Photoassimilate translocation in the petiole of Cyclamen and Primula is independent of lateral retrieval

Phloem translocation of photoassimilates between source andsink is considered to be linked with active retrieval of sugarsleaked to the vascular apoplast. This hypothesis was evaluatedby studying photo-assimilate movement in petioles of intactplants of Cyclamen persicum and Primula obconica in the presenceof inhibitors affecting sucrose retrieval (PCMBS, CCCP). Inhibitorsolutions were applied by rinsing locally isolated petiole bundlesor by injection into the petioe parenchyma. PCMBS and CCCP reduced[¹⁴C]sucrose retrieval from the petiole apoplast by the vascularcells and altered the distribution pattern of ¹⁴C-photoassimilateswithin the petiole tissues. However, these treatments did notaffect translocation through the petiole phloem. Evidence isprovided that the reagents were present in the vascular apoplastsurrounding the translocating phloem. It was concluded thatassimilate movement in the petiole of Cyclamen and Primula wasindependent of apoplastic retrieval. Key words: Cyclamen, Primula, phloem, transport, path, sucrose, retrieval     

Retention of xenobiotics along the phloem path

Detached leaves of Cyclamen persicum Mill. can be used as a simple source-sink system. Phloem transport in the excised material was monitored by the noninvasive ¹¹C-technique. Assimilate movement stopped immediately when the petiole was cut off. However, within 20 min a recovery of transport was observed. The translocation rate in the detached leaf was only 13% of that in the intact plant. ¹⁴C-Xenobiotics and [³H]sucrose were injected into the upper petiole parenchyma (source). They moved downstream by a symplastic route. The stump of the petiole was inserted into a buffer solution containing ethylenediaminetetraacetic acid (sink). After 3 h, the distribution of sucrose and xenobiotics was determined in five subsequent segments of the petiole (path). The retention coefficient (r) was calculated from the ratio of radioactivity in the vascular bundle to that in the petiole parenchyma. The distribution along the vascular path was given by a geometric progression, whereas its constant was the transport coefficient (q). Values of r and q corresponded with the degree of phloem mobility and ambimobility. Four groups of compounds were classified: (i) acidic substances with log K_ow = — 2 to — 2.4 (K_ow is the partition coefficient octanol/water) at pH 8 (pH of sieve tube sap), retained by ion trapping and exhibiting small lateral efflux (q0.7; maleic hydrazide, dalapon); (ii) acidic substances with log K_ow = — 0.7 to — 0.8 at pH 8, retained by ion trapping and subjected to a moderate lateral efflux (0.7>q> 0.5; 2,4-dichlorophenoxyacetic acid, 2-methyl-4-chlorophenoxyacetic acid, bromoxynil); (iii) nonionised substances retained by optimum permeability, exhibiting a considerable lateral leakage (q<0.5; glyphosate, amitrole); (iv) substances without basipetal transport in the phloem (atrazine, diuron). Retention of sucrose corresponded quantitatively with that shown in group (i). This classification was also supported by results of uptake and efflux experiments using the isolated conducting tissue. Theoretical translocation profiles were calculated from the determined transport coefficients (q).Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - K_ow partition coefficient octanol/water - MCPA 2-methyl-4-chloro-phenoxyacetic acid - q transport coefficient in the vascular bundle - r retention coefficient in the vascular bundle The authors gratefully acknowledge the assistance of H. Fiedler and M. Neugebauer. We are particularly grateful to K. Dutschka, G. Hudepokl, and Dr. J. Knust for producing ¹¹CO₂.     

Changes in soluble carbohydrates during phytochrome-regulated petiole elongation in watermelon seedlings

Changes in soluble carbohydrate composition and concentration in leavesand petioles of watermelon (Citrullus lanatus (Thunb)Matsum and Nakai cv. Sugar Baby) seedlings during early stages ofphytochrome-regulated petiole elongation were investigated. Watermelon seedlingswere grown in a controlled environment with 350 molm^–2 s^–1 photosynthetically activeradiation (PAR) during a 12-h photoperiod. Low intensity end-of-day(EOD) light treatments (for 15 min) of red (R), far-red (FR) and FRfollowed by R (FR/R) were initiated when the seedlings were 14 days old.Seedling growth, and soluble carbohydrate concentration and composition inleaves and petioles were determined after 3 and 6 days of EOD light treatments.The EOD FR increased the petiole length and dry mass partitioned to petioles asearly as 3 days into the treatment. This increased petiole dry mass inFR-treated plants was accompanied with an increase in reducing sugar (glucoseand fructose) concentration in the petioles. Although both leaves and petiolesshowed this effect, the relative increase was greater in petioles than leaves.While the most abundant sugars in petioles were fructose and glucose, thepredominant sugars in leaves were sucrose, raffinose, and stachyose. Thephotoreversion of FR induced changes in growth and sugar concentrations by Rindicates the involvement of phytochrome in these processes.     

Movement of virus and photoassimilate in the phloem: A comparative analysis

Recent progress in the study of short-distance (cell-to-cell) movement of plant virus, facilitated by ‘movement proteins’, has led to a resurgence of interest in long-distance virus transport in the phloem. Relatively little is known about phloem-specific barriers to virus movement or about the form in which virus enters, travels within and exits this tissue. Progress in understanding virus and photoassimilate transport is limited by a paucity of information on the substructure and properties of plasmodesmata at specific interfaces. The direction of virus movement, once it has entered the phloem, can be understood by following photoassimilate translocation, a complex and dynamic process influenced by plant growth, development and vascular topology.     

Inorganic ions modulate the path of phloem loading of sucrose in Ricinus communis L. seedlings

The impact of inorganic ions on sucrose fluxes in the cotyledons and on the pathway of phloem loading was studied in Ricinus communis L. seedlings. The cotyledons were incubated in defined solutions which contained either potassium, sodium, magnesium or calcium as chloride salts, or the sodium salts of sulphate or phosphate. Sucrose uptake from the medium into the cotyledons was only slightly affected by the salts. Sucrose efflux to the medium was increased by phosphate and sulphate and to a lesser extent by sodium and potassium. Phloem loading from the apoplasm and the symplasm was analysed by addition of labelled sucrose to the medium, determination of the specific radioactivity of sucrose in sieve-tube exudate and quantification of export into the seedling axis. Potassium and sodium stimulated the apoplasmic route of phloem loading of sucrose, mostly at the expense of loading from cotyledon sucrose pools. In contrast, sulphate and phosphate strongly inhibited the apoplasmic route whereas the (small) symplasmic flux from the cotyledon sucrose pools was less affected. Magnesium ions inhibited phloem loading by both pathways. The potential of ions in modulating the pathways of sucrose export in day to day operation of plants is discussed.     

Anatomy and lignin distribution in reaction phloem fibres of several Japanese hardwoods

Background and Aims Although tension wood formation and the structure of gelatinous fibres (G-fibres) have been widely investigated, studies of the influence of the reaction phenomenon on phloem fibres have been few and incomplete in comparison with those of xylem wood fibres. This study was undertaken to clarify the influence of stem inclination on phloem fibres using several Japanese hardwood species that produce different G-fibre types in tension wood. Methods Eight hardwood species were inclined at 30-45° at the beginning of April. Specimens were collected in July and December. The cell-wall structure and lignin distribution of phloem fibres on both the tension and opposite sides were compared by light microscopy, ultraviolet microscopy, confocal laser scanning microscopy after staining with acriflavine, and transmission electron microscopy after staining with potassium permanganate. Key Results Three types of changes were found in tension-side phloem fibres: (1) increases in the proportion of the syringyl unit in lignin in the S(1) and S(2) layers and compound middle lamella (Cercidiphyllum japonicum), (2) formation of unlignified gelatinous layers (Melia azedarach and Acer rufinerve) and (3) increases in the number of layers (n) in the multi-layered structure of S(1) + S(2) + n (G + L) (Mallotus japonicus). Other species showed no obvious change in cell-wall structure or lignin distribution. Conclusions Phloem fibres of the tree species examined in our study showed three types of changes in lignin distribution and cell-wall structure. The reaction phenomenon may vary with tree species and may not be closely related to G-fibre type in tension wood.     

Cellular distribution of alkaloids and their translocation via phloem and xylem: the importance of compartment pH

The physico‐chemical background of alkaloid allocation within plants is outlined and discussed exemplarily for pyrrolizidine alkaloids (PAs) and nicotine. The trigger for this discourse is the finding that, for example, PAs, which are taken up from the soil, are translocated in the xylem, whereas – when genuinely present in plants – they are allocated as N‐oxides via phloem. Special emphasis is put on the impact of different pH values in certain compartments, as this entails significant changes in the relative lipophilic character of alkaloids: tertiary alkaloids diffuse readily through biomembranes, while the corresponding protonated alkaloids are retained in acidic compartments, i.e. vacuoles or xylem. Therefore, this phenomenon, well known as the ‘ion trap mechanism’, is also relevant for long‐distance transport of alkaloids. Any efficient allocation of typical tertiary alkaloids within the phloem can thus be excluded. In contrast, due to their strongly increased hydrophilic properties, alkaloid‐N‐oxides or quarternary alkaloids cannot diffuse through biomembranes and, consequently, would be retained in the acidic xylem during translocation. The major aim of this paper is to sharpen the mind for the chemical peculiarities of alkaloids and to consider them adequately in forthcoming investigations on allocation of alkaloids.     

环剥对红松(Pinus koraiensis)韧皮部和木质部碳水化合物的影响

树干环剥可以阻碍韧皮部光合产物的运输并进一步影响光合产物的分配。长时期内,环剥能够导致环痕上部可溶性糖和淀粉的积累,但对于短期内如何影响碳水化合物在木质部和韧皮部内的运输模式所知甚少。以38年生红松(Pinus koraiensis Sieb.etZucc.)为研究材料,分别对环剥上部、下部每隔1~2d采样,区分木质部和韧皮部(树皮)进行可溶性糖和淀粉含量及树干糖呼吸消耗速率测定,确定环剥后的日变化和周变化,并对木质部可溶性糖、淀粉含量与韧皮部中相应指标进行相关关系的回归分析。结果发现:(1)环剥后4周内,在环剥痕上、下部间木质部可溶性糖和淀粉含量,韧皮部中淀粉含量均不存在显著差异(p>0.05),而韧皮部内可溶性糖含量,环剥后第2周出现显著差异,从第4周出现环剥上部显著高于下部的碳水化合物积累现象(p<0.05);(2)环剥阻隔了韧皮部可溶性糖的纵向运输,但是并不影响木质部的纵向运输,而且环剥并没有影响木质部和韧皮部之间的糖和淀粉的相关关系;(3)环剥第1周内环剥上部和下部呼吸消耗速率差异不显著,第2周环剥上部显著高于环剥下部,从第3周开始环剥下部呼吸消耗速率显著下降。推断认为,在环剥处理的4周内,环剥上部冠层新形成的碳水化合物很大一部分均被呼吸消耗掉,导致环剥上部较环剥下部可溶性糖稍有增加;红松胸高直径以下部分所储藏的碳水化合物足以保障2周内红松树干呼吸。     

The distribution of plasmodesmata in the phloem ofHevea brasiliensis in relation to laticifer loading

Summary Cell to cell connections, including plasmodesmata and perforations, were examined in the non-conducting secondary phloem ofHevea brasiliensis. Samples were taken from trunks of numerous trees, from several clones, and prepared for thin sectioning and transmission or scanning electron microscopy and as optical sections for fluorescence microscopy. Numerous plasmodesmata were found clustered in primary pit-fields between the ray and axial parenchyma cells. Between the laticifers and adjacent parenchyma sheath cells, structures corresponding to functional plasmodesmata were not observed. But some unusual structural features were occasionally seen in these walls. These observations are discussed in relation to the possible function of the cell types, and to the loss of latex on the tapping ofHevea. It is suggested that the loading of the laticifer might first require a symplastic pathway for the transport of metabolites, at the end of which the assimilates must enter the apoplast. A transmembrane active transport system then transfers the metabolites in the laticifer. The presumable role of parenchyma cells in the loading of laticifers is emphasized.     

芹菜韧皮部蛋白基因的分离与表达分析

韧皮部蛋白在维持植物形态,物质转运以及植物伤口保护等方面起着重要作用。本研究以地域来源和性状特性差异均较大的两个芹菜品种‘六合黄心芹’和‘美国西芹’为试验材料,利用RT-PCR技术获得这两种芹菜韧皮部蛋白基因的cDNA序列。结果显示：这两种芹菜来源的韧皮部蛋白基因全长均为546 bp,编码181个氨基酸。两者核苷酸序列有3个位点的不同,分别为：88G/A、399T/C和489T/C;在氨基酸序列上有一个位点的不同,为30T/A。预测其蛋白质分子量为19 kD,pI值为9.18。‘六合黄心芹’和‘美国西芹’的韧皮部蛋白与忽地笑等植物的韧皮部蛋白相似度较高,在保守位置分别具有5个亮氨酸残基和4个色氨酸残基。实时定量PCR表达分析表明,该基因主要在芹菜的茎和根等部位表达,具有明显的组织特异性。     

Computation techniques in the conformational analysis of carbohydrates

A growing number of modern studies of carbohydrates is devoted to spatial mechanisms of their participation in the cell recognition processes and directed design of inhibitors of these processes. No progress in this field is possible without the development of theoretical conformational analysis of carbohydrates. In this review, we generalize literature data on the potentialities of using various molecular-mechanic force fields, the methods of quantum mechanics, and molecular dynamics to study the conformation of glycoside linkage. A possibility of analyzing the reactivity of carbohydrates with the computation techniques is also discussed in brief.     

Computation techniques in the conformational analysis of carbohydrates

A growing number of modern studies of carbohydrates is devoted to spatial mechanisms of their participation in the cell recognition processes and directed design of inhibitors of these processes. Any progress in this field is impossible without the development of theoretical conformational analysis of carbohydrates. In this review, we generalize literature data on the potentialities of using of different molecular-mechanic force fields, the methods of quantum mechanics, and molecular dynamics to study the conformation of glycoside bond. A possibility of analyzing the reactivity of carbohydrates with the computation techniques is also discussed in brief.     

Correction to: Three-dimensional quantification of twisting in the Arabidopsis petiole

Organisms have a variety of three-dimensional (3D) structures that change over time. These changes include twisting, which is 3D deformation that cannot happen in two dimensions. Twisting is linked to important adaptive functions of organs, such as adjusting the orientation of leaves and flowers in plants to align with environmental stimuli (e.g. light, gravity). Despite its importance, the underlying mechanism for twisting remains to be determined, partly because there is no rigorous method for quantifying the twisting of plant organs. Conventional studies have relied on approximate measurements of the twisting angle in 2D, with arbitrary choices of observation angle. Here, we present the first rigorous quantification of the 3D twisting angles of Arabidopsis petioles based on light sheet microscopy. Mathematical separation of bending and twisting with strict definition of petiole cross-sections were implemented; differences in the spatial distribution of bending and twisting were detected via the quantification of angles along the petiole. Based on the measured values, we discuss that minute degrees of differential growth can result in pronounced twisting in petioles.

     

The evolution of minor vein phloem and phloem loading

Phylogenetic analysis provides a rational basis for comparative studies of phloem structure and phloem loading. Although several types of minor vein companion cell have been identified, and progress has been made in correlating structural features of these cells with loading mechanisms, little is known about the phylogenetic relationships of the different types. To add to the available data on companion cells, we analyzed the ultrastructure of minor veins in Euonymus fortunei and Celastrus orbiculatis (Celastraceae) leaves and determined that in these species they are specialized as intermediary cells. This cell type has been implicated in symplastic phloem loading. The data were added to published data sets on minor vein phloem characteristics, which were then mapped to a well-supported molecular tree. The analysis indicates that extensive plasmodesmatal continuity between minor vein phloem and surrounding cells is ancestral in the angiosperms. Reduction in plasmodesmatal frequency at this interface is a general evolutionary trend, punctuated by instances of the reverse. This is especially true in the case of intermediary cells that have many plasmodesmata, but other distinguishing characteristics as well, and have arisen independently at least four, and probably six, times in derived lineages. The character of highly reduced plasmodesmatal frequency in minor vein phloem, common in crop plants, has several points of origin in the tree. Thus, caution should be exercised in generalizing results on apoplastic phloem loading obtained from model species. Transfer cells have many independent points of origin, not always from lineages with reduced plasmodesmatal frequency.     

Interxylary phloem: Diversity and functions

Interxylary phloem is here defined as strands or bands of phloem embedded within the secondary xylem of a stem or root of a plant that has a single vascular cambium. In this definition, interxylary phloem differs from intraxylary phloem, bicollateral bundles, pith bundles, and successive cambia. The inclusive but variously applied terms included phloem and internal phloem must be rejected. Histological aspects of interxylary phloem are reviewed and original data are presented. Topics covered include duration of interxylary phloem; relationship in abundance between sieve tubes in external phloem and interxylary phloem; distinctions between interxylary and intraxylary phloem; presence of parenchyma, fibers, and crystals in the interxylary phloem strands; development of cambia within interxylary phloem strands; three-dimensionalization and longevity of phloem, systematic distribution of interxylary phloem; physiological significance; and habital correlations. No single physiological phenomenon seems to explain all instances of interxylary phloem occurrence, but rapidity and volume of photosynthate transport seem implicated in most instances.