C]-triterpenoid productionfrom [¹⁴C]-ethanol was greatly reduced. This observation isconsistent with the proposition that at high sucrose uptakerates, some sucrose is converted into ethanol, so that the incorporationof [¹⁴C]-ethanol into triterpenoids is reduced by competitionwith endogenously formed ethanol. A calculation based on theputative daily ethanol production in the cotyledons and thedaily triterpenoid production of seedlings indicates that about10 % of the triterpenoid synthesis in vivo may be from ethanol. Ethanol, Euphorbia lathyris, fermentation, seedling, triterpenoid biosynthesis     

Pathway of Phloem unloading of sucrose in corn roots

The pathway of phloem unloading and the metabolism of translocated sucrose were determined in corn (Zea mays) seedling roots. Several lines of evidence show that exogenous sucrose, unlike translocated sucrose, is hydrolyzed in the apoplast prior to uptake into the root cortical cells. These include (a) presence of cell wall invertase activity which represents 20% of the total tissue activity; (b) similarity in uptake and metabolism of [¹⁴C]sucrose and [¹⁴C]hexoses; and (c) randomization of ¹⁴C within the hexose moieties of intracellular sucrose following accumulation of [¹⁴C] (fructosyl)sucrose. Conversely, translocated sucrose does not undergo apoplastic hydrolysis during unloading. Asymmetrically labeled sucrose ([¹⁴C](fructose)sucrose), translocated from the germinating kernels to the root, remained intact indicating a symplastic pathway for unloading. In addition, isolated root protoplasts and vacuoles were used to demonstrate that soluble invertase activity (V_max = 29 micromoles per milligram protein per hour, K_m = 4 millimolar) was located mainly in the vacuole, suggesting that translocated sucrose entered via the symplasm and was hydrolyzed at the vacuole prior to metabolism.     

Starch synthesis in tomato remains constant throughout fruit development and is dependent on sucrose supply and sucrose synthase activity

Studies designed to investigate the cellular pathway of phloem unloading were conducted on two tomato lines with either high or low fruit invertase activities. Experiments were based on determination of the degree to which ³H label from [³H]-(fructosyl)-sucrose was randomized between fructose and glucose following exposure of excised fruit to a pulse of labelled sucrose delivered through pedicels. Fruit from the low invertase line harvested 10, 20 and 40 d after anthesis had similar sucrose uptake kinetics to the high invertase line. A positive correlation was found between sucrose synthase activity and sucrose uptake in both low and high invertase lines. In contrast, no correlation was observed between acid or neutral invertase activities and sucrose uptake. Within the putative apoplasmic sap collected from fruit, label in [³H]-(fructosyl)-sucrose was randomized between the free hexoses and sucrose hexose moieties. Label asymmetry was retained in sucrose on arrival within the tissues. Randomization patterns were similar in both the low and high acid invertase lines. These data support the view that sucrose imported into the fruit was not exposed to extracellular hydrolysis. This suggests that movement from the phloem is likely to occur predominantly through a symplastic pathway. About 25% of the sucrose taken up by the fruit was converted into starch regardless of fruit age, suggesting that starch turnover remains constant throughout fruit development and that starch synthesis was dependent on sucrose supply.     

Differential sugar uptake by cell suspension cultures of <Emphasis Type="Italic">Rudgea jasminoides</Emphasis>, a tropical woody Rubiaceae

The primary utilization of carbohydrates by cell suspension cultures of Rudgea jasminoides, a native woody Rubiaceae from tropical forests, was investigated. Sucrose, glucose + fructose, glucose, or fructose were supplied as carbon sources. The growth curves of R. jasminoides cultured in glucose + fructose, glucose, or fructose showed similar patterns to that observed when sucrose was supplied to the cells, except that an increase in dry mass was observed at the beginning of the stationary growth phase in the media containing only one monosaccharide. The increase in hexose levels in the media during the early stages of the cultures indicated extracellular hydrolysis of sucrose, which was further supported by the increase in the activity of acid invertase bound to the cell wall. Glucose was preferentially taken up, whereas uptake of fructose was delayed until glucose was nearly depleted from the medium. Measurements of intracellular sucrose content and cytoplasmatic and vacuolar invertases indicate that the enzymatic activity seems to be correlated with a decrease in the hexose flux into the cells of R. jasminoides. Our results indicate that the behavior of cell suspension cultures of R. jasminoides regarding sugar utilization seems to be similar to other dicotyledonous undifferentiated cell suspension cultures.     

Sucrose suppression of chlorophyll synthesis in carrot tissue cultures: The role of invertase

Summary Free space invertase activities were determined in carrot callus strains CRT1 and CRT2 grown under conditions in which sucrose suppression of chlorophyll synthesis occurred in CRT1 but not CRT2. CRT2 possessed a high free space acid invertase activity (pH optimum 5.0 K_m for sucrose 3.1×10^-3M) while CRT1 lacked this enzyme. [U-¹⁴C] sucrose introduced into the free space of calluses was rapidly inverted by CRT2, but not by CRT1.Despite their different invertase levels, CRT1 and CRT2 showed similar sucrose uptake rates and took up [U-¹⁴C-glucosyl] sucrose and [5-T-glucosyl] sucrose from external bathing media essentially without prior inversion.It is concluded that acid invertase in callus tissue relieves the suppression of chlorophyll synthesis caused by sucrose in the free space. The invertase may in some circumstances hydrolyse sucrose before uptake, but is not an essential part of the sucrose uptake mechanism in carrot tissue cultures.     

Sugar uptake by maize endosperm suspension cultures

Maize (Zea mays L.) endosperm suspension cultures are a useful model system for studying biochemical and physiological events in developing maize endosperm. In this report, sugar uptake by the cultures is characterized. Uptake of ¹⁴C-labeled fructose and l-glucose was linear with time, while the rate of uptake of radioactivity from sucrose increased over a 120 min period. Both saturable and linear components of uptake were observed for fructose, glucose, sucrose, 1′-deoxy-1′-fluorosucrose, and maltose. Uptake of mannitol, sorbitol, and l-glucose took place at lower rates and was linear with concentration. Rates of incorporation of radioactivity from fructose and glucose exceeded that of sucrose at all concentrations tested. Kinetics of 1′-deoxy-1′-fluorosucrose uptake indicated that ¹⁴C from sucrose can be taken up by a saturable carrier of intact sucrose as well as by invertase hydrolysis and subsequent uptake of hexoses. Cell wall invertase was demonstrated histochemically. Further study of fructose uptake at a concentration at which the saturable component predominated revealed sensitivity to metabolic inhibitors, respiratory uncouplers, the nonpermeant sulfhydryl reagent p-chloromercuribenzenesulfonic acid, and nigericin. Uptake was not affected by valinomycin plus K⁺ and was stimulated by fusicoccin. Fructose and glucose uptake was not pH-sensitive below pH 7.0, whereas uptake of radioactivity from sucrose and 1′-deoxy-1′-fluorosucrose declined as the pH was increased above 5.0. Fructose uptake was not completely inhibited by glucose and vice versa, suggesting the presence of specific carriers. These results indicate that maize endosperm suspension cultures (a) absorb fructose via a typical, energy-requiring, carrier-mediated proton cotransport system; (b) possess saturable carriers for glucose and sucrose; and (c) also absorb sucrose via hexose uptake after sucrose hydrolysis by extracellular invertase.     

Metabolism of Radioactive Sugars by Tobacco leaf Disks

Destarched tobacco-leaf disks were floated on per cent. (w/v)solutions of sucrose uniformly labelled with ¹⁴C in either theglucose or fructose moiety, and on invert sugar in which onehexose only was so labelled. The experiments were carried outin an atmosphere of oxygen at 25° C. Seventy-five per cent,of the sugar lost from the external solutions was recoveredas starch, sucrose, fructose, glucose, and CO₂. With sucroseas the substrate, 30 per cent, of the material was recoveredas CO₂ and 17 per cent. each as starch, sucrose, fructose, andglucose. With invert sugar as the substrate, 30 per cent, wasagain recovered as CO₂ only 20 per cent. as the three sugarstogether, and 50 per cent. as starch. Whichever hexose was initiallylabelled and whether the sugar was supplied as sucrose or hexose,the relative specific activities of starch and sucrose in theleaf disks and of the CO₂ evolved were equal or nearly equalto that of the sugar supplied. With sucrose as the substratethe sucrose in the disks retained its asymmetry of label, andfree hexoses produced were similarly asymmetrically labelled.When invert sugar was the substrate the sucrose synthesizedwas strongly labelled in both moieties, as also were the freehexosea. It is concluded that fructose and glucose free or combinedin sucrose were equally available for starch synthesis and CO₂,formation, and that there can be no question of preferentialutilization of one or other hexose. Starch and CO₂ must arisefrom a common source in which readily formed derivatives ofthe hexoses are rapidly equilibrated. Free hexose cannot participatedirectly in either sucrose or starch synthesis. Accumulationof sugar not immediately metabolized and inversion of sucrosetake place at a site remote from the common pool. A scheme toaccommodate the results is discussed.     

Evidence for the presence of a sucrose carrier in immature sugar beet tap roots

The objectives of this work were to determine the path of phloem unloading and if a sucrose carrier was present in young sugar beet (Beta vulgaris L.) taproots. The approach was to exploit the characteristics of the sucrose analog, 1'-fluorosucrose (F-sucrose) which is a poor substrate for acid invertase but is a substrate for sucrose synthase. Ten millimolar each of [³H]sucrose and [¹⁴C]F-sucrose were applied in a 1:1 ratio to an abraded region of an attached leaf for 6 hours. [¹⁴C]F-sucrose was translocated and accumulated in the roots at a higher rate than [³H]sucrose. This was due to [³H]sucrose hydrolysis along the translocation path. Presence of [³H]hexose and [¹⁴C]F-sucrose in the root apoplast suggested apoplastic sucrose unloading with its subsequent hydrolysis. Labeled F-sucrose uptake by root tissue discs exhibited biphasic kinetics and was inhibited by unlabeled sucrose, indicating that immature roots have the ability for carrier-mediated sucrose transport from the apoplast. Collectively, in vivo and in vitro data indicate that despite sucrose hydrolysis by the wall-bound invertase, sucrose hydrolysis is not entirely essential for sugar accumulation in this tissue.     

Pathways of Uptake and Accumulation of Sugars in Tomato Fruit

The route of sucrose unloading from the conducting tissue, theregulation of sucrose hydrolysis and the uptake and subsequentmetabolism of sugars were investigated in the rapidly growingtomato fruit. During the first two weeks of fruit enlargement, the vacuoleaccounted for more than 85% of the protoplast volume and theintercellular space accounted for 20% of the fruit placentaltissue. The plasmodesmatal frequency was highest between phloemparenchyma cells and lowest between phloem sieve cells and phloemparenchyma. The total invertase activity was about 8 µmolglucose g^–1 d. wt min^–1 during the rapid growingperiod and increased six-fold at ripening. The wall-bound invertaseaccounted for less than 11% of the total activity. Invertaseactivity increased with increasing sucrose concentrations (upto 50 mM) in the incubation medium, but decreased at higherconcentrations. Sucrose synthase activity could only be detectedwhen fruit was older than 19 d. The uptake and metabolism of sugars by fruit cells were investigatedby incubation of fruit slices with ¹⁴C-sugars for 3 h. The uptakeof sucrose increased with the sucrose concentration up to 200mM. The rate of glucose uptake and its conversion to the ethanol-insolublefraction were higher than those of sucrose. The uptake of sucrosedid not compete with that of glucose or vice versa, providedthe osmotic potential of the incubation solution was maintainedconstant. The uptake of sucrose was not inhibited by metabolicinhibitors such as PCMBS, CCCP, sodium azide or vanadate. TheATPase activity in the fruit tissue was low. These findings did not identify conclusively the mode of sucroseunloading. However, the uptake of sugars by fruit cells is non-specificand does not appear to require a membrane carrier or plasmalemmaATPase to provide energy for sucrose uptake. Fruit, invertase, Lycopersicon esculentum, phloem unloading, plasmodesmata, sucrose     

Invertase Activity and its Relation to Hexose Accumulation in Potato Tubers

Hexose accumulation was shown to occur in freshly harvestedmature potato tubers (Solanum tuberosum L.) both after storageat 10 ?C and when subsequently transferred to low temperature(3 ?C) storage. In general, changes in hexoses and sucrose werefound to be related to changes in acid invertase activity. Totalacid invertase activity (i.e. assayed after destroying the endogenousinvertase inhibitor present in the extracts) generally reflectedsugar changes more closely than did basal activity (i.e. assayedwith the inhibitor present). There was no evidence of a specificalkaline invertase. A comparison of the temperature responsesof cultivar Record with that of two SCRI² clones demonstrateddistinct genotypic variation in the extent of hexose accumulation.However, these differences were not always reflected by genotypicdifferences in total invertase activity. Key words: Invertase inhibitor, glucose, fructose, sucrose     

Sucrose in the free space of translocating maize leaf bundles

Following exposure of portions of mature maize (Zea mays L.) leaf strips to ¹⁴CO₂, xylem exudate from the leaf strips contained [¹⁴C]sucrose. Sucrose was the only sugar in the xylem exudate which was obtained from the cut surface of the leaf strips by reducing the external pressure. The sucrose found in the xylem exudate apparently was obtained from the free space of the vascular bundles, its concentration amounting up to 0.25%. When [¹⁴C]glucose or [¹⁴C]fructose was supplied in the dark to one end of a maize leaf strip, each was taken up by the xylem, and transported to the opposite end. Xylem exudate from such leaf strips contained ¹⁴C-labeled sucrose in addition to the ¹⁴C-labeled hexose. The results of this study support the view that sucrose is loaded into the companion cell-sieve tube complexes from the apoplast of the vascular bundles in the maize leaf.     

The Effects of Leaf Age and Senescence on the Distribution of Carbon in Lolium temulentum

Assimilate distribution in leaves of Lolium temulentum was establishedby root absorption of [¹⁴C]sucrose and after exposure to ¹⁴CO₂.Age determined the amount of carbon assimilated, with more labelbeing incorporated during expansion than at maturity. Duringsenescence ¹⁴C assimilation was much lower. Ethanol-solubleextracts from various tissues of root-labelled plants containedmost of the radioactivity chiefly in basic and acidic compounds.The neutral fraction was composed predominantly of sucrose. Sucrose was comparably labelled in leaves from plants fed equalamounts of either [¹⁴C]sucrose, glucose, or fructose and onlytraces of labelled monosaccharides appeared in extracts. Radioactive sucrose was translocated rapidly from mature leaveswhereas, in the expanding leaf, carbon incorporation was directedtowards growth and the greater proportion of label present atligule formation was in ethanol-insoluble material. Induced senescence, of a mature leaf fed during expansion, produceda rapid loss from the pool of insoluble ¹⁴C. This was accompaniedby a reduction in the contents of chlorophyll and soluble proteinand an accumulation of amino acids. The onset of senescencecaused changes in leaf sugar levels which were correlated withincreased rates of respiration.     

Import of sucrose and its transformation to starch in the developing sorghum caryopsis

Levels of soluble and bound invertases and amylases were studied in relation to the changes in the free sugars and the accumulation of starch in the developing sorghum [Sorghum bicolor (L.) Moench, cv. spv. 351] caryopsis and its associated bractspedicel. Besides sucrose, glucose and fructose as the principal sugars, small amounts of sugars of the raffinose series were detected in the developing caryopsis. Through out the period of caryopsis development, the amount of reducing sugars was higher than that of sucrose. With the advancement in the development of the caryopsis, the contents and levels of sucrose rose with a concomitant fall in the activity of soluble acid (pH 4.8) invertase (EC 3.2.1.26) in the endosperm. In the pericarp-aleurone layer, the activity of soluble acid invertase predominated over soluble neutral (pH 7.5) invertase (EC 3.2.1.27). The activity of bound acid invertase declined with the ageing of the caryopsis. In bracts-pedicel, the activity of bound invertase and the levels of reducing sugars peaked around 18 days post anthesis. In these organs, the level of starch gradually decreased concomitantly with an increase in its level in the developing caryopsis. Amylases (EC 3.2.1.1 and 3.2.1.2) are distributed in the endosperm as well as in the pericarp-aleurone layer. On culturing detached ears in [U-¹⁴C]-sucrose solution for 6 h in the dark at 25°C, 80–90% of the ¹⁴C of extracted major sugars (i.e. sucrose + glucose + fructose) of the caryopsis appeared in sucrose alone. In comparison with the effects of glucose or fructose, transport into the caryopsis of ¹⁴C from [U-¹⁴C]-sucrose supplied to detached ears was promoted by the addition to the radiolabelled sucrose solution of 1% unlabelled sucrose. Addition to the [U-¹⁴C]-sucrose solution fed to the detached ears of 20 mM NaN₃ or HgCl₂ or galactose, lowered the amount of ¹⁴C in the free sugars and starch of the earyopsis.     

Living on a high sugar diet: the fate of sucrose ingested by a phloem-feeding insect,the pea aphid Acyrthosiphon pisum

The natural diet of aphids, plant phloem sap, generally contains high concentrations of sucrose. When pea aphids (Acyrthosiphon pisum) were fed on chemically defined diets containing sucrose radiolabelled in the glucose or fructose moiety, 2 to 12-fold and 87 to 110-fold more radioactivity was recovered from the tissues and honeydew, respectively, of aphids that ingested [U-(14)C-glucose]-sucrose than from those ingesting [U-(14)C-fructose]-sucrose. The total radioactivity recovered was 70% of the ingested [U-(14)C-glucose]-sucrose and <5% of ingested [U-(14)C-fructose]-sucrose. The dominant honeydew sugars produced by aphids feeding on 0.75 M sucrose diets were oligosaccharides comprising glucose. In vitro the guts of pea aphids had high sucrase activity, 1-5 U mg(-1) protein, generating equimolar glucose and fructose except at high sucrose concentrations where glucose production was inhibited (K(si)=0.1 M). These data suggest that the fructose moiety of ingested sucrose is assimilated very efficiently and may be preferentially respired by the aphid, and that the glucose moiety of sucrose is incorporated into oligosaccharides by the transglucosidase activity of the gut sucrase at high sucrose concentrations. These differences in the fate of sucrose-derived glucose and fructose are important elements in both the carbon nutrition and osmoregulation of aphids.     

Glucofructosan metabolism in Cichorium intybus roots

A 10-fold purification of sucrose sucrose fructosyl transferase from Cichorium intybus roots was achieved by ammonium sulphate fractionation and DEAE-cellulose column chromatography. The energy of activation for this enzyme was ca 48 kJ/mol sucrose. Sucrose sucrose fructosyl transferase and invertase were prominent during early months of growth. Evidence obtained from: (1) the changes in carbohydrate composition at monthly intervals; (2) comparative studies on fructosyl transferase and invertase at different stages of root growth; and (3) incubation studies with [¹⁴C]glucose, [¹⁴C]fructose and [¹⁴C]sucrose revealed that, during the later stages of root growth, fructosan hydrolase is responsible for fructosan hydrolysis. No evidence for the direct transfer of fructose from sucrose to high M_r glucofructosans was obtained.     

The regulation of sugar uptake and accumulation in bean pod tissue

The identity, localization and physiological significance of enzymes involved in sugar uptake and accumulation were determined for endocarp tissue of pods of Kentucky Wonder pole beans (Phaseolus vulgaris). An intracellular, alkaline invertase (pH optimum, 8) was assayed in extracted protein, as well as enzymes involved in sucrose synthesis, namely, uridinediphosphate (UDP-glucose pyrophosphorylase and UDP-glucose-fructose transglucosylase). Indirect evidence indicated the presence also of hexokinase, phosphohexoseisomerase and phosphoglucomutase. The data suggested that sucrose synthesis occurred in the cytoplasm, and that both sugar storage and an alkaline invertase occurred in the vacuole. The latter functions to hydrolyze accumulated sucrose. An outer space invertase (pH optimum, 4.0) was detected, but was variable in occurrence. Although its activity at the cell surface enhanced sucrose uptake, sucrose may be taken up unaltered.

Over a wide range of concentrations of exogenous glucose the sucrose/reducing sugar ratio of accumulated sugars remained unchanged at about 20. Synthesis of sucrose appears to be requisite to initial accumulation from glucose or fructose, as free hexoses do not increase at the apparent saturating concentration for uptake. Sucrose accumulation from exogenous hexose represents a steady-state value, in which sucrose is transported across the tonoplast into the vacuole at a rate equivalent to its rate of synthesis. Evidence indicates that this component of the accumulation process involves active transport of sucrose against a concentration gradient. The ratio of sucrose/reducing sugars in the accumulated sugars immediately after a period of uptake was inversely related to the level of inner space invertase. Within 16 hours after a period of accumulation, practically all of the sugar occurs as glucose and fructose.

The absence of competition among hexoses and sucrose indicated that a common carrier was not involved in their uptake. From a series of studies on the kinetics of uptake of glucose and fructose, including competition studies, the effects of inhibitors, radioactive assay of accumulated sugars and the distribution of label in accumulated sucrose it appeared that rate limitation for glucose or fructose uptake resides in the sequence of reactions leading to sucrose synthesis, rather than in a process mediated by a carrier protein.

     

Import of Sucrose and its Partitioning in the Synthesis of Galactomannan and Raffinose-oligosaccharides in the Developing Guar (Cyamopsis tetragonolobus) Seed

Import of sucrose and its transformation to galactomannan andraffinose-oligosaccharides have been studied in the developingguar seed. The amount of galactomannan gradually increased withthe ageing of the seed. During the entire period of pod development,sucrose constituted the major portion of the free sugars inthe seed (both endosperm and cotyledons) as well as in the podwall. Besides myo-inositol, the free sugars detected in thedeveloping endosperm and cotyledons were glucose, fructose,raffinose and stachyose. Some compounds, possibly glycosides(RG values higher than that of fructose), were also detectedin the endosperm. In the later stages of seed development, therelative proportion of raffinose in the free sugars increased,reaching 50% of the total free sugars in 77-d-old cotyledons.With pod maturity, the activities of soluble acid and boundacid invertases in the pod wall increased manifold with a concomitantdecline in the non-reducing sugar content. These enzymes seemto be involved in the mobilization of sucrose from this fruitingstructure into the seed. An increased synthesis of raffinose-oligosaccharidesboth in the endosperm and cotyledons was associated with highactivities of soluble acid invertase (pH 4.8) and sucrose-UDPglucosyl transferase in these tissues. Feeding uniformly labelled¹⁴C-sugars to the detached intact pods as well as to the isolatedendosperm and cotyledons resulted in labelling of all endogenousfree sugars and galactomannan. The uptake and incorporationinto galactomannan of ¹⁴C was stimulated by Co²⁺, Mn²⁺ and Mg²⁺.Except for mannose, a major proportion of the ¹⁴C from glucose,fructose and sucrose appeared in sucrose in both endosperm andcotyledons indicating a fast reconstitution of sucrose in situ.Based on the present results, a possible mode of transformationof sucrose to galactomannan and raffinose-oligosaccharides hasbeen proposed. Key words: Sucrose, galactomannan, raffinose-oligosaccharides, invertase, sucrose-UDP glucosyl transferase, ¹⁴C-incorporation, guar seed