Sucrose carrier RcSCR1 is involved in sucrose retrieval, but not in sucrose unloading in growing hypocotyls of Ricinus communis L

The transport of assimilates from source to sink tissues is mediated by the phloem. Along the vascular system the phloem changes its physiological function from loading phloem to transport and unloading phloem. Sucrose carrier proteins have been identified in the transport phloem, but it is unclear whether the physiological role of these transporters is phloem unloading of sucrose or retrieval of apoplasmic sucrose back into the sieve element/companion cell complex. Here, we describe the dynamic expression of the Ricinus communis sucrose carrier RcSCR1 in the hypocotyl at different sink strengths. Our results indicate that phloem unloading in castor bean is not catalysed by the phloem loader RcSCR1. However, this sucrose carrier represents the molecular basis of the sucrose retrieval mechanism along the transport phloem, which is dynamically adjusted to the sink strength. As a consequence, we assume that other release carrier(s) exist in sink tissues, such as the hypocotyl, in R. communis.     

On sieve tube function

Summary Several theories of phloem transport are currently being considered in various laboratories on the basis of recent evidence. Proponents of the activated diffusion or protoplasmic theories claim support in the disclosure of fibrils, longitudinally arranged, in the connecting strands passing across sieve plates, and in the close connection between respiratory activity and transport. Those favoring a surface migration theory claim support in the demonstrated systems of sieve element lamellae, along whose surfaces one could imagine solute transport to occur. Proponents of the pressure flow theory point to results of exudation studies, tracer investigations, and to histochemical evidence indicating that sieve elements are relatively inactive, metabolically, as well as to the suggestion that perhaps the connecting strands are more open (tubular) than they have been considered to be up to now.Callose formation is stimulated by turgor changes, promoted by foreign chemicals, viruses, and, in the sieve element, by a relatively alkaline pH, a high sucrose concentration, and doubtless by the unique unbalanced character of sieve sap composition. The function of callose in older or wounded elements appears to be a constricting or plugging action, but its function in young mature elements is essentially obscure.Recent evidence augments the view that sieve elements display an extraordinary sensitivity towards experimental manipulation.     

Surface-templated fibril growth of peptide fragments from the shaft domain of the adenovirus fibre protein

Here we present a study of five analogues of a fragment from the shaft domain of the adenovirus fibre protein that readily form fibrils under a range of conditions. Using atomic force microscopy the fibrillisation of these peptides at the liquid/solid interface utilizing ordered crystalline substrates has been investigated. Our results demonstrate that the assembly pathway at the liquid/solid interface enables only the formation of truncated fibrillar structures, which align along the substrate's underlying atomic lattice during growth. Furthermore, that the concentration and volume of solution applied can be used to directly control the density of fibrillar coverage at the surface.     

Phloem loading and unloading of sugars and amino acids

In terrestrial higher plants, phloem transport delivers most nutrients required for growth and storage processes. Some 90% of plant biomass, transported as sugars and amino nitrogen (N) compounds in a bulk flow of solution, is propelled though the phloem by osmotically generated hydrostatic pressure differences between source (net nutrient export) and sink (net nutrient import) ends of phloem paths. Source loading and sink unloading of sugars, amino N compounds and potassium largely account for phloem sap osmotic concentrations and hence pressure differences. A symplasmic component is characteristic of most loading and unloading pathways which, in some circumstances, may be interrupted by an apoplasmic step. Raffinose series sugars appear to be loaded symplasmically. However, sucrose, and probably certain amino acids, are loaded into minor veins from source leaf apoplasms by proton symporters localized to plasma membranes of their sieve element/companion cell (se/cc) complexes. Sucrose transporters, with complementary kinetic properties, are conceived to function as membrane transporter complexes that respond to alterations in source/sink balance. In contrast, symplasmic unloading is common for many sink types. Intervention of an apoplasmic step, distal from importing phloem, is reserved for special situations. Effluxers that release sucrose and amino acids to the surrounding apoplasm in phloem loading and unloading are yet to be cloned. The physiological behaviour of effluxers is consistent with facilitated membrane transport that can be energy coupled. Roles of sucrose and amino acid transporters in phloem unloading remain to be discovered along with mechanisms regulating symplasmic transport. The latter is hypothesized to exert significant control over phloem unloading and, in some circumstances, phloem loading.     

Quantification of plasmodesmatal endoplasmic reticulum coupling between sieve elements and companion cells using fluorescence redistribution after photobleaching

Transgenic tobacco (Nicotiana tabacum) was studied to localize the activity of phloem loading during development and to establish whether the endoplasmic reticulum (ER) of the companion cell (CC) and the sieve element (SE) reticulum is continuous by using a SUC2 promoter-green fluorescent protein (GFP) construct targeted to the CC-ER. Expression of GFP marked the collection phloem in source leaves and cotyledons as expected, but also the transport phloem in stems, petioles, midveins of sink leaves, nonphotosynthetic flower parts, roots, and newly germinated seedlings, suggesting that sucrose retrieval along the pathway is an integral component of phloem function. GFP fluorescence was limited to CCs where it was visualized as a well-developed ER network in close proximity to the plasma membrane. ER coupling between CC and SEs was tested in wild-type tobacco using an ER-specific fluorochrome and fluorescence redistribution after photobleaching (FRAP), and showed that the ER is continuous via pore-plasmodesma units. ER coupling between CC and SE was quantified by determining the mobile fraction and half-life of fluorescence redistribution and compared with that of other cell types. In all tissues, fluorescence recovered slowly when it was rate limited by plasmodesmata, contrasting with fast intracellular FRAP. FRAP was unaffected by treatment with cytochalasin D. The highest degree of ER coupling was measured between CC and SE. Intimate ER coupling is consistent with a possible role for ER in membrane protein and signal exchange between CC and SE. However, a complete lack of GFP transfer between CC and SE indicated that the intraluminal pore-plasmodesma contact has a size exclusion limit below 27 kD.     

Effects of low light on phloem ultrastructure and subcellular localization of sucrose synthase in <Emphasis Type="Italic">Prunus persica</Emphasis> L. var. <Emphasis Type="Italic">nectarina</Emphasis> Ait. fruit

Using electron microscopy, the ultrastructure of phloem unloading zone was examined in the Prunus persica L. var. nectarina Ait. fruit. Our study showed that, in the SE/CC (sieve element/companion cell) complexes, CC developing under low light had a thin cytoplasm layer with few mitochondria and numerous small vacuoles, and not clearly seen nuclei. The cytoplasm vacuolation indicated that the cytoskeleton was destroyed at low light. The effects of low light on CC development suggest that unloading evidently linked to the low accumulation of soluble sugars by fruit. At the young fruit stage, flesh parenchyma around the phloem tissue had no starch grains in the plastids in fruit developing under low light. This is a further indication that less photoassimilates was translocated from source leaves to fruit sinks under low light during the young fruit developmental stage. The activity of sucrose synthase (SuSy), the key enzyme of sucrose metabolism in fruit, increased dramatically during fruit maturation. The highest SuSy activity during the rapid fruit growth phase suggests that sink strength could be correlated with the SuSy activity. The high SuSy activity under normal light possibly indicates that fruit had a capacity to utilize sucrose irrespective of their site of phloem unloading. Immunogold electron microscopy showed that SuSy was localized mainly in the vacuole of flesh parenchyma cells. The vacuole-localized SuSy can hydrolyze sucrose imported from the phloem, which may explain the apparent correlation between SuSy activity and phloem unloading. The double sieve element (SE/SE) complexes occurred in a greater number and had thicker cell walls under normal light intensity than under low light intensity. These data demonstrate clearly that low light decreased SuSy activity in the control of phloem unloading. Published in Russian in Fiziologiya Rastenii, 2009, Vol. 56, No. 4, pp. 509–517. This text was submitted by the authors in English.     

Expression of a yeast-derived invertase in companion cells results in long-distance transport of a trisaccharide in an apoplastic loader and influences sucrose transport

Companion cell-specific expression of a cytosolic invertase from yeast (Saccharomyces cerevisiae) was used as a tool to synthesise oligosaccharides in the sieve element/companion cell complex and study whether oligosaccharides could be transported in the phloem of an apoplastically loading species. Potato (Solanum tuberosum L.) plants expressing the invertase under the control of the Agrobacterium tumefaciens rolC promoter produced the trisaccharide 6-kestose in leaves, which was transported via the phloem and accumulated in tubers of transgenic plants. In graft experiments with rolC invertase plants as scion and wild-type rootstocks, 6-kestose accumulated in tubers to levels comparable to sucrose. This shows that long-distance transport of oligosaccharides is possible in apoplastically loading plants, which normally transport only sucrose. The additional transport route for assimilates neither led to elevated photosynthetic activity nor to increased tuber yield. Enhanced sucrose turnover in companion cells caused large amounts of glucose and fructose to be exuded from leaf petioles, and elevated levels of sucrose were detected in phloem exudates. While the latter indicates a higher capacity for sucrose loading into the phloem due to increased metabolic activity of companion cells, the massive release of hexoses catalysed by the invertase seemed to interfere with assimilate delivery to sink organs.Abbreviations HPAEC High-performance liquid anion-exchange chromatography - SE–CCC Sieve element/companion cell complex - WT Wild type     

Phloem loading in two Scrophulariaceae species. What can drive symplastic flow via plasmodesmata?

To determine the driving forces for symplastic sugar flux between mesophyll and phloem, gradients of sugar concentrations and osmotic pressure were studied in leaf tissues of two Scrophulariaceae species, Alonsoa meridionalis and Asarina barclaiana. A. meridionalis has a typical symplastic configuration of minor-vein phloem, i.e. intermediary companion cells with highly developed plasmodesmal connections to bundle-sheath cells. In A. barclaiana, two types of companion cells, modified intermediary cells and transfer cells, were found in minor-vein phloem, giving this species the potential to have a complex phloem-loading mode. We identified all phloem-transported carbohydrates in both species and analyzed the levels of carbohydrates in chloroplasts, vacuoles, and cytoplasm of mesophyll cells by nonaqueous fractionation. Osmotic pressure was measured in single epidermal and mesophyll cells and in whole leaves and compared with calculated values for phloem sap. In A. meridionalis, a 2-fold concentration gradient for sucrose between mesophyll and phloem was found. In A. barclaiana, the major transported carbohydrates, sucrose and antirrhinoside, were present in the phloem in 22- and 6-fold higher concentrations, respectively, than in the cytoplasm of mesophyll cells. The data show that diffusion of sugars along their concentration gradients is unlikely to be the major mechanism for symplastic phloem loading if this were to occur in these species. We conclude that in both A. meridionalis and A. barclaiana, apoplastic phloem loading is an indispensable mechanism and that symplastic entrance of solutes into the phloem may occur by mass flow. The conditions favoring symplastic mass flow into the phloem are discussed.     

Nonvascular,Symplasmic Diffusion of Sucrose Cannot Satisfy the Carbon Demands of Growth in the Primary Root Tip of Zea mays L

Nonvascular, symplasmic transport of sucrose (Suc) was investigated theoretically in the primary root tip of maize (Zea mays L. cv WF9 x Mo 17) seedlings. Symplasmic diffusion has been assumed to be the mechanism of transport of Suc to cells in the root apical meristem (R.T. Giaquinta, W. Lin, N.L. Sadler, V.R. Franceschi [1983] Plant Physiol 72: 362-367), which grow apical to the end of the phloem and must build all biomass with carbon supplied from the shoot or kernel. We derived an expression for the growth-sustaining Suc flux, which is the minimum longitudinal flux that would be required to meet the carbon demands of growth in the root apical meristem. We calculated this flux from data on root growth velocity, area, and biomass density, taking into account construction and maintenance respiration and the production of mucilage by the root cap. We then calculated the conductivity of the symplasmic pathway for diffusion, from anatomical data on cellular dimensions and the frequency and dimensions of plasmodesmata, and from two estimates of the diffusive conductance of a plasmodesma, derived from independent data. Then, the concentration gradients required to drive a growth-sustaining Suc flux by diffusion alone were calculated but were found not to be physiologically reasonable. We also calculated the hydraulic conductivity of the plasmodesmatal pathway and found that mass flow of Suc solution through plasmodesmata would also be insufficient, by itself, to satisfy the carbon demands of growth. However, much of the demand for water to cause cell expansion could be met by the water unloaded from the phloem while unloading Suc to satisfy the carbon demands of growth, and the hydraulic conductivity of plasmodesmata is high enough that much of that water could move symplasmically. Either our current understanding of plasmodesmatal ultrastructure and function is flawed, or alternative transport mechanisms must exist for Suc transport to the meristem.     

Sieve element unloading: cellular pathway, mechanism and control

The transport and distribution of phloem – mobile solutes is predominantly determined by transport processes located at the sink end of the source – transport – sink system. Transport across the sieve element boundary, sieve element unloading, is the first of a series of sink transport processes. Unloading of solutes from the sieve elements may follow an apo- or symplastic route. It is speculated that the unloading pathway is integrated with sink function and that apoplastic unloading is restricted to situations in which movement through the symplast is not compatible with sink function. These situations include axial transport and storage of osmotically active solutes against concentration and turgor gradients between the sieve elements and sink cells. Coupled with alteration in sink function, the cellular pathway of unloading can switch in stems and possibly other sinks. Experimental systems and approaches used to elucidate the mechanism of sieve element unloading are reviewed. Unloading fluxes to the apoplast can largely be accounted for by membrane diffusion in axial sinks. However, the higher fluxes in storage sinks suggests dependence on some form of facilitated transport. Proton sucrose symport is assessed to be a possible mechanism for facilitated efflux of solutes across the sieve element plasma membrane to the sink apoplast. Unloading through the symplast may occur by diffusion or mass flow. The latter mechanism serves to dissipate phloem water and hence prevent the potential elevation of sieve element turgor that would otherwise slow phloem import into the sink. The possibility of energised plasmodesmatal transport is raised. Sieve element unloading must be integrated with subsequent compartmentation and metabolism of the unloaded solute. Solute levels are an obvious basis for control of sieve element unloading, but are found to offer limited scope for a mass action mechanism. Apoplastic, cellular pathway, sieve element, solute transport, symplastic. Translated into a turgor signal, solute levels could regulate the rate of unloading, metabolism and compartmentation forming part of a turgor homeostat irrespective of the pathway of unloading.     

Molecular chain orientation of DNA films induced by both the magnetic field and the interfacial effect

DNA films showing highly homogeneous orientation of molecular chains were successfully prepared by drying a semidiluted solution in a horizontal magnetic field. Most of the molecular chain elements in the obtained film were found to be one-dimensionally oriented, as shown by X-ray diffraction, polarization microscopy, and linear dichroism spectroscopy. Because a DNA chain is theoretically expected to orientate only in divergent directions perpendicular to a magnetic field, this result suggests that the DNA chains were aligned not only by a magnetic field but also by the interfacial effect that induced the chains to fit along the air-liquid interface. The descent speed of an air-liquid interface by evaporation was faster than the estimated diffusion rate of DNA, suggesting an emergence of a concentrated layer near the surface. As proved by polarization microscopy, this emergence led to the transitional formation of a nematic-like liquid crystalline phase, which resulted in a DNA film with good chain alignment and unitary orientation. This mechanism underlying chain alignment was supported by molecular weight dependency, in which higher molecular weight DNA is more likely to evince chain alignment that exhibits a higher degree of birefringence. Low molecular weight components have such high thermal motility that it would be difficult to fit them along the air-liquid interface in the early stage of drying. For chain alignment, it was preferable to use an initial concentration of DNA lower than a critical concentration for liquid crystal formation so that the possible diffusion and assembly in a diluted solution would be essential for chain alignment. The DNA film exhibited obvious linear dichroism, indicating the potential for further applications.     

The mechanism of phloem loading in rice (Oryza sativa)

Carbohydrates, mainly sucrose, that are synthesized in source organs are transported to sink organs to support growth and development. Phloem loading of sucrose is a crucial step that drives long-distance transport by elevating hydrostatic pressure in the phloem. Three phloem loading strategies have been identified, two active mechanisms, apoplastic loading via sucrose transporters and symplastic polymer trapping, and one passive mechanism. The first two active loading mechanisms require metabolic energy, carbohydrate is loaded into the phloem against a concentration gradient. The passive process, diffusion, involves equilibration of sucrose and other metabolites between cells through plasmodesmata. Many higher plant species including Arabidopsis utilize the active loading mechanisms to increase carbohydrate in the phloem to higher concentrations than that in mesophyll cells. In contrast, recent data revealed that a large number of plants, especially woody species, load sucrose passively by maintaining a high concentration in mesophyll cells. However, it still remains to be determined how the worldwide important cereal crop, rice, loads sucrose into the phloem in source organs. Based on the literature and our results, we propose a potential strategy of phloem loading in rice. Elucidation of the phloem loading mechanism should improve our understanding of rice development and facilitate its manipulation towards the increase of crop productivity.     

Sucrose Concentration Gradients along the Post-Phloem Transport Pathway in the Maternal Tissues of Developing Wheat Grains

Sucrose concentrations were measured in serial frozen sections of the post-phloem transport pathway in developing wheat (Triticum aestivum L.) grains. In normally importing grains, there was an approximately linear concentration gradient along the pathway, with a difference between the ends of the pathway of about 180 mM. This indicates an unusually low resistance for cell-to-cell transport, due perhaps to the large size-exclusion limit for the pathway. However, the existence of concentration gradients raises presently unresolvable questions about the relative contributions of diffusion versus bulk flow to transport within the symplast. The concentration gradient disappeared when sucrose movement ceased (i.e. in excised grains or when endosperm cavities of attached grains were perfused with p-chloromercuribenzene sulfonate [PCMBS] or with 1660 mOsm sorbitol). PCMBS appeared to block solute release into the endosperm cavity, whereas the sorbitol treatment, previously shown to cause localized plasmolysis in the chalaza, appeared to block movement across the chalaza. Sieve element/companion cell unloading appears to be an important control point for assimilate import. The sucrose concentration gradient and, probably, turgor and osmotic gradients are extremely steep there. PCMBS blocked import without affecting the sucrose concentration in the vascular parenchyma around the phloem. Thus, blockage of unloading was more complex than a simple "backing up" of solutes in the vascular parenchyma.     

The fungal UmSrt1 and maize ZmSUT1 sucrose transporters battle for plant sugar resources

The biotrophic fungus Ustilago maydis causes corn smut disease, inducing tumor formation in its host Zea mays. Upon infection, the fungal hyphae invaginate the plasma membrane of infected maize cells, establishing an interface where pathogen and host are separated only by their plasma membranes. At this interface the fungal and maize sucrose transporters, UmSrt1 and ZmSUT1, compete for extracellular sucrose in the corn smut/maize pathosystem. Here we biophysically characterized ZmSUT1 and UmSrt1 in Xenopus oocytes with respect to their voltage-,pH-and substrate-dependence and determined affinities toward protons and sucrose. In contrast to ZmSUT1,UmSrt1 has a high affinity for sucrose and is relatively pH-and voltage-independent. Using these quantitative parameters, we developed a mathematical model to simulate the competition for extracellular sucrose at the contact zone between the fungus and the host plant. This approach revealed that UmSrt1 exploits the apoplastic sucrose resource, which forces the plant transporter into a sucrose export mode providing the fungus with sugar from the phloem. Importantly, the high sucrose concentration in the phloem appeared disadvantageous for the ZmSUT1, preventing sucrose recovery from the apoplastic space in the fungus/plant interface.     

Sugar concentrations along and across the Ricinus communis L. hypocotyl measured by single cell sampling analysis

Single cell sap sampling and analysis were used to measure the longitudinal and radial distribution of sucrose, glucose and fructose in the apical cell division zone and in the basal, elongated zone of the Ricinus hypocotyl. Sucrose and hexose increased in concentration from the apex to the base of the seedling axis. In the cell division zone low hexose and sucrose concentrations prevailed in cortex and pith, with a slightly higher hexose concentration in pith cells. The sucrose concentrations in sieve tubes and in phloem were much higher than in the cortex and pith cells. In the basal zone of the hypocotyl high levels of sucrose in phloem, cortex and pith were found, therefore radial, diffusional sucrose flow away from the phloem was considered unlikely. It is proposed that radial flow of growth-water to the hypocotyl periphery together with the down-regulation of a sucrose transporter at the phloem leads to a preferential sucrose flow to the expanding cortex. The pith cells, which do not experience flow of growth-water, are probably insufficiently supplied with sucrose from the phloem resulting eventually in cell death as the plant grows. Shortage of sucrose supply, experimentally achieved by removal of the endosperm, led to sucrose hydrolysis in the pith. The sucrose levels in the other tissues decreased less. It appears that the hydrolysis to hexose was initiated to maintain the osmotic value in the pith cell sap. It is speculated that high hexose levels in the cells are indicative of insufficient sucrose supply via the phloem and that the pith cells are confronted with that situation during early seedling development.     

The Function of Phloem Connections in Regenerating In Vitro-Grafts*

In order to answer the question whether functioning phloem connections exist between graft partners, phloem transport has been studied in cultured explant-grafts after application of ¹⁴C-sucrose and carboxyfluorescein (CF) to the scion. Autografts of Lycopersicon esculentum and Helianthus annuus were investigated at various regeneration periods. Ungrafted internodes served as controls. A segmental analysis was used to determine the tissue distribution of ¹⁴C-sucrose in a graft. The ¹⁴C-profiles obtained show that sucrose translocation across the graft interface started 4 days after grafting and increased later. The observed translocation appears to occur via wound phloem, since at this time the first complete wound-phloem bridges (shown as files of aniline-blue-positive sieve plates) traverse the graft interface. In 7-d-old autografts, sucrose transport across the graft interface returned to normal again, as indicated by the distribution of the label. In addition, ¹⁴C-profiles reveal accumulation of label in sink tissues. Here the basal callus of the stock, and temporarily the graft union itself, represent the main sinks for labelled sucrose. Translocation of CF was analyzed in hand sections of the grafts. The beginning of translocation into the stock was confirmed with the dye. Moreover, effective phloem translocation across the graft interface, visualized with CF, could undoubtedly be assigned to wound-phloem bridges reconnecting the cut vascular bundles of scion and stock. Thus, the function of phloem connections in regenerated in vitro-grafts is directly shown.     

Sucrose transport by seedlings of Ricinus communis L: the export of sucrose from the cotyledons to the hypocotyl as a function of sucrose concentration in the cotyledons

Abstract Using seedlings of Ricinus communis L. sucrose export from the cotyledons to the hypocotyl and roots was measured at different levels of sucrose concentration in the cotyledons. Sucrose export followed Michaelis-Menten kinetics with a half-saturation of export at 35 mM sucrose in the cotyledons. A maximal export flux of 90 μmol h^?l g^?1 fresh weight of the cotyledons was obtained. Both these figures coincide with those obtained for sucrose uptake into the cotyledons. It is postulated that sucrose uptake and sucrose export occurs by the same mechanism and possibly by the same cells which then would have to be part of the phloem. Since sucrose uptake has been shown to proceed as proton-sucrose co-transport, phloem loading might also be energized by the protonmotive potential difference. The data, furthermore, are difficult to reconcile with the symplastic route of phloem loading.     

A Reanalysis of the Two-Component Phloem Loading System in Beta vulgaris

Kinetic analysis of [¹⁴C]sucrose loading into sugar beet leaf discs revealed the presence of two transport components. At low exogenous sucrose concentrations, a saturable component, which exhibited Michaelis-Menten characteristics, was the main mode of transport. At concentrations greater than 50 millimolar, phloem loading was dominated by a linear component which appeared to operate as a first order kinetic transport process. Over the exogenous sucrose concentrations employed, influx could be described by the equation v = V_maxS/(S + K_m) + kS. Influx via both processes was strongly pH-dependent. Evidence is presented that the linear component was not explicable in terms of simple diffusion, or exchange diffusion, into either mesophyll or minor vein phloem tissue. Extensive metabolic conversion of sucrose was not a factor contributing to influx at high external sucrose concentrations. At present, it is believed that both components operate in parallel at the membrane bounding the sieve element-companion cell complex. The saturable component is identified with sucrose-H⁺ cotransport. While the significance of the linear component has been established, its nature remains to be elucidated.     

Freeze-fracture analysis of phloem structure in plant tissue cultures. I. The sieve element reticulum

During the differentiation of phloem sieve elements, the endoplasmic reticulum undergoes unique modifications to form the sieve element reticulum (SER) which persists in mature, functioning sieve tubes. Cisternae of the SER lack ribosomes and are restricted to the periphery of the sieve element at late stages of development. Some of the SER is seen as single cisternae that are in close contact with the sieve element plasma membrane. Thin sections and freeze-fracture images of sieve elements formed in tissue cultures demonstrate that the SER consists of both single cisternae and regions of stacked cisternae at some stages of maturity. The unstacked regions of the SER are continuous with the cisternae of the stacked regions. In freeze-fracture images the single cisternae adjacent to the plasma membrane are seen to be fenestrated and the openings allow continuity between the plasma membrane and the cell lumen. It is concluded that the interface between the SER and the plasma membrane of the sieve element serves to allow membrane functions such as proton efflux, proton-sucrose cotransport and compensating movements of ions to occur in a microenvironment that is separated from the moving translocation stream in the sieve element lumen. Passage of water and translocated solutes from the plasma membrane or the SER/PM interface to the interior of the cell is enhanced by the openings in the fenestrated regions of the SER. It is suggested tha the SER may also play a role in channeling ATP from mitochondria associated with the SER to the proton-pumping ATPase in the plasma membrane and that the SER may function in the uptake and release of potassium ions in the sieve element.     

Ultrastructure and Differentiation of Protein-storing Cells in Secondary phloem of Hevea brasiliensis Stem

The ultrastructure and differentiation of the protein-storingcells in secondary phloem of terminal branchlets of Hevea brasiliensisMull. Arg. were studied using electron microscopy. The cellsare parenchyma tissue in the axial system and characterizedby the presence of a large amount of proteinaceous fibrils inthe central vacuole. The fibrils of the protein-storing cellsare straight, about 9 nm in diameter and arranged in a directionroughly parallel to the longitudinal axis of the stem. At theearly differentiation stage of the protein-storing cells, amass of proteinaceous fibrils appears in the cytoplasm, thenis separated from the peripheral cytoplasm by the endomembranesystem derived from endoplasmic reticulum, resulting in theformation of the central vacuole with the fibrils inside asthe vacuolar content The peripheral cytoplasm may continue toproduce proteinaceous fibrils with which the fibrils of thecentral vacuole is supplemented after the protein-storing cellsare formed. Hevea brasiliensis, storage protein, proteinaceous fibrils, vacuole, secondary phloem