The Induction of Fibre Differentiation in Peas

The problem studied in this work was that of the internal controlof the formation of strands of fibres in Pisum sativum. It isshown that fibre differentiation is dependent on stimuli originatingin young leaf primordia. Removing these primordia early enoughprevents fibre differentiation; changing the position of theleaves experimentally changes the position of the fibres aswell. It was demonstrated that some stimuli for fibre differentiationmust flow through the strands at the time they differentiate.The evidence for this flow is in experiments concerning theability of very young fibre strands to regenerate after cutsas well as in experiments concerning their pattern of joining.The stimuli which originate in the leaves and control the differentiationof fibres and xylem are shown to differ in at least one component:auxin does not cause fibre differentiation and no surgical treatments,carried out on very young tissues, caused the replacement ofpart of a strand of fibres byor xylem or vice versa.     

Changes of Plastids During Xylem Differentiation in Barley Root

Plastids (eoplasts) are present in meristematic cells of prospectivecentral metaxylem in the barley root. Starch starts to be formedin plastids precisely after the cessation of mitotic activityand at the beginning of endomitotic growth. During secondarywall formation, the starch is gradually lost. Cavities are formedin plastids and signs of plastid degeneration are present fromthis stage of cell development. However, some intact globularplastids without starch are present until shortly before thefinal step of ontogeny, i.e. total destruction of protoplast. Hordeum distichum L., root, xylem, plastids, endomitotic growth, starch     

Potassium Translocation into the Root Xylem

Abstract: Potassium is the most abundant cation in cells of higher plants and plays vital roles in plant growth and develop ment. Since the soil is the only source of potassium, plant roots are well adapted to exploit the soil for potassium and supply it to the leaves. Transport across the root can be divided into three stages: uptake into the root symplast, transport across the symplast and release into the xylem. Uptake kinetics of potassium have been studied extensively in the past and sug gested the presence of high and low affinity systems. Molecular and electrophysiological techniques have now confirmed the existence of discrete transporters encoded by a number of genes. Surprisingly, detailed characterisation of the transpor ters using reverse genetics and heterologous expression shows that a number of the transporters (AKT and AtKUP family) func tion both in the low (μM) and high (mM) K⁺ range. Electrophy siological studies indicate that K⁺ uptake by roots is coupled to H⁺, to drive uptake from micromolar K⁺. However, thus far only Na⁺ coupled K⁺ transport has been demonstrated (HKT1). Ion channels play a major role in the exchange of potassium be tween the symplast and the xylem. An outward rectifying chan nel (KORC) mediates potassium release. Cloning of the gene en coding this channel (SKOR) shows that it belongs to the Shaker super-family. Both electrophysiological and genetic studies demonstrate that K⁺ release through this channel is controlled by the stress hormone abscisic acid. Interestingly, xylem par enchyma cells of young barley roots also contain a number of in ward rectifying K⁺ channels that are controlled by G-proteins. The involvement of G-proteins emphasises once more that po tassium transport at the symplast/xylem boundary is under hor monal control. The role of the electrical potential difference across the symplastxylem boundary in controlling potassium release is discussed.     

木质部细胞分化的程序

本文主要对近十几年来有关木质部细胞分化研究中使用的实验系统及用这些系统所取得的重要进展作了评述.并以作者实验室的研究成果为基础,结合国内外研究进展,提出木质部细胞分化程序由参与细胞编程死亡(PCD)和次生壁构建的全部基因综合编制而成.以PCD过程各阶段的划分标准来看,木质部细胞分化中从IAA诱导形成层细胞平周分裂到细胞扩大前为PCD的起始阶段,其间包括死亡信号的发生、接受和传导,以及启始caspase(半胱氨酰基天门冬氨酸蛋白酶)类似物(例如caspase-8类似物)的活化;木质部母细胞的径向扩大为PCD的效应阶段,而效应caspase类似物(例如caspase-3类似物)活化DNase、DNA的片段化及次生细胞壁的构建和各种细胞器的解体则为PCD的清除降解阶段.至今还无法将DNase活化及其引起的DNA断裂过程与次生细胞壁构建过程分开.     

The Development of Root Nodule Xylem Transfer Cells in Trifolium repens

Stereological analysis of micrographs of developing and functioningxylem transfer cells of Trifolium repens indicates that theER and dictyosomes may be involved in wall ingrowth deposition,and the ER in intracellular solute transport.     

Mechanism of Xylem Differentiation in Pinus silvestrisL.

The duration and daily rate of radial growth and secondary-wallformation of all consecutive, radially-formed tracheids throughoutthe season were investigated in stems of adult trees of Pinussilvestris L. It was found that (1) the variation in radial diameter of tracheidswas probably dependent on seasonal changes in the rate of growthduring the phase of radial enlargement, (2) the daily rate ofcell-wall formation determined the final cell-wall thicknessof tracheids only at the beginning and the end of the season,(3) both rates were affected by temperature, (4) seasonal changesin cell-wall thickness were dependent mostly upon seasonal changesin the duration of the maturation period, (5) the changes inthe duration of the maturation period which brought about transitionfrom early to late wood were determined mostly by the delayin onset of autolysis of cytoplasm which terminates the phaseof tracheid maturation. This process, unlike the xylem productionfrom cambium and the termination of radial enlargemnt, was foundnot to be affected by the seasonal variation of temperature. An attempt to correlate these processes with the activity ofnatural auxin extracted from the cambial region gave negativeresults. On the basis of the results obtained, auxin and environmentalfactors such as precipitation and temperature seem not to bespecific for xylem differentiation. They may seriously affectwood differentiation if they become limiting or exceed the limitof tolerance, but probably they do not determine differentiationof the annual ring of conifers into early and late wood.     

木质部细胞分化的研究进展

本文主要从研究木质部细胞分化常用的实验系统,木质部分的诱导,木质部细胞的编程性死亡以及次生壁的构建4个方面阐述了木质细胞分的研究进展,并对目前研究的热点也是难点问题进行了展望,希望引起同行的兴趣。     

Some Constituents of Xylem Sap and their Possible Relationship to Xylem Differentiation

Bleeding sap of Actinidia chinensis and Betula populifolia andguttation fluidof Avena sativa were analysed for sugars, amino-acids,auxin, and certain enzymes. A wide range of amino-acids wasfound in all three. Auxin was not detected in the bleeding sap,but was present in Avena guttation fluid (5.1 µg IAA equivalent/l).‘IAA oxidase’, acid phosphatase, ribonuclease, deoxyribonuclease,and protease were detected in the bleeding sap and guttationfluid. The possibility that some of the substances found insap and guttation fluid are products of autolysing, differentiatingxylem cells in the roots is discussed.     

木质部细胞分化的研究进展

本文主要从研究木质部细胞分化常用的实验系统、木质部分化的诱导、木质部细胞的编程性死亡以及次生壁的构建4个方面阐述了木质部细胞分化的研究进展。并对目前研究的热点也是难点问题进行了展望,希望引起同行的兴趣。     

Field Estimates of Root and Xylem Resistances in Pinus contorta using Root Excision

A root excision technique was used to estimate the proportionof total resistance to water flux residing in the soil, theroot, and the xylem of lodgepole pine (Pinus contorta Douglex. Loud.) trees in the field. Root excision at mid-day alwaysresulted in rapid recovery of leaf water potential when waterwas supplied to the cut stem, suggesting a high soil-root resistance.Transpiration was unaffected if leaf water potential beforecutting was not limiting leaf conductance. By mid-June wateruptake by the excised stem always exceeded calculated crowntranspiration indicating recharge of internal sapwood storage.Predawn leaf water potential before root excision was highlycorrelated with total soil-plant resistance (r² = 0·89)and calculated root water uptake (r² = 0·92).     

Proteins and Carbohydrates in Xylem Sap from Squash Root

The xylem sap from squash roots was collected from the cut surfaceof stems, and the proteins and carbohydrates in the sap wereanalyzed. The sap contained 18.6 µg ml^–1 proteinand the major polypeptides were as follows: 1) two polypeptides,of 75 and 40 kDa, with high-mannose glycans, the levels of whichincreased for about 24 h after cutting and then decreased; 2)a 32-kDa polypeptide, which appeared soon after cutting, disappearedand then reappeared again 48–64 h after cutting; and 3)a 19-kDa and a 14-kDa polypeptide, which were present constitutively.The carbohydrates contained in the xylem sap were fractionatedinto 80% ethanol-soluble and -insoluble material, and whichwere analyzed by high-performance liquid chromatography, gaschromatography and enzymatic mathods. The former fraction containedconsiderable amounts of myo-inositol and fructose as free sugarsand oligosaccharides composed mainly of galactose, arabinoseand glucose. The latter contained polysaccharides composed mainlyof uronic acids, galactose and arabinose. The possible significanceof these substances, which may mediate the interactions betweenthe root and the aerial organs, is discussed. (Received April 20, 1992; Accepted July 4, 1992)     

The Differentiation of Contact Cells and Isolation Cells in the Xylem Ray Parenchyma of Populus maximowiczii

A histochemical analysis was made of the differentiation ofcontact cells and isolation cells in the xylem ray parenchymaof Populus maximowiczii. The contact cells formed secondarywalls at approximately the same time as adjoining vessel elements.The lignification of the cell walls of contact cells and vesselelements began earlier than that of wood fibres and isolationcells. Thus, the formation of the secondary wall, includinglignification, of the contact cells might occur at the sametime as that of the vessel elements to which they are directlyconnected. By contrast, the isolation cells began to form secondarywalls later than the vessel elements and wood fibres in thevicinity of the isolation cells. After the deposition of thesecondary wall, a protective layer was formed in contact cellsbut no isotropic layer was observed in isolation cells. Theresults suggest the importance of vessel elements in the determinationof the differentiation of adjoining ray parenchyma cells.Copyright1999 Annals of Botany Company Contact cell, isolation cell, vessel element, xylem differentiation, Populus maximowiczii Henry.     

Nitrate Reductase Activity: Phytochrome Mediation of Induction in Etiolated Peas

THE extractable activity of nitrate reductase from higher plant leaves is inducible by light and shows, under natural growth conditions, a pattern of diurnal variation¹. Studies on the nature of light involvement have generally used the green leaf as experimental material, implying that photosynthesis supports the induction process^1,2. We have examined the role of light for the induction of nitrate reductase activity in the etiolated terminal buds of field peas (Pisum arvense cv. Century). Treatments consisted of brief exposure of intact plants to broad bands of light, followed by a period in darkness before extraction for enzyme assay. These light treatments exclude the possibility of photosynthesis as a process contributing to induction. Under these conditions, induction is shown to be reversibly controlled by red and far red light, an effect ascribable to the pigment phytochrome.     

Step by Step: Deciphering Ion Transport in the Root Xylem Parenchyma

Proton pumps produce electrical potential differences and differences in pH across the plasma membrane of cells which drive secondary ion transport through sym- and antiporters. We used the patch-clamp technique to characterize an H⁺-pump in the xylem parenchyma of barley roots. This cell type is of special interest with respect to xylem loading. Since it has been an ongoing debate whether xylem loading is a passive or an active process, the functional characterization of the H⁺-pump is of major interest in the context of previous work on ion channels through which passive salt efflux into the xylem vessels could occur. Cell-type specific features like its Ca²⁺ dependence were determined, that are important to interpret its physiological role and eventually to model xylem loading. We conclude that the electrogenic pump in the xylem parenchyma does not participate directly in the transfer of KCl and KNO₃ to the xylem but, in combination with short-circuiting conductances, plays a crucial role in controlling xylem unloading and loading through modulation of the voltage difference across the plasma membrane. Here, our recent results on the H⁺ pump are put in a larger context and open questions are highlighted.Key Words: plant nutrition, H⁺-ATPase, anion conductance, K⁺ channel, electrophysiology, signaling networkThe root xylem parenchyma is of major interest with respect to nutrient (and signal) traffic between root and shoot. One of its main functions appears to be xylem loading. However, the cell walls of the vascular tissue provide apoplastic paths between xylem and phloem that represent the upward and downward traffic lanes, allowing nutrient circulation¹ (Fig. 1). Therefore mechanisms for ion uptake and for ion release must exist side by side. In the last 15 years major progress has been made in the investigation of transport properties of xylem-parenchyma cells, and both uptake and release channels and transporters were identified. Today, we have good knowledge on the role of K⁺ and anion conductances in xylem loading with salts.² Note, that from the functionally well characterized conductances only the molecular structure of K⁺ channels is known. In contrast, many transporters are identified on the molecular level, but functional data are scarce.Open in a separate windowFigure 1Distribution of tissues in the periphery of the stele. The stippled area marks the region from which early metaxylem protoplasts originated. E, Endodermis with Casparian strip; eMX, ‘early’ metaxylem vessel; IMX, ‘late’ metaxylem vessel; Mph, metaphloem (sieve tube); Pph, protophloem (sieve tube); P, pericycle; Cx, cortex. Symplasmic and apoplasmic transport routes are indicated in red and black, respectively. The Casparian strip prevents apoplastic transport into the stele. Plasmodesmata are shown exemplarily for the indicated symplastic pathway. All cells of the symplast are connected via plasmodesmata. Sites of active uptake into the root symplast and of release into the stelar apoplast are indicated by a black and an orange arrow. Modified from Wegner and Raschke, 1994.³A challenging question to deal with was the dispute about xylem loading with ions being a passive or active process. While it is clear that energy through electrogenic H⁺ efflux is needed to take up nutrient ions from the soil against their electrochemical gradient into the cortical symplast, it has been a matter of debate if ion release into xylem vessels also is energy-linked or if the electrochemical potentials of ions are raised high enough to allow a thermodynamically passive flux.^2,3 The Casparian strip prohibits apoplastic transport of nutrients into the stele and electrically insulates the stelar from the cortical apoplast. Therefore the electrical potential difference of the cells in the xylem parenchyma could be independent from the cortical potential difference but be subject to control, for instance, from the shoot.⁴ Indeed, evidence points to xylem loading as a second control point in nutrient transfer to the shoot.^5,6 The identification and characterization of K⁺ and anion conductances clearly showed that release of KCl and KNO₃ into the xylem can be passive through voltage-dependent ion channels.^2,3,7–9 No need appeared for a pump energizing the transfer of salts to the xylem.However, H⁺ pumps are ubiquitous. H⁺-ATPases are encoded by a multigene family and heterologous expression in yeast showed that isoforms have distinct enzymatic properties.^10,11 As the example of the amino acid transporter AAP6 from the xylem parenchyma shows, a cell-type specific functional characterization of transporters is essential to draw conclusions on their physiological role. AAP6 is the only member of a multigene family with an affinity for aspartate in the physiologically relevant range. The actual apoplastic concentration of amino acids and the pH will determine what is transported in vivo.^12,13 Xylem-parenchyma cells of barley roots were strongly labelled by antibodies against the plasma membrane H⁺-ATPase.¹⁴ In a recent publication in Physiologia Plantarum we report the functional analysis of the electrogenic pump from the plasma membrane of xylem parenchyma from barley roots that was done with the patch-clamp technique after specific isolation of protoplasts from this cell type. It displayed characteristics of an H⁺-ATPase: current-voltage relationships were characteristic for a ‘rheogenic’ pump¹⁵ and currents were stimulated by fusicoccin or by an enlarged transmembrane pH gradient and inhibited by dicyclohexylcarbodiimide (DCCD). Importantly, it also showed distinct characteristics. Neither intracellular pH nor the intracellular Ca²⁺ concentration affected its activity. Noteworthy, K⁺ and anion conductances from the same cell type are controlled by intracellular [Ca²⁺]^7,9 (Fig. 2). It was proposed that the effect of abscisic acid (ABA) on anion conductances is mediated via an increase in the cytosolic Ca²⁺ concentration.¹⁶ Very likely stelar H⁺ pumps are stimulated by ABA.¹⁷ Thus, a Ca²⁺ independent control has to be hypothesized in this case.Open in a separate windowFigure 2Control of ion conductances in the plasma membrane of xylem-parenchyma cells. Arrowheads indicate stimulation and bars indicate inhibition by an increase in cytosolic [Ca²⁺],^7,9,16 by ABA,^16,17,21 by cytosolic and apoplastic acidification,^4,22 by G-proteins²³ and by an increase in apoplastic [K⁺]⁷ and [NO₃⁻].²⁴ Apoplastic [K⁺] and [NO₃⁻] modify the voltage dependence exerting negative feedback on K⁺ efflux and a positive feedback on NO₃⁻ efflux. Abscisic acid has an immediate effect on ion channel activity, most likely via [Ca²⁺], and causes a change in gene expression as indicated by circles (up) and bars (down). ABA perception is not clear. A Ca²⁺ influx could occur through a hyperpolarization activated cation conductance (HACC).^16,25 Cation transporters are NORC, nonselective cation conductance, KORC, K⁺-selective outwardly rectifying conductance (=SKOR⁸), and KIRC, K⁺-selective inwardly rectifying conductance, and anion conductances with different voltage-dependencies and gating characteristics are X-QUAC, quickly activating anion conductance, X-SLAC, slowly activating anion conductance, and X-IRAC, inwardly rectifying anion channel.^2,3,9,16,26 Transported ions and direction of flux are plotted.To date, we know that besides Ca²⁺ and abscisic acid also the pH, nonhydrolyzable GTP analogs and extracellular NO₃⁻ and K⁺ affect membrane transport capacities of root xylem-parenchyma cells (Fig. 2). Other control mechanisms by metabolites, the redox potential and phytohormones have to be included, especially if they represent signals in xylem loading or root-shoot communication. The composition of the xylem sap changes during the course of a day, depending on nutrient supply and various stresses, and the apoplastic ion concentration is considered to be an important factor in ion circulation.^6,18,19 ABA is such a signal. It is known to increase solute accumulation within the root by inhibiting release of ions into the xylem.¹⁷ Any change in transport activity has an impact on the membrane potential. This again determines whether salt release or uptake takes place. Passive salt release is restricted to a limited range of membrane potentials in which conductances for anions and cations are active simultaneously, that is with depolarization. Negative membrane voltages will be required for reabsorption of NO₃⁻ by a putative NO₃⁻/H⁺-symporter and for the uptake of K⁺ and amino acids.^3,13 As shown in our recent paper, the balance between the activities of the H⁺-pump and the anion conductances could affect the position between a depolarized and a hyperpolarized state of the parenchymal membrane. Thus, H⁺ pump activity is crucial in membrane voltage control. Furthermore, the simultaneous activities of H⁺ pumps and anion conductances make the generation of a high pH gradient possible, whilst maintaining electroneutrality. The proton gradient could be used for ion transport through cotransporters and antiporters as suggested for the loading of borate into the xylem through the boron transporter BOR1.²⁰ So we are on the way to decipher xylem loading in roots and this exciting field will also provide information about small-scale nutrient cycling and root-shoot communication. To determine how the activities of pumps, channels and transporters are adjusted among each other is the next challenge. Further insight has to be obtained by experimentation as well as by biophysical modeling.     

Investigation of Growth Hormones in Xylem Exudate and Root Tissue 1

Infection of the root system in tomato by the root-knot nematodewas found to alter the gibberellins and cytokinina extractedfrom root tissue and xylum exudate. Gibberellins from the roottissue and xylem exudate of healthy plants occurred in the neutral,acidic, and aqueous fractions. With increasing levels of infection,gibberellins were primarily extracted in the slightly acidicfraction. Gibberellin activity in the neutral, acidic, and aqueousfractions was decreased in diseased plants. Cytokinins, whichwere also extracted from root tissue and xylem exudate, werelower in diseased plants than in uninfected plants.     

土壤干旱条件下木质部汁液成分变化及其在根冠信号传递中的作用

介绍了植物木质部汁液收集方法和干旱条件下 ,蒸腾流中碳水化合物、蛋白质、pH等的变化 ,并对这些成分变化在根冠信号传递中的可能作用进行了讨论     

杜仲次生木质部分化过程中的细胞编程死亡

通过电子显微镜观察、DNA断裂检测及类似半胱氨酸蛋白酶（caspase-like proteases,CLPs)降解检测等技术,对杜仲（Eucommia ulmoides Oliv.）次生木质部分化过程的细胞编程死亡进行了研究。分化中的次生木质部细胞总DNA凝胶电泳检测到DNA ladder,并通过TUNEL检测进一步确定了DNA被降解。Western blot结果表明：caspase-8和caspase-3状蛋白酶（caspase-8-和caspase-3-like proteases,CLPs)及多聚ADP-核糖聚合酶（poly(ADP-ribose) polymerase,PARP)在次生木质部分化过程中被降解。这些研究结果表明,杜仲次生木质部的细胞分化是一个典型的编程性死亡（Programmed cell death,PCD)过程,CLPs可能参与了此过程。     

杜仲次生木质部分化过程中的细胞编程死亡

通过电子显微镜观察、DNA断裂检测及类似半胱氨酸蛋白酶(caspase-like proteases,CLPs)降解检测等技术,对杜仲(Eucommia ulmoides Oliv.)次生木质部分化过程的细胞编程死亡进行了研究.分化中的次生木质部细胞总DNA凝胶电泳检测到DNA ladder,并通过TUNEL检测进一步确定了DNA被降解.Western blot结果表明;caspase-8和caspase-3状蛋白酶(caspase-8-和caspase-3-like proteases,CLPs)及多聚ADP-核糖聚合酶(poly(ADP-ribose)polymerase,PARP)在次生木质部分化过程中被降解.这些研究结果表明,杜仲次生木质部的细胞分化是一个典型的编程性死亡(Programmed cell death,PCD)过程,CLPs可能参与了此过程.