Comparative analysis of protein transport in the N. benthamiana vasculature reveals different destinations.

We investigated the vascular transport of exogenously applied proteins and compared their delivery to various aerial parts of the plant with carboxy fluorescein dye. Alexafluor tagged bovine serum albumin (Alexa-BSA) moves at a low level to upper parts of the plant and unloads to the apoplast. Alexafluor tagged Histone H1 (Alexa-Histone) moves rapidly throughout the plant and is retained in the phloem and phloem parenchyma. Both Alexa-Histone and -BSA were exported from leaf veins class II and III but they unloaded completely into the leaf lamina with barely any residual fluorescence left inside the leaf veins. Fluorescein tagged hepatitis C virus core protein (fluorescein-HCV) moves more rapidly than BSA through the plant and was restricted to the leaf veins. Fluorescein-HCV failed to unload to the leaf lamina. These combined data suggest that there is not a single default pathway for the transfer of exogenous proteins through the plant. Specific protein properties appear to determine their destination and transport properties within the phloem.     

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      

Tie-dyed1 encodes a novel, phloem-expressed transmembrane protein that functions in carbohydrate partitioning

Carbon is partitioned between export from the leaf and retention within the leaf, and this process is essential for all aspects of plant growth and development. In most plants, sucrose is loaded into the phloem of carbon-exporting leaves (sources), transported through the veins, and unloaded into carbon-importing tissues (sinks). We have taken a genetic approach to identify genes regulating carbon partitioning in maize (Zea mays). We identified a collection of mutants, called the tie-dyed (tdy) loci, that hyperaccumulate carbohydrates in regions of their leaves. To understand the molecular function of Tdy1, we cloned the gene. Tdy1 encodes a novel transmembrane protein present only in grasses, although two protein domains are conserved across angiosperms. We found that Tdy1 is expressed exclusively in phloem cells of both source and sink tissues, suggesting that Tdy1 may play a role in phloem loading and unloading processes. In addition, Tdy1 RNA accumulates in protophloem cells upon differentiation, suggesting that Tdy1 may function as soon as phloem cells become competent to transport assimilates. Monitoring the movement of a fluorescent, soluble dye showed that tdy1 leaves have retarded phloem loading. However, once the dye entered into the phloem, solute transport appeared equal in wild-type and tdy1 mutant plants, suggesting that tdy1 plants are not defective in phloem unloading. Therefore, even though Tdy1 RNA accumulates in source and sink tissues, we propose that TDY1 functions in carbon partitioning by promoting phloem loading. Possible roles for TDY1 are discussed.     

Vascular invasion routes and systemic accumulation patterns of tobacco mosaic virus in Nicotiana benthamiana

Plant viruses must enter the host vascular system in order to invade the young growing parts of the plant rapidly. Functional entry sites into the leaf vascular system for rapid systemic infection have not been determined for any plant/virus system. Tobacco mosaic virus (TMV) entry into minor, major and transport veins from non-vascular cells of Nicotiana benthamiana in source tissue and its exit from veins in sink tissue was studied using a modified virus expressing green fluorescent protein (GFP). Using a surgical procedure that isolated specific leaf and stem tissues from complicating vascular tissues, we determined that TMV could enter minor, major or transport veins directly from non-vascular cells to produce a systemic infection. TMV first accumulated in abaxial or external phloem-associated cells in major veins and petioles of the inoculated leaf and stems below the inoculated leaf. It also initially accumulated exclusively in internal or adaxial phloem-associated cells in stems above the inoculated leaf and petioles or major veins of sink leaves. This work shows the functional equivalence of vein classes in source leaves for entry of TMV, and the lack of equivalence of vein classes in sink leaves for exit of TMV. Thus, the specialization of major veins for transport rather than loading of photoassimilates in source tissue does not preclude virus entry. During transport, the virus initially accumulates in specific vascular-associated cells, indicating that virus accumulation in this tissue is highly regulated. These findings have important implications for studies on the identification of symplasmic domains and host macromolecule vascular transport.     

Improvement of pea biomass and seed productivity by simultaneous increase of phloem and embryo loading with amino acids

The development of sink organs such as fruits and seeds strongly depends on the amount of nitrogen that is moved within the phloem from photosynthetic‐active source leaves to the reproductive sinks. In many plant species nitrogen is transported as amino acids. In pea (Pisum sativum L.), source to sink partitioning of amino acids requires at least two active transport events mediated by plasma membrane‐localized proteins, and these are: (i) amino acid phloem loading; and (ii) import of amino acids into the seed cotyledons via epidermal transfer cells. As each of these transport steps might potentially be limiting to efficient nitrogen delivery to the pea embryo, we manipulated both simultaneously. Additional copies of the pea amino acid permease PsAAP1 were introduced into the pea genome and expression of the transporter was targeted to the sieve element‐companion cell complexes of the leaf phloem and to the epidermis of the seed cotyledons. The transgenic pea plants showed increased phloem loading and embryo loading of amino acids resulting in improved long distance transport of nitrogen, sink development and seed protein accumulation. Analyses of root and leaf tissues further revealed that genetic manipulation positively affected root nitrogen uptake, as well as primary source and sink metabolism. Overall, the results suggest that amino acid phloem loading exerts regulatory control over pea biomass production and seed yield, and that import of amino acids into the cotyledons limits seed protein levels.     

Callose deposition in the phloem plasmodesmata and inhibition of phloem transport in citrus leaves infected with “Candidatus Liberibacter asiaticus”

Huanglongbing (HLB) is a destructive disease of citrus trees caused by phloem-limited bacteria, Candidatus Liberibacter spp. One of the early microscopic manifestations of HLB is excessive starch accumulation in leaf chloroplasts. We hypothesize that the causative bacteria in the phloem may intervene photoassimilate export, causing the starch to over-accumulate. We examined citrus leaf phloem cells by microscopy methods to characterize plant responses to Liberibacter infection and the contribution of these responses to the pathogenicity of HLB. Plasmodesmata pore units (PPUs) connecting companion cells and sieve elements were stained with a callose-specific dye in the Liberibacter-infected leaf phloem cells; callose accumulated around PPUs before starch began to accumulate in the chloroplasts. When examined by transmission electron microscopy, PPUs with abnormally large callose deposits were more abundant in the Liberibacter-infected samples than in the uninfected samples. We demonstrated an impairment of symplastic dye movement into the vascular tissue and delayed photoassimilate export in the Liberibacter-infected leaves. Liberibacter infection was also linked to callose deposition in the sieve plates, which effectively reduced the sizes of sieve pores. Our results indicate that Liberibacter infection is accompanied by callose deposition in PPUs and sieve pores of the sieve tubes and suggest that the phloem plugging by callose inhibits phloem transport, contributing to the development of HLB symptoms.     

<Superscript>11</Superscript>C-imaging: methyl jasmonate moves in both phloem and xylem,promotes transport of jasmonate,and of photoassimilate even after proton transport is decoupled

The long-distance transport and actions of the phytohormone methyl jasmonate (MeJA) were investigated by using the short-lived positron-emitting isotope ¹¹C to label both MeJA and photoassimilate, and compare their transport properties in the same tobacco plants (Nicotiana tabacum L.). There was strong evidence that MeJA moves in both phloem and xylem pathways, because MeJA was exported from the labeled region of a mature leaf in the direction of phloem flow, but it also moved into other parts of the same leaf and other mature leaves against the direction of phloem flow. This suggests that MeJA enters the phloem and moves in sieve tube sap along with photoassimilate, but that vigorous exchange between phloem and xylem allows movement in xylem to regions which are sources of photoassimilate. This exchange may be enhanced by the volatility of MeJA, which moved readily between non-orthostichous vascular pathways, unlike reports for jasmonic acid (which is not volatile). The phloem loading of MeJA was found to be inhibited by parachloromercuribenzenesulfonic acid (PCMBS) (a thiol reagent known to inhibit membrane transporters), and by protonophores carbonyl cyanide 3-chlorophenylhydrazone (CCCP) and 2,4-dinitrophenol (DNP) suggesting proton co-transport. MeJA was found to promote both its own transport and that of recent photoassimilate within 60 min. Furthermore, we found that MeJA can counter the inhibitory effect of the uncoupling agent, CCCP, on sugar transport, suggesting that MeJA affects the plasma membrane proton gradient. We also found that MeJA’s action may extend to the sucrose transporter, since MeJA countered the inhibitory effects of the sulfhydryl reagent, PCMBS, on the transport of photoassimilate.     

Leaf structure and translocation in sugar beet

Anatomical and ultrastructural details of a translocating 10-cm leaf of sugar beet (Beta vulgaris L. var. Klein Wanzleben) were correlated with translocation rate data. The minor veins were found to be 13 times as extensive as the major veins and measure 70 cm/cm² leaf lamina. Measurements disclosed that a 33-μ length of minor vein services 29 mesophyll cells with the result that translocate moves an average of 73 μ or 2.2 cell diameters during transport from mesophyll cells to a minor vein. High-resolution, freeze-dry autoradiography revealed that assimilates accumulate in organelle-rich cells of the minor vein phloem. Correlation of phloem volume and loading rate for minor veins yielded an uptake rate of 735 μmoles of sucrose per g fresh weight of phloem. The arrangement and structural features of minor veins appeared to be consistent with the concept that vein loading precedes translocation.     

Radiosynthesis of 6’-Deoxy-6’[18F]Fluorosucrose via Automated Synthesis and Its Utility to Study In Vivo Sucrose Transport in Maize (Zea mays) Leaves

Sugars produced from photosynthesis in leaves are transported through the phloem tissues within veins and delivered to non-photosynthetic organs, such as roots, stems, flowers, and seeds, to support their growth and/or storage of carbohydrates. However, because the phloem is located internally within the veins, it is difficult to access and to study the dynamics of sugar transport. Radioactive tracers have been extensively used to study vascular transport in plants and have provided great insights into transport dynamics. To better study sucrose partitioning in vivo, a novel radioactive analog of sucrose was synthesized through a completely chemical synthesis route by substituting fluorine-18 (half-life 110 min) at the 6’ position to generate 6’-deoxy-6’[¹⁸F]fluorosucrose (¹⁸FS). This radiotracer was then used to compare sucrose transport between wild-type maize plants and mutant plants lacking the Sucrose transporter1 (Sut1) gene, which has been shown to function in sucrose phloem loading. Our results demonstrate that ¹⁸FS is transported in vivo, with the wild-type plants showing a greater rate of transport down the leaf blade than the sut1 mutant plants. A similar transport pattern was also observed for universally labeled [U-¹⁴C]sucrose ([U-¹⁴C]suc). Our findings support the proposed sucrose phloem loading function of the Sut1 gene in maize, and additionally demonstrate that the ¹⁸FS analog is a valuable, new tool that offers imaging advantages over [U-¹⁴C]suc for studying phloem transport in plants.     

Towards the proteome of Brassica napus phloem sap

The soluble proteins in sieve tube exudate from Brassica napus plants were systematically analyzed by 1-DE and high-resolution 2-DE, partial amino acid sequence determination by MS/MS, followed by database searches. 140 proteins could be identified by their high similarity to database sequences (135 from 2-DE, 5 additional from 1-DE). Most analyzed spots led to successful protein identifications, demonstrating that Brassica napus, a close relative of Arabidopsis thaliana, is a highly suitable model plant for phloem research. None of the identified proteins was formerly known to be present in Brassica napus phloem, but several proteins have been described in phloem sap of other species. The data, which is discussed with respect to possible physiological importance of the proteins in the phloem, further confirms and substantially extends earlier findings and uncovers the presence of new protein functions in the vascular system. For example, we found several formerly unknown phloem proteins that are potentially involved in signal generation and transport, e.g., proteins mediating calcium and G-protein signaling, a set of RNA-binding proteins, and FLOWERING LOCUS T (FT) and its twin sister that might be key components for the regulation of flowering time.     

Symplastic Transfer of Fluorescent Dyes from Mesophyll to Sieve Tube in Stripped Leaf Tissue and Partly Isolated Minor Veins of Commelina benghalensis

We have stripped small (3 × 3 mm) fields of the upper and the opposite lower epidermis of Commelina benghalensis leaves. Pectinase treatment of the resulting chlorenchyma windows produced free-lying viable minor veins with small lumps of mesophyll cells attached. These veins were still connected with the intact remainder of the leaf. Fluorescent dyes were injected into mesophyll cells or mestome sheath cells. Continuous following of the dye from the moment of injection and use of the simple vein system allowed an unhindered and precise assessment of the cell-to-cell route of dye transfer. Disodium fluorescein and Lucifer Yellow CH injected into mesophyll or mestome sheath cells readily moved to the sieve tube. This symplastic dye transfer from mesophyll to sieve tube was also observed after injection into unmacerated stripped leaf tissue. The displacement of fluorescent dyes substantiates a symplastic continuity between mesophyll and sieve tube and therefore supports the possibility of symplastic phloem loading.     

Export of amino acids to the phloem in relation to N supply in wheat

The effect of different N supply on amino acid export to the phloem was studied in young plants of wheat (Triticum aestivum L. cv. Klein Chamaco), using the exudation in EDTA technique. Plants were grown in a growth cabinet in pots with sand, and supplied with nutrient solutions of different NO₃^? concentrations. When plants were grown for 15 days with nutrient solutions containing 1.0, 3.0, 5.0, 10.0, 15.0 or 20.0 mM KNO₃, the exudation rate of sugars from the phloem was unaffected by N supply, but sugars accumulated in the leaf tissue when the N supply was limiting for growth. On the other hand, the rate of exudation of amino acids was proportional to the NO₃^? concentration in the nutrient solution. When the supply of N to plants grown for 15 days with 15.0 mM NO₃^? was interrupted, the exudation of sugars was again unaffected, but there was a fast decrease in the amount of amino acids exudated, and of the concentration of amino acids and nitrogen in the tissues. Also, when 10-day-old plants grown without N were supplied with 15.0 mM NO₃^?, there was a sharp increase in the exudation of amino acids, without changes in the amount of sugar exudated. The rate of exudation of amino acids to the phloem was independent of the concentration of free amino acids in the leaves in all three types of experiment. Asp was the most abundant amino acid in the leaf tissue, while Glu was the one most abundant in the phloem exudate. Asp and Ala were exported to the phloem at a rate lower than expected from their leaf tissue concentrations, indicating some discrimination. On the contrary, Glu showed a preferential export at low N supply. It is concluded that the rate of amino acid export from the leaf to the phloem is dependent on the N available to the plant. This N is used for synthesis of leaf protein when the supply is low, exported to the phloem when supply is adequate, and accumulated in the storage pool when supply is above plant demand.     

Transport of Cadmium via Xylem and Phloem in Maturing Wheat Shoots: Comparison with the Translocation of Zinc, Strontium and Rubidium

The toxic heavy metal cadmium is taken up by plants and maycontaminate harvested parts of agricultural crops. In the experimentsreported here, cadmium was introduced together with markersfor phloem (rubidium) and xylem (strontium) transport, eitherinto intact shoots via a flap below the flag leaf node, or intodetached shoots via the cut stem. Cadmium introduced into intactplants was redistributed during maturation from the peduncleand the flag leaf lamina to the grain. In detached shoots, somecadmium was removed from the transpiration stream, as judgedfrom the comparison of shoots steam-girdled below the ear andof controls with an intact phloem in the peduncle. A minor quantityof cadmium was transported to the grain via the phloem in controlshoots while a high percentage of this element was retainedin the peduncle. The cadmium content of the grain increasedin response to the increased cadmium concentrations in the feedingsolutions (0.1 to 10 µM). The cadmium content of the grainwas slightly lower when zinc (>10 µM) was introducedat the same time as cadmium (1 µM).Copyright 1997 Annalsof Botany Company Triticum aestivumL.; cadmium; phloem transport; wheat; zinc     

Structure and function of leaf minor veins in trees and herbs

Summary The structure of leaf minor veins in 700 species from 140 families of dicotyledons, monocotyledons and conifers has been studied by light and electron microscopy. The presence of several structural types of minor veins has been shown. The main types are open and closed veins characteristic of trees and herbs, respectively. These vein types differ by the structure of intermediate cells, and by the mechanisms of phloem loading and sugar transport. Most woody plants have intermediate cells with numerous plasmodesmal fields, symplastic transport as the main phloem loading mechanism, as well as oligosaccharides and other complex sugars as the main phloem transport substances. By contrast, the majority of herbs have intermediate cells without plasmodesmal connections, and apoplastic loading of sucrose occurs only by membrane proton cotransport. The closed type is divided into three subtypes, differing in the degree of development of the structures used for sugar uptake from the apoplast. A list of the plants investigated with their vein types is given. The evolution of the minor vein structure and phloem loading mechanism are discussed in relation to the evolution of life forms of higher plants.     

6(5)CARBOXYFLUORESCEIN AS A TRACER OF PHLOEM SAP TRANSLOCATION

6(5)carboxyfluorescein (6(5)CF), a polar fluorescein with an apparent pK of 6.3, was introduced, as a pH 6.3 solution, into the apoplast of lamina or petioles of mature soybean leaves. Freehand sections were prepared at various times and immediately observed with a fluorescence microscope. 6(5)CF-associated fluorescence appeared in all sink organs, from shoot apex to roots. It was strictly confined to the phloem regions, even after 4 days. Its transport into young leaves ceased at approximately the time they underwent sink-to-source transition. It was never transported between two leaflets of the same leaf. Its transport was interrupted by phloem destruction. All these transport characteristics were highly reproducible, and were paralleled by those of ¹⁴C transport after application of (¹⁴C)sucrose to leaf surfaces. In contrast with 6(5)CF, fluorescein was transported between mature leaves, and between leaflets of the same leaf. It was not restricted to phloem, and often appeared in the xylem region. These results indicate that 6(5)CF can be used to monitor phloem sap translocation in real time, in short- and long-term experiments.     

Responses of plant vascular systems to auxin transport inhibition.

To assess the role of auxin flows in plant vascular patterning, the development of vascular systems under conditions of inhibited auxin transport was analyzed. In Arabidopsis, nearly identical responses evoked by three auxin transport inhibitor substances revealed an enormous plasticity of the vascular pattern and suggest an involvement of auxin flows in determining the sites of vascular differentiation and in promoting vascular tissue continuity. Organs formed under conditions of reduced auxin transport contained increased numbers of vascular strands and cells within those strands were improperly aligned. In leaves, vascular tissues became progressively confined towards the leaf margin as the concentration of auxin transport inhibitor was increased, suggesting that the leaf vascular system depends on inductive signals from the margin of the leaf. Staged application of auxin transport inhibitor demonstrated that primary, secondary and tertiary veins became unresponsive to further modulations of auxin transport at successive stages of early leaf development. Correlation of these stages to anatomical features in early leaf primordia indicated that the pattern of primary and secondary strands becomes fixed at the onset of lamina expansion. Similar alterations in the leaf vascular responses of alyssum, snapdragon and tobacco plants suggest common functions of auxin flows in vascular patterning in dicots, while two types of vascular pattern alterations in Arabidopsis auxin transport mutants suggest that at least two distinct primary defects can result in impaired auxin flow. We discuss these observations with regard to the relative contributions of auxin transport, auxin sensitivity and the cellular organisation of the developing organ on the vascular pattern.     

Movement of protein and macromolecules between host plants and the parasitic weed Phelipanche aegyptiaca Pers.

Little is known about the translocation of proteins and other macromolecules from a host plant to the parasitic weed Phelipanche spp. Long-distance movement of proteins between host and parasite was explored using transgenic tomato plants expressing green fluorescent protein (GFP) in their companion cells. We further used fluorescent probes of differing molecular weights to trace vascular continuity between the host plant and the parasite. Accumulation of GFP was observed in the central vascular bundle of leaves and in the root phloem of transgenic tomato plants expressing GFP under the regulation of AtSUC2 promoter. When transgenic tomato plants expressing GFP were parasitized with P. aegyptiaca, extensive GFP was translocated from the host phloem to the parasite phloem and accumulated in both Phelipanche tubercles and shoots. No movement of GFP to the parasite was observed when tobacco plants expressing GFP targeted to the ER were parasitized with P. aegyptiaca. Experiments using fluorescent probes of differing molecular weights to trace vascular continuity between the host plant and the parasite demonstrated that Phelipanche absorbs dextrans up to 70 kDa in size from the host and that this movement can be bi-directional. In the present study, we prove for the first time delivery of proteins from host to the parasitic weed P. aegyptiaca via phloem connections, providing information for developing parasite resistance strategies.     

Allocation,stress tolerance and carbon transport in plants: how does phloem physiology affect plant ecology?

Despite the crucial role of carbon transport in whole plant physiology and its impact on plant–environment interactions and ecosystem function, relatively little research has tried to examine how phloem physiology impacts plant ecology. In this review, we highlight several areas of active research where inquiry into phloem physiology has increased our understanding of whole plant function and ecological processes. We consider how xylem–phloem interactions impact plant drought tolerance and reproduction, how phloem transport influences carbon allocation in trees and carbon cycling in ecosystems and how phloem function mediates plant relations with insects, pests, microbes and symbiotes. We argue that in spite of challenges that exist in studying phloem physiology, it is critical that we consider the role of this dynamic vascular system when examining the relationship between plants and their biotic and abiotic environment.     

Movement of lucifer yellow in leaves of Coleus blumei Benth

Abstract Individual spongy mesophyll cells in green areas of variegated Coleus blumei leaves were injected with the symplast-mobile dye lucifer yellow and its movement to other cell types was followed with fluorescence microscopy. In 13 trials, the dye remained in the injected cell twice, moved only to other mesophyll cells five times, and moved up to and along minor veins six times. Where extensive movement of the dye occurred, the tissue was fixed with 4% glutaraldehyde, dehydrated, embedded in plastic, sectioned at 3 μm, and examined again with fluorescence microscopy. The dye was found in abaxial bundle-sheath cells for up to 200 μm or more distant from the site of injection near the minor vein, but no convincing evidence was found for its presence in the vascular tissue itself. It thus appears that superficial whole-mount views of lucifer yellow movement along leaf minor veins cannot be taken as certain evidence for symplastic transport of the dye into and along the vascular tissues.