Pathway of Phloem unloading of sucrose in corn roots

In Vitro Sugar Transport in Zea mays L. Kernels : I. Characteristics of Sugar Absorption and Metabolism by Developing Maize Endosperm

Short-term transport studies were conducted using excised whole Zea mays kernels incubated in buffered solutions containing radiolabeled sugars. Following incubation, endosperms were removed and rates of net (14)C-sugar uptake were determined. Endogenous sugar gradients of the kernel were estimated by measuring sugar concentrations in cell sap collected from the pedicel and endosperm. A sugar concentration gradient from the pedicel to the endosperm was found. Uptake rates of (14)C-labeled glucose, fructose, and sucrose were linear over the concentration range of 2 to 200 millimolar. At sugar concentrations greater than 50 millimolar, hexose uptake exceeded sucrose uptake. Metabolic inhibitor studies using carbonylcyanide-m-chlorophenylhydrazone, sodium cyanide, and dinitrophenol and estimates of Q(10) suggest that the transport of sugars into the developing maize endosperm is a passive process. Sucrose was hydrolyzed to glucose and fructose during uptake and in the endosperm was either reconverted to sucrose or incorporated into insoluble matter. These data suggest that the conversion of sucrose to glucose and fructose may play a role in sugar absorption by endosperm. Our data do not indicate that sugars are absorbed actively. Sugar uptake by the endosperm may be regulated by the capacity for sugar utilization (i.e. starch synthesis).

Phloem unloading in tobacco sink leaves: insensitivity to anoxia indicates a symplastic pathway

Phloem unloading in transition sink leaves of tobacco (Nicotiana tabacum L.) was analyzed by quantitative autoradiography. Detectable levels of labeled photoassimilates entered sink leaves approx. 1 h after source leaves were provided with (14)CO2. Samples of tissue were removed from sink leaves when label was first detected and further samples were taken at the end of an experimental phloem-unloading period. The amount of label in veins and in surrounding cells was determined by microdensitometry of autoradiographs using a microspectrophotometer. Photoassimilate unloaded from first-, second-and third-order veins but not from smaller veins. Import termination in individual veins was gradual. Import by the sink leaf was completely inhibited by exposing the sink leaf to anaerobic conditions, by placing the entire plant in the cold, or by steam-girdling the sink-leaf petiole. Phloem unloading was completely inhibited by cold; however, phloem unloading continued when the sink-leaf petiole was steam girdled or when the sink leaf was exposed to a N2 atmosphere. Compartmental efflux-analysis indicated that only a small percentage of labeled nutrients was present in the free space after unloading from sink-leaf veins in a N2 atmosphere. The results are consistent with passive symplastic transfer of photoassimilates from phloem to surrounding cells.

Diurnal Changes in Maize Leaf Photosynthesis : III. Leaf Elongation Rate in Relation to Carbohydrates and Activities of Sucrose Metabolizing Enzymes in Elongating Leaf Tissue

Maize (Zea mays L. cv. Pioneer 3184) leaf elongation rate was measured diurnally and was related to diurnal changes in the activities of sucrose metabolizing enzymes and carbohydrate content in the elongating portion of the leaf. The rate of leaf elongation was greatest at midday (1300 hours) and was coincident with the maximum assimilate export rate from the distal portion of the leaf. Leaf elongation during the light period accounted for 70% of the total observed increase in leaf length per 24 hour period. Pronounced diurnal fluctuations were observed in the activities of acid and neutral invertase and sucrose phosphate synthase. Maximum activities of sucrose phosphate synthase and acid invertase were observed at 0900 hours, after which activity declined rapidly. The activity of sucrose phosphate synthase was substantially lower than that observed in maize leaf source tissue. Neutral invertase activity was greatest at midday (1200 hours) and was correlated positively with diurnal changes in leaf elongation rate. There was no significant change in the activity of sucrose synthase over the light/dark cycle. Sucrose accumulation rate increased during a period when leaf elongation rate was maximal and beginning to decline. Maximum sucrose concentration was observed at 1500 hours, when the activities of sucrose metabolizing enzymes were low. At no time was there a significant accumulation of hexose sugars. The rate of starch accumulation increased after the maximum sucrose concentration was observed, continuing until the end of the light period. There was no delay in the onset of starch mobilization at the beginning of the dark period, and essentially all of the starch was depleted by the end of the night. Mobilization of starch in the elongating tissue at night could account for a significant proportion of the calculated increase in the tissue dry weight due to growth. Collectively, the results suggested that leaf growth may be controlled by the activities of certain sucrose metabolizing enzymes and may be coordinated with assimilate export from the distal, source portion of the leaf. Results are discussed with reference to diurnal photoassimilation and export in the distal, source portion of the leaf.

Assimilate Unloading from Maize (Zea mays L.) Pedicel Tissues : I. Evidence for Regulation of Unloading by Cell Turgor

Sugar and (14)C-assimilate release from the pedicel tissue of attached maize (Zea mays L.) kernels was studied following treatment with solute concentrations of up to 800 millimolal. Exposure and collection times ranged from 3 to 6 hours. Sugar and (14)C-assimilate unloading and collection in agar traps was reduced by 25 and 43%, respectively, following exposure to 800 millimolal mannitol. Inhibition of unloading was not specific to mannitol, since similar concentrations of glucose, fructose, or equimolar glucose plus fructose resulted in comparable inhibition. Ethylene glycol, a rapidly permeating solute which should not greatly influence cell turgor, did not inhibit (14)C-assimilate unloading. Based on these results, we suggest that inhibition of unloading by high concentrations of sugar or mannitol was due to reduced pedicel cell turgor. Changes in pedicel cell turgor may play a role in the regulation of assimilate transfer within the maize kernel.

Labeling of fructans in winter wheat stems

Fructans synthesized from newly formed assimilates accumulate in wheat stems as nonstructural carbohydrates. Experiments performed tested the hypothesis that the fructose moiety from translocated sucrose is used preferentially in biosynthesis of these fructans. Results indicated: (a) a large percentage of labeled sucrose was translocated and unloaded in an unaltered state; and (b) sucrose contributed its fructose moiety to fructan synthesis in stems.

STRUCTURAL ASPECTS OF THE PRIMARY TISSUES OF THE CUCURBITA PEPO L. ROOT WITH SPECIAL REFERENCE TO THE PHLOEM

The primary phloem and associated tissues in the root of Cucurbita pepo L. were examined by light and transmission electron microscopy to provide information on the feasibility of symplastic transport from the sieve-tube members to the cortex in this organ. The structure, distribution and frequency of cytoplasmic connections between the various cell types of the root are reported as well as the cytological characteristics of the various cells. The protoplasts of contiguous cells of the root are joined by various numbers of cytoplasmic connections: plasmodesmata (simple and branched) between parenchymatous elements; pore-plasmodesmata between sieve-tube members and parenchymatous elements (companion, phloem parenchyma or pericycle cells in the Cucurbita root); and sieve-area pores between contiguous sieve elements. The plasmodesmata associated with secondary- and tertiary-state endodermal cells are not constricted by the suberin lamellae. Differences in the frequency of cytoplasmic connections between the various cell types of the primary phloem, pericycle and ground tissue do occur with the highest frequency between sieve-tube members and companion cells. The results indicate that the structure of the C. pepo root may be compatible with a symplastic pathway of phloem unloading and transport to the cells of the ground tissue.

Phloem Unloading in Developing Leaves of Sugar Beet : I. Evidence for Pathway through the Symplast

Physiological and transport data are presented in support of a symplastic pathway of phloem unloading in importing leaves of Beta vulgaris L. (;Klein E multigerm'). The sulfhydryl reagent p-chloromercuribenzene sulfonic acid (PCMBS) at concentration of 10 millimolar inhibited uptake of exogenous [(14)C]sucrose by sink leaf tissue over sucrose concentrations of 0.1 to 5.0 millimolar. Inhibited uptake was 24% of controls. The same PCMBS treatment did not affect import of (14)C-label into sink leaves during steady state labeling of a source leaf with (14)CO(2). Lack of inhibition of import implies that sucrose did not pass through the free space during unloading. A passively transported xenobiotic sugar, l-[(14)C]glucose, imported by a sink leaf through the phloem, was evenly distributed throughout the leaf as seen by whole-leaf autoradiography. In contrast, l-[(14)C]glucose supplied to the apoplast through the cut petiole or into a vein of a sink leaf collected mainly in the vicinity of the major veins with little entering the mesophyll. These patterns are best explained by transport through the symplast from phloem to mesophyll.

Carbon Transport and Root Respiration of Split Root Systems of Phaseolus vulgaris Subjected to Short Term Localized Anoxia

The influence of anoxia on carbon transport and root respiration was evaluated by applying [U-(14)C]sucrose to the foliage. Translocation patterns to the root systems of two dry edible bean genotypes (Phaseolus vulgaris L.) were examined after a 3-day exposure to aerated and nonaerated environments. Localized anoxia of root systems was simulated by growing roots in split configurations and exposing half of the system to anoxic conditions. Anoxia of the root system for 72 hours reduced the movement of (14)C label into the roots with concurrent accumulations in the hypocotyl region. The translocation of (14)C label to anoxic roots was less than 50% of the aerated controls of both genotypes. Most of the (14)C label translocated to anoxic root systems was excluded from respiratory metabolism during the 3-hour pulse/chase period and was an order of magnitude less than the aerated controls. These observations suggest that the bulk of (14)C label which entered the root during the anoxic period was unavailable for metabolism by the enzymes of glycolysis and/or was diluted by a relatively large metabolite pool. A higher percentage of (14)C label was translocated to the aerated half of the localized anoxia treatment relative to the half of the aerated controls. The proportion of (14)C label translocated to the root system in the aerated control was 20 and 16% compared to 28 and 25% in the aerated localized anoxia treatment for the genotypes Seafarer and line 31908, respectively. Line 31908 partitioned a greater percentage of (14)C-labeled compounds to the actively growing fraction of the root system in the localized anoxia treatment than did Seafarer. This suggests a greater reliance on previously stored carbohydrate for immediate root growth in Seafarer than in line 31908.

Sugar Efflux from Maize (Zea mays L.) Pedicel Tissue

Sugar release from the pedicel tissue of maize (Zea mays L.) kernels was studied by removing the distal portion of the kernel and the lower endosperm, followed by replacement of the endosperm with an agar solute trap. Sugars were unloaded into the apoplast of the pedicel and accumulated in the agar trap while the ear remained attached to the maize plant. The kinetics of (14)C-assimilate movement into treated versus intact kernels were comparable. The rate of unloading declined with time, but sugar efflux from the pedicel continued for at least 6 hours and in most experiments the unloading rates approximated those necessary to support normal kernel growth rates. The unloading process was challenged with a variety of buffers, inhibitors, and solutes in order to characterize sugar unloading from this tissue.Unloading was not affected by apoplastic pH or a variety of metabolic inhibitors. Although p-chloromercuribenzene sulfonic acid (PCMBS), a nonpenetrating sulfhydryl group reagent, did not affect sugar unloading, it effectively inhibited extracellular acid invertase. When the pedicel cups were pretreated with PCMBS, at least 60% of sugars unloaded from the pedicel could be identified as sucrose. Unloading was inhibited up to 70% by 10 millimolar CaCl(2). Unloading was stimulated by 15 millimolar ethyleneglycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid which partially reversed the inhibitory effects of Ca(2+). Based on these results, we suggest that passive efflux of sucrose occurs from the maize pedicel symplast followed by extracellular hydrolysis to hexoses.

Effect of path or sink anoxia on sugar translocation in roots of maize seedlings

After feeding the scutellum of young maize seedlings with a labeled analog of glucose, 2-deoxy-d-glucose, the progress of radioactivity along the root was followed when either 70% of the path or the whole root were in strict anoxic conditions, and was compared with the translocation pattern of aerobic seedlings. Special care was taken to suppress the internal O(2) transport and to control its occurrence.In air, the radioactive compounds accumulated from 30 minutes in the root tip mainly as an analog of sucrose. When the whole root was anoxic, the progress of the radioactivity was very slow and never reached the tip which did not survive more than 8 hours. When 70% of the path was in strict anoxia and the sink (root tip) in air, the translocation was not impaired and the radioactivity accumulated in the tips as fast as in aerobic controls. The addition of 3 millimolar NaF, which inhibits the fermentative energy production, did not modify these results. It is concluded that long distance transport in maize sieve tubes has no special energy requirements and is controlled by source-sink relationships. The inhibition of sugar supply in anoxic root tips is attributed to an effect on unloading processes rather than on sink metabolism.

Transport and metabolism of 1'-fluorosucrose, a sucrose analog not subject to invertase hydrolysis

The novel sucrose derivative 1'-fluorosucrose (alpha-d-glucopyranosyl-beta- d-1-deoxy-1-fluorofructofuranoside) was synthesized in order to help define mechanisms of sucrose entry into plant cells. Replacement of the 1'-hydroxyl by fluorine very greatly reduces invertase hydrolysis of the derivative (hydrolysis at 10 millimolar 1'-fluorosucrose is less than 2% that of sucrose) but does not reduce recognition, binding, or transport of 1'-fluorosucrose by a sucrose carrier. Transport characteristics of 1'-fluorosucrose were studied in three different tissues. The derivative is transported by the sucrose carrier in the plasmalemma of developing soybean cotyledon protoplasts with a higher affinity than sucrose (K(m) 1'-fluorosucrose 0.9 millimolar, K(m) sucrose 2.0 millimolar). 1'-Fluorosucrose is a competitive inhibitor of sucrose uptake with an apparent K(i) also of 0.9 millimolar, while the K(i) of sucrose competition of 1'-fluorosucrose uptake was 2.0 millimolar. Thus, both sugars are recognized at the same binding site in the plasmalemma. Both sucrose and 1'-fluorosucrose show very similar patterns of phloem translocation from an abraded leaf surface through the petiole indicating that recognition of 1'-fluorosucrose by sucrose carriers involved in phloem loading is likely as well.1'-Fluorosucrose is a very poor substrate for invertase and as such is absorbed only slowly by corn root segments, a tissue in which sucrose hydrolysis by a cell wall invertase is required prior to active hexose uptake.The kinetics of 1'-fluorosucrose uptake by soybean cotyledon protoplasts indicate that membrane passage and substrate release to the protoplast interior are rate limiting to transport. Recognition of sucrose at the inner membrane surface of the carrier protein is apparently different than recognition and binding at the external surface.

Sugar transport in isolated corn root protoplasts

Isolated corn (Zea mays L.) root protoplasts were used to study sucrose and hexose uptake. It is found that glucose was preferentially taken up by the protoplasts over sucrose and other hexoses. Glucose uptake showed a biphasic dependence on external glucose concentration with saturable (K(m) of 7 millimolar) and linear components. In contrast, sucrose uptake only showed a linear kinetic curve. Sucrose and glucose uptake were linear over a minimum of 1 hour at pH 6.0 and 1 millimolar exogenous sugar concentration. Glucose uptake showed a sharp 42 degrees C temperature optimum, while sucrose uptake showed a lower temperature sensitivity which did not reach a maximum below 50 degrees C. Uptake of both sugars was sensitive to several metabolic inhibitors and external pH. Differences between sucrose and glucose uptake in two different sink tissue (i.e. protoplasts from corn roots and soybean cotyledons) are discussed.

Crop Productivity and Photoassimilate Partitioning

The photosynthetic basis for increasing the yield of major field crops is examined in terms of improving the interception of seasonal solar radiation by crop foliage, the efficiency of conversion of intercepted light to photosynthetic assimilates, and the partitioning of photoassimilates to organs of economic interest. It is concluded that, in practice, genetic and chemical manipulation of light interception over the season and of partitioning offer the most potential for achieving further increases in yield. During the history of improvement of genetic yield potential of crops, increase in the partitioning of photoassimilates to harvested organs has been of primary importance.

Sugar transport into protoplasts isolated from developing soybean cotyledons : I. Protoplast isolation and general characteristics of sugar transport

A procedure is described to isolated functional protoplasts from developing soybean (Glycine max L. Merr. cv Wye) cotyledons. Studies of sucrose and hexose uptake into these protoplasts show that the plasmalemma of cotyledons during the stage of rapid seed growth contains a sucrose-specific carrier which is energetically and kinetically distinct from the system(s) involved in hexose transport. For example, sucrose, but not hexose uptake: (a) is inhibited by alkaline pH and the nonpermeant SH modifier, p-chloromercuribenzene sulfonic acid; (b) is stimulated by fusicoccin; (c) shows both a saturable and a linear component of uptake in response to substrate concentration; and (d) displays a sharp temperature response (high Q(10) value and high activation energies).

Sugar uptake by cotton tissues: leaf disc versus cultured roots

The tissue accumulation of sucrose, glucose, and fructose has been studied in cultured cotton (Gossypium hirsutum L.) roots and leaf discs. Sucrose uptake by both tissues from high apoplastic concentrations was independent of pH but has a slightly acidic pH optimum from low concentrations. Like other higher plant tissues, cotton root cells accumulate sucrose via a ;saturable,' inhibitor-sensitive mechanism and a linear, inhibitor-resistant mechanism. The linear mechanism of sucrose uptake is not as pronounced in leaf disc data as it is in root data. Further, sucrose uptake by cotton leaf discs is more resistant than uptake by root cells to pH alterations, inhibitors, and monosaccharides in the uptake medium. The saturable phase of sucrose influx into cotton root is eliminated by glucose, fructose, and high pH. Sucrose influx into both tissues is not altered by osmotica up to 200 milliOsmolar. Sucrose accumulated by both tissues is rapidly converted to other chemical forms, especially in root tissue where only approximately 50% remains as neutral sugars 1 hour following the start of radiolable exposure. Although the entry of radiolabeled sucrose is faster in abraded leaf discs, they give the same response patterns to pH, inhibitors, and monosaccharide as do unabraded discs.The sucrose accumulation kinetics of cotton roots and leaf discs differ. These differences may be related to the physiological roles (source versus sink) of the two tissues in the intact plant.