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.     

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.     

植物体内糖分子的长距离运输及其分子机制

植物器官(如叶、叶鞘、绿色的茎等)可以通过光合作用将CO₂合成为碳水化合物, 并经过长距离运输到达库组织(如新生组织、花粉、果实等)中进行贮存或利用。蔗糖是高等植物长距离运输碳水化合物的主要形式。蔗糖分子从源到库的运输包括源组织韧皮部的装载、维管束的运输和库组织韧皮部的卸载3个步骤。遗传学和分子生物学研究证明, 蔗糖转运蛋白、转化酶和单糖转运蛋白在糖分子的装载和卸载过程中发挥重要作用。该文综述了目前对光合产物运输过程及其调控分子机制的最新研究进展。     

Evidence for symplastic phloem unloading in sink leaves of barley

The pathway of phloem unloading in sink barley (Hordeum vulgare) leaves was studied using a combination of electron microscopy, carboxyfluorescein transport, and systemic movement of barley stripe mosaic virus expressing the green fluorescent protein. Studies of plasmodesmatal frequencies between the phloem and mesophyll indicated a symplastic sieve element- (SE) unloading pathway involving thick-walled and thin-walled SEs. Phloem-translocated carboxyfluorescein was unloaded rapidly from major longitudinal veins and entered the mesophyll cells of sink leaves. Unloading was "patchy" along the length of a vein, indicating that sieve element unloading may be discontinuous along a single vascular bundle. This pattern was mirrored precisely by the unloading of barley stripe mosaic virus expressing the green fluorescent protein. Transverse veins were not utilized in the unloading process. The data collectively indicate a symplastic mechanism of SE unloading in the sink barley leaf.     

Phloem transport and differential unloading in pea seedlings after source and sink manipulations

Phloem transport was investigated in pea seedlings after application of [¹⁴C]sucrose to the cotyledons. The accumulation of the label in segments of young seedlings shows a differential unloading along the plant axis. Shoot and root exhibit tip-to-base gradients of sink strength. In the primary root, the sink-strength profiles reflect not only the importance of the apical meristem, but show also the starting points of cambial activity and production of secondary vascular elements. Experiments including partial removal of the source and manipulations of the sink strength indicate that translocation of pea seedlings is sink-regulated and responds rapidly to changed apoplastic conditions in the apical root region. Here, a lowered water potential leads to an increase of phloem unloading that is suggested to supply the assimilate demand for the short-term osmoregulation of affected cells via the symplasmic pathway.Abbreviation PCMBS parachloromercuribenzenesulfonic acid Discussions with Prof. R. Kollmann, Botanisches Institut, Universität Kiel, and financial support from the Deutsche Forschungsgemeinschaft are gratefully acknowledged.     

Altered xylem-phloem transfer of amino acids affects metabolism and leads to increased seed yield and oil content in Arabidopsis

Seed development and nitrogen (N) storage depend on delivery of amino acids to seed sinks. For efficient translocation to seeds, amino acids are loaded into the phloem in source leaves and along the long distance transport pathway through xylem-phloem transfer. We demonstrate that Arabidopsis thaliana AMINO ACID PERMEASE2 (AAP2) localizes to the phloem throughout the plant. AAP2 T-DNA insertion lines showed changes in source-sink translocation of amino acids and a decrease in the amount of seed total N and storage proteins, supporting AAP2 function in phloem loading and amino acid distribution to the embryo. Interestingly, in aap2 seeds, total carbon (C) levels were unchanged, while fatty acid levels were elevated. Moreover, branch and silique numbers per plant and seed yield were strongly increased. This suggests changes in N and C delivery to sinks and subsequent modulations of sink development and seed metabolism. This is supported by tracer experiments, expression studies of genes of N/C transport and metabolism in source and sink, and by phenotypic and metabolite analyses of aap2 plants. Thus, AAP2 is key for xylem to phloem transfer and sink N and C supply; moreover, modifications of N allocation can positively affect C assimilation and source-sink transport and benefit sink development and oil yield.     

The developmental and organ specific expression of sucrose cleaving enzymes in sugar beet suggests a transition between apoplasmic and symplasmic phloem unloading in the tap roots

Sucrose utilisation in sink tissues depend on its cleavage and is mediated by two different classes of enzymes, invertase and sucrose synthase, which determine the mechanism of phloem unloading. Cloning of two extracellular (BIN35 and BIN46) and one vacuolar invertase (BIN44) provided the basis for a detailed molecular analysis of the relative contribution of the sucrose cleaving enzymes to the sink metabolism of sugar beets (Beta vulgaris) during development. The determination of the steady state levels of mRNAs has been complemented by the analysis of the corresponding enzyme activities. The present study demonstrates an inverse regulation of extracellular invertase and sucrose synthase during tap root development indicating a transition between functional unloading pathways. Extracellular cleavage by invertase is the dominating mechanism to supply hexoses via an apoplasmic pathway at early stages of storage root development. Only at later stages sucrose synthase takes over the function of the key sink enzyme to contribute to the sink strength of the tap root via symplasmic phloem unloading. Whereas mRNAs for both extracellular invertase BIN35 and sucrose synthase were shown to be induced by mechanical wounding of mature leaves of adult plants, only sucrose synthase mRNA was metabolically induced by glucose in this source organ supporting the metabolic flexibility of this species.     

Transport of photoassimilates in plants under unfavourable environmental conditions

Transport of photoassimilates linking functionally plant, as a whole system, is discussed as a target for different environmental stresses. Anatomical, physiological and biochemical aspects of phloem transport, phloem loading and unloading are taken into consideration. In the light of modern theoríes of assimilate transport some historical hypotheses are also shown, due to their input into the progress of transport science. The role of phloem unloading in plant acclimation to environment stress is not clear, however changes in source/sink ratio was often observed as the effect of stress. The blockage of sieve tubes found as the result of given stress may be of secondary importance. On the other hand, phloem loading process seems to be an important target for different environmental stresses.     

Metabolic networks of Cucurbita maxima phloem

Metabolomic analysis aims at a comprehensive characterization of biological samples. Yet, biologically meaningful interpretations are often limited by the poor spatial and temporal resolution of the acquired data sets. One way to remedy this is to limit the complexity of the cell types being studied. Cucurbita maxima Duch. vascular exudates provide an excellent material for metabolomics in this regard. Using automated mass spectral deconvolution, over 400 components have been detected in these exudates, but only 90 of them were tentatively identified. Many amino compounds were found in vascular exudates from leaf petioles at concentrations several orders of magnitude higher than in tissue disks from the same leaves, whereas hexoses and sucrose were found in far lower amounts. In order to find the expected impact of assimilation rates on sugar levels, total phloem composition of eight leaves from four plants was followed over 4.5 days. Surprisingly, no diurnal rhythm was found for any of the phloem metabolites that was statistically valid for all eight leaves. Instead, each leaf had its own distinct vascular exudate profile similar to leaves from the same plant, but clearly different from leaves harvested from plants at the same developmental stage. Thirty to forty per cent of all metabolite levels of individual leaves were different from the average of all metabolite profiles. Using metabolic co-regulation analysis, similarities and differences between the exudate profiles were more accurately characterized through network computation, specifically with respect to nitrogen metabolism.     

菜豆茎韧皮部卸出的细胞途径以及激素的促进作用(英文)

通过缩小叶面积和去茎尖改变源库比率,以调节韧皮部卸出的途径,证明了韧皮部卸出的共质体与质外体途径的季节变化,和由对氯高汞苯磺酸所诱发的从质外体向共质体途径的转变,是与光合产物的输入有关。缩小叶面积而降低源库比率,能增加夏季生长植株茎韧皮部的质外体卸出,但对冬季生长植株无影响。去尖而增加源库比率,则促进共质体卸出。赤霉酸和激动素能促进共质体的横向转运,但对质外体转运无作用。当质外体为主要运输途径时,赤霉酸和激动素开启共质体途径。赤霉酸和激动素刺激光合产物,通过共质体从筛管一伴胞复合体向韧皮部薄壁纽胞输送,并可能在韧皮部薄壁细胞被动扩散到自由空间。由此可进一步说明蔗糖在激素处理部位自由空间的增加。     

Symplastic connection is required for bud outgrowth following dormancy in potato (Solanum tuberosum L.) tubers

To gain greater insight into the mechanism of dormancy release in the potato tuber, an investigation into physiological and biochemical changes in tuber and bud tissues during the transition from bud dormancy (immediately after harvest) to active bud growth was undertaken. Within the tuber, a rapid shift from storage metabolism (starch synthesis) to reserve mobilization within days of detachment from the mother plant suggested transition from sink to source. Over the same period, a shift in the pattern of [U-(14)C]sucrose uptake by tuber discs from diffuse to punctate accumulation was consistent with a transition from phloem unloading to phloem loading within the tuber parenchyma. There were no gross differences in metabolic capacity between resting and actively growing tuber buds as determined by [U-(14)C]glucose labelling. However, marked differences in metabolite pools were observed with large increases in starch and sucrose, and the accumulation of several organic acids in growing buds. Carboxyfluorescein labelling of tubers clearly demonstrated strong symplastic connection in actively growing buds and symplastic isolation in resting buds. It is proposed that potato tubers rapidly undergo metabolic transitions consistent with bud outgrowth; however, growth is initially prevented by substrate limitation mediated via symplastic isolation.     

Metabolic diversification of nitrogen‐containing metabolites by the expression of a heterologous lysine decarboxylase gene in Arabidopsis

Lysine decarboxylase converts l ‐lysine to cadaverine as a branching point for the biosynthesis of plant Lys‐derived alkaloids. Although cadaverine contributes towards the biosynthesis of Lys‐derived alkaloids, its catabolism, including metabolic intermediates and the enzymes involved, is not known. Here, we generated transgenic Arabidopsis lines by expressing an exogenous lysine/ornithine decarboxylase gene from Lupinus angustifolius (La‐L/ODC) and identified cadaverine‐derived metabolites as the products of the emerged biosynthetic pathway. Through untargeted metabolic profiling, we observed the upregulation of polyamine metabolism, phenylpropanoid biosynthesis and the biosynthesis of several Lys‐derived alkaloids in the transgenic lines. Moreover, we found several cadaverine‐derived metabolites specifically detected in the transgenic lines compared with the non‐transformed control. Among these, three specific metabolites were identified and confirmed as 5‐aminopentanal, 5‐aminopentanoate and δ‐valerolactam. Cadaverine catabolism in a representative transgenic line (DC29) was traced by feeding stable isotope‐labeled [α‐¹⁵N]‐ or [ε‐¹⁵N]‐l ‐lysine. Our results show similar ¹⁵N incorporation ratios from both isotopomers for the specific metabolite features identified, indicating that these metabolites were synthesized via the symmetric structure of cadaverine. We propose biosynthetic pathways for the metabolites on the basis of metabolite chemistry and enzymes known or identified through catalyzing specific biochemical reactions in this study. Our study shows that this pool of enzymes with promiscuous activities is the driving force for metabolite diversification in plants. Thus, this study not only provides valuable information for understanding the catabolic mechanism of cadaverine but also demonstrates that cadaverine accumulation is one of the factors to expand plant chemodiversity, which may lead to the emergence of Lys‐derived alkaloid biosynthesis.     

Symplastic continuity in Agrobacterium tumefaciens-induced tumours

The Agrobacterium tumefaciens-induced plant tumour is regarded as a strong sink, containing a well-developed vascular system that guarantees an efficient supply of water and nutrients from the host plant into the tumour. The phloem transport and unloading of the fluorescent dye carboxyfluorescein (CF) was studied to examine the potential pathways for unloading of a low-molecular-mass solute, and was compared with the symplastic movement of potato virus X expressing a green fluorescent protein-coat protein fusion (PVX.GFP-CP). The distribution of both CF and PVX.GFP-CP in the host plant, Nicotiana benthamiana, demonstrated a clear symplastic pathway between the phloem of the host stem and the cells of the tumour, and also a considerable capacity for subsequent cell-to-cell transport between tumour cells. This same pattern of CF transport was also demonstrated independently for the host species Cucurbita maxima and Ricinus communis. In addition to entering the tumour, CF and PVX both moved through the vascular rays of the host stem towards the stele. The results confirm that host and tumour tissues in the Agrobacterium gall are in direct symplastic continuity and emphasize an important symplastic pathway for radial solute transport in stems.Key words: Agrobacterium tumefaciens, carboxyfluorescein, GFP, symplastic phloem unloading, plant tumour, vascular rays      

The sucrose transporter StSUT1 localizes to sieve elements in potato tuber phloem and influences tuber physiology and development

The sucrose (Suc) H(+)-cotransporter StSUT1 from potato (Solanum tuberosum), which is essential for long-distance transport of Suc and assumed to play a role in phloem loading in mature leaves, was found to be expressed in sink tubers. To answer the question of whether SUT1 serves a function in phloem unloading in tubers, the promoter was fused to gusA and expression was analyzed in transgenic potato. SUT1 expression was unexpectedly detected not in tuber parenchyma but in the phloem of sink tubers. Immunolocalization demonstrated that StSUT1 protein was present only in sieve elements of sink tubers, cells normally involved in export of Suc from the phloem to supply developing tubers, raising the question of the role of SUT1 in tubers. SUT1 expression was inhibited by antisense in transgenic potato plants using a class I patatin promoter B33, which is primarily expressed in the phloem of developing tubers. Reduced SUT1 expression in tubers did not affect aboveground organs but led to reduced fresh weight accumulation during early stages of tuber development, indicating that in this phase SUT1 plays an important role for sugar transport. Changes in Suc- and starch-modifying enzyme activities and metabolite profiles are consistent with the developmental switch in unloading mechanisms. Altogether, the findings may suggest a role of SUT1 in retrieval of Suc from the apoplasm, thereby regulating the osmotic potential in the extracellular space, or a direct role in phloem unloading acting as a phloem exporter transferring Suc from the sieve elements into the apoplasm.     

Systemic delivery of siRNA in pumpkin by a plant PHLOEM SMALL RNA‐BINDING PROTEIN 1–ribonucleoprotein complex

In plants, the vascular system, specifically the phloem, functions in delivery of small RNA (sRNA) to exert epigenetic control over developmental and defense‐related processes. Although the importance of systemic sRNA delivery has been established, information is currently lacking concerning the nature of the protein machinery involved in this process. Here, we show that a PHLOEM SMALL‐RNA BINDING PROTEIN 1 (PSRP1) serves as the basis for formation of an sRNA ribonucleoprotein complex (sRNPC) that delivers sRNA (primarily 24 nt) to sink organs. Assembly of this complex is facilitated through PSRP1 phosphorylation by a phloem‐localized protein kinase, PSRPK1. During long‐distance transport, PSRP1–sRNPC is stable against phloem phosphatase activity. Within target tissues, phosphatase activity results in disassembly of PSRP1–sRNPC, a process that is probably required for unloading cargo sRNA into surrounding cells. These findings provide an insight into the mechanism involved in delivery of sRNA associated with systemic gene silencing in plants.     

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.     

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.     

Are plant growth substances involved in the partitioning of assimilate to developing reproductive sinks?

The possible involvement of plant growth substances in assimilate production in source leaves, loading into phloem and unloading in sink tissues is discussed in relation to their sites of formation and transport characteristics. The relative importance and possible functions of imported and locally-synthesised plant growth substances in postfertilisation development of reproductive sinks is briefly reviewed.Abbreviations ABA Abscisic acid - PA Phaseic acid - DPA Dihydrophaseic acid - GA Gibberellin - IAA Indole-3-acetic acid     

植物体内光合同化物韧皮部装载和卸出研究进展

近年来研究表明,植物体内光合同化物的韧皮部装载和卸出均有其本途径和质外体途径,装载转运的糖类主要有：（２）棉子糖及其人类似物（以共质体方式装载）;（２）蔗糖（以质外体方式装载）。同化物的共质体卸出可通过扩散和集中作用实现,而质外体卸出则根据蔗糖在质外体是否水解而分为两种类型。卸出和装载的途径、机理因植物种类及库源关系而不同,也会受生长发育阶段及环境的变化而调整。深入研究韧皮部装载和帛出调控机制,对