Are developing xylem vessels the site of ion exudation from root to shoot?

Abstract This paper describes experiments to test the suggestion that developing xylem vessels are the site of exudation of ions from the root to the shoot. Electron microscopy is used to define the stage of development of xylem vessels in young barley roots along the length of the root. The amino acid analogue p-fluorophenyl-alanine (FPA) is used to inhibit ion transport from the stele to the xylem vessels at varied distances from the apex. In the presence of FPA protein synthesis is not inhibited but ineffective proteins are formed. It is shown that exudation of Cl^? from the root can be inhibited in this way in parts of the root where all the xylem vessels are mature. This is in contradiction to the suggestion that root exudation is due to the activity of developing vessels. The hypothesis is thus strengthened that ion transport proceeds into the xylem vessels, which are fully mature and devoid of cytoplasm, and is due to release from the xylem parenchyma cells.     

Organic substances in xylem sap delivered to above-ground organs by the roots

Squash (Cucurbita maxima) xylem sap, an apoplastic fluid, contains t-zeatin riboside, glutamine, methylglycine, myo-inositol, fructose, oligosaccharides of arabinogalactan, glucan, galacturonan, and pectins (rhamnogalacturonan-I and rhamnogalacturonan-II), as well as various proteins, including arabinogalactan and pathogen-related proteins. These substances are mainly produced in stele (xylem) parenchyma and the pericycle in the root-hair zone where ion transporter genes are expressed. Glycine-rich protein genes (CRGRPs) cloned by antiserum raised against whole xylem sap of cucumber (Cucumis sativus) were abundantly expressed in the parenchyma cells surrounding xylem vessels in the root-hair zone. CRGRP proteins accumulated and immobilized in the lignified walls of metaxylem vessels and perivascular fibers in shoots, suggesting a systemic delivery mechanism of wall materials via xylem sap. A major 30-kDa protein (XSP30) found in cucumber xylem sap was homologous to the B chains of a lectin (ricin) and bound to a nonfucosylated core N-acetylglucosamine dimer of N-linked glycoproteins abundant in leaf parenchyma cells. XSP30 gene expression, abundant in root xylem parenchyma and pericycle, and the level of XSP30 protein fluctuated diurnally under the control of a circadian clock, and the amplitude was up-regulated by gibberellic acid produced in young leaves, suggesting a long-distance control system between organs.     

Hydraulic properties of living late metaxylem and interactions between transpiration and xylem pressure in maize

A new approach to study dynamic interactions between transpiration and xylem pressure in intact plants is presented. Pressure probe measurements were preformed in living (immature) late metaxylem of maize roots rather than in adjacent mature xylem. This eliminated technical limitations related to the measurement of negative pressures. Water relations of single cells showed that turgor and volumetric elastic modulus were significantly larger in living metaxylem than in cortical cells; hydraulic conductivity was similar in both types of root cells. Increasing transpiration induced an immediate decrease of xylem pressure, and vice versa. Turgor in the living metaxylem could be continuously recorded for more than 1 h. The relationship between xylem pressure and transpiration yielded a root hydraulic resistance of 1.3 x 10⁹ MPa s m^-3. Control experiments indicated that the response of living xylem in the positive pressure range essentially paralleled that of mature root xylem in the negative range. In mature xylem, pressures as low as -0.55 MPa were recorded for short periods (several minutes). Several tests verified that the pressure probe was in contact with mature xylem during the measurements of tensions. The results demonstrate convincingly that transpiration generates an effective driving force for water uptake in roots, a central feature of the cohesion theory.Key words: Hydraulic conductivity, negative pressure, root development, turgor, water transport, Zea mays.      

Hydraulic propagation of pressure along immature and mature xylem vessels of roots of Zea mays measured by pressure-probe techniques

Turgor pressure was measured in cortical cells and in xylem elements of excised roots and roots of intact plants of Zea mays L. by means of a cell pressure probe. Turgor of living and hence not fully differentiated late metaxylem (range 0.6–0.8 MPa) was consistently higher than turgor of cortical cells (range 0.4–0.6 MPa) at positions between 40 and 180 mm behind the root tip. Closer to the tip, no turgor difference between the cortex and the stele was measured. The turgor difference indicated that late-metaxylem elements may function as nutrient-storage compartments within the stele. Excised roots were attached to the root pressure probe to precisely manipulate the xylem water potential. Root excision did not affect turgor of cortical cells for at least 8 h. Using the cell pressure probe, the propagation of a hydrostatic pressure change effected by the root pressure probe was recorded in mature and immature xylem elements at various positions along the root. Within seconds, the pressure change propagated along both early and late metaxylems. The half-times of the kinetics, however, were about five times smaller for the early metaxylem, indicating they are likely the major pathway of longitudinal water flow. The hydraulic signal dissipated from the source of the pressure application (cut end of the root) to the tip of the root, presumably because of radial water movement along the root axis. The results demonstrate that the water status of the growth zone and other positions apical to 20 mm is mainly uncoupled from changes of the xylem water potential in the rest of the plant.Abbreviations and Symbols CPP cell pressure probe - EMX early metaxylem - LMX Late metaxylem - P_c cell turgor - P_r root pressure - RPP root pressure probe - t_1/2,c half-time of water exchange across a single cell - t_1/2 half-time of water exchange across multiple cells We thank Antony Matista for his expert assistance in the construction and modification of instruments. The work was supported by grant DCB8802033 from the National Science Foundation and grant 91-37100-6671 from USDA, and by the award of a Feodor Lynen-Fellowship from the Alexander von Humboldt-Foundation (Germany) to J.F.     

Late Maturation of Large Metaxylem Vessels in Soybean Roots: Significance for Water and Nutrient Supply to the Shoot

The large, late metaxylem (LMX) in the roots of soybean beginsdevelopment in the centre of the stele after lignification ofthe early metaxylem poles. Subsequent maturation of the firstappearing LMX elements is gradual. They were never mature inthe 8-d-old seedlings examined. In 10 to 15-d-old plants thefirst LMX matured to open vessels at a mean of 17 cm proximalto the root tip. Additional LMX vessels developed in more proximalregions of the roots and these also matured gradually. Based on calculations from relative vessel diameters, the potentialflow of xylem sap in a single central LMX vessel is 50 timesthat in the total of all the early metaxylem (EMX) vessels ofa typical primary root of soybean. There was a marked dependence of relative leaf area on the lengthof primary root with open LMX vessels. This may result fromthe predicted increased water and nutrient flow to the shoot,facilitated by the opening of the large vessels. It is suggestedthat, as in maize, the living LMX elements may function in ionaccumulation. Dicotyledonous roots, soybean, Glycine max, xylem vessels, xylem maturation, water conduction     

Accumulation of potassium by differentiating metaxylem elements of maize roots

The previous demonstration that the large late metaxylem vessels of field-grown maize ( Zea mays L. cv. Rosella) roots do not lose their crosswalls until they are 20–30 cm from the tip, and that the presence of a soil sheath outside the root was indicative of immature vessels within, greatly strengthened the hypothesis that ion accumulation into these roots was by uptake into living xylem element vacuoles. Proposals that salt movement into the xylem was by leakage or secretion into dead vessels became much less plausible. Potassium concentration in the vacuoles of late metaxylem elements was measured by X-ray microanalysis in unetched fracture faces of bulk, frozen-hydrated pieces of sheathed roots, and found to be in the range 150–400 m M . Potassium concentration in open vessels of bare roots, measured both with the microprobe and by spectrophotometry of aspirated sap, was in the range of 5 to 25 m M . It is concluded that uptake of potassium (and possibly other ions) is into living xylem elements, and that its release to the transpiration stream occurs by the breakdown of their crosswalls and the addition of their vacuoles to the solution in the vessels above.     

Aluminium localization in root tips of the aluminium-accumulating plant species buckwheat (Fagopyrum esculentum Moench)

Aluminium (Al) uptake and transport in the root tip of buckwheat is not yet completely understood. For localization of Al in root tips, fluorescent dyes and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were compared. The staining of Al with morin is an appropriate means to study qualitatively the radial distribution along the root tip axis of Al which is complexed by oxalate and citrate in buckwheat roots. The results compare well with the distribution of total Al determined by LA-ICP-MS which could be reliably calibrated to compare with Al contents by conventional total Al determination using graphite furnace atomic absorption spectrometry. The Al localization in root cross-sections along the root tip showed that in buckwheat Al is highly mobile in the radial direction. The root apex predominantly accumulated Al in the cortex. The subapical root section showed a homogenous Al distribution across the whole section. In the following root section Al was located particularly in the pericycle and the xylem parenchyma cells. With further increasing distance from the root apex Al could be detected only in individual xylem vessels. The results support the view that the 10 mm apical root tip is the main site of Al uptake into the symplast of the cortex, while the subapical 10-20 mm zone is the main site of xylem loading through the pericycle and xylem parenchyma cells. Progress in the better molecular understanding of Al transport in buckwheat will depend on the consideration of the tissue specificity of Al transport and complexation.     

Exogenous zeatin accumulation in wheat-root cells and its role in regulation of cytokinin transport

It was shown that the cytokinin content in the xylem sap of a wheat plant treated with exogenous zeatin was about ten times lower than in the nutrient solution in 24 h. Cytokinins were accumulated in roots rather shoots of treated plants. These data demonstrate the existence of a barrier in the cytokinin pathway from the nutrient solution to plant shoots. The deposition of lignin and suberin in stele detected with Sudan @III is enlarged with an increase in the distance from the tip of the root. The augmented content of suberin and lignin coincided with reduced cytokinin immunolabeling in root cells revealed by monoclonal antibodies to cytokinin and secondary gold-labeled antibodies. The accumulation of exogenous cytokinin in root stele cells shows that Casparian bands are not the only barrier on the cytokinin pathway to plant shoots. Intensive cytokinin immunolabeling in parenchyma cells surrounding stele vessels indicates the accumulation of cytokinin by these cells and suggests that there are mechanisms that limit the hormone loading in xylem vessels during transport to the shoot. The role of cytokinin transporters in this process is discussed.     

Accumulation of high levels of potassium in the developing xylem elements in roots of soybean and some other dicotyledons

Summary Potassium concentrations have been determined by cryo-analytical scanning microscopy in vacuoles of cells of the roots of soybean and six other dicotyledons. Developing vessel elements accumulate the highest concentration of potassium in any cell type in these roots, and those of the secondary xylem have more (median 190 mM normalized to 120%) than those of either the early (median 100 mM, normalized 80%) or late metaxylem (median 110 mM, normalized 100%). Potassium concentration in these developing vessels always exceeds those of their adjacent parenchyma (60–80%), which in turn are higher than those in cells of the cortex (30–40%), including the endodermis. Potassium concentration in the vessels increases during their development until cell death and maturation when it drops dramatically (normalized 4–12%). Developing vessel elements are clearly major sites of potassium accumulation in roots and need to be considered in any models of ion uptake, accumulation, circulation, or exudation.Abbreviations EDX energy dispersive X-ray microanalysis - EMX early metaxylem - K elemental potassium - LMX late metaxylem - CSEM cryo-scanning electron microscopy - SX secondary xylem Dedicated to the memory of Professor John G. Torrey     

A microautoradiographic study of Ca45 and S35 distribution in the intact bean root

Summary Microautoradiographic techniques were used to determine the distribution of Ca⁴⁵ and S³⁵ in regions of the bean root where anatomical features may influence the processes of ion uptake and translocation. Root tissue from intact plants was prepared by methods that preserve both soluble and insoluble Ca and S. Ca⁴⁵ distribution was determined after 1 hour and 15 min, of uptake, after 2 efflux periods, and after replacement by non-tracer Ca.S³⁵ distribution was determined after 1 hour and 15 min of uptake.The quantity of Ca⁴⁵ that entered the root was greater than the quantity of S³⁵. Ca⁴⁵ concentration within the root increased with linear distance from the 8-mm level behind the tip. The pathways of Ca and S across the cortex appeared to be different since Ca⁴⁵ was particularly associated with cell walls and S³⁵ was distributed more evenly through the cells. There was no evidence that the endodermis was a diffusion barrier for Ca; the small parenchyma cells associated with conducting elements acquired a high concentration of Ca⁴⁵ and thus appear to be implicated in absorption and perhaps in transfer to the xylem. The evidence suggests that the endodermis may have been a barrier for S, but if so, certain parenchyma cells inside the stele, especially at xylem poles, were equally involved. The region from 30 to 80 mm from the tip appeared to participate in Ca uptake and transfer to the xylem; because of tissue immaturity the 8-mm region, which contained the least Ca⁴⁵, was thought not to translocate to the shoot. Deposition of Ca⁴⁵ in oxalate crystals represented almost complete immobilization. Calcium oxalate metabolism was most active in the 30-mm region of secondary roots and in their small branches. S³⁵-labelled nuclei occurred in the cortex 2.5 to 3 mm behind the root tip.     

Xylem perfusion of tap root segments of Plantago maritima: the physiological significance of electrogenic xylem pumps

Abstract A method is described for perfusing xylem vessels in tap root segments of the halophyte P. maritima. Use of excised segments allowed recording of the trans-root potential (TRP) at both ends of a segment. It was shown that there can be a spatial variation of electrogenic ion pump activity along the xylem in one root segment. The pH of perfusion solutions, differing in buffering capacity, was adjusted by the root segment to pH 5.1–5.6 during How through the xylem. This pH range was similar to that of sap produced by root pressure. The K⁺ activity in the outflow solution (K⁺_out) was rather constant at 12–13 mol m^?l3 despite input K⁺ activities ranging from 8 to 20 mol m^?l3. Addition of fusicoccin (10^?l2 mol m^?l3) to the perfusion solution induced a strong acidification of the xylem sap, a decrease in K⁺_out and an increase in Na⁺_out. Inhibition of aerobic respiration through anoxia inhibited electrogenic proton pumping into the xylem and led to an increase in K⁺_out and a decrease in Na⁺_out. It is suggested that transport of K⁺ and Na⁺ to the shoot of the halophyte P. maritima is regulated in the tap root by means of ion exchange between xylem vessels and xylem parenchyma and that this exchange is energized by proton translocating ATPases.     

The haustorium of the root parasite Triphysaria (Scrophulariaceae), with special reference to xylem bridge ultrastructure

Haustoria of Triphysaria pusilla and T. versicolor subsp. faucibarbata from a natural habitat were analyzed by light and electron microscopy. Secretory trichomes (root hairs) participate in securing the haustorium to the surface of the host root. The keel-shaped intrusive part of the secondary haustorium penetrates to the depth of the vascular tissue of the host. Some of the epidermal interface cells differentiate into xylem elements. A significant number of haustoria do not differentiate further, but in most haustoria one to five of the epidermal xylem elements terminate a similar number of xylem strands. The strands mostly consist of vessel members and they connect host xylem or occasionally host parenchyma to the plate xylem adjacent to the stele of the parasite root. Each strand of this xylem bridge is accompanied by highly protoplasmic parenchyma cells with supposed transfer cell function. Increased surface area of the plasmalemma occurs in these cells as it does in interface parenchyma cells. Graniferous tracheary elements are restricted to the haustorium and occur most frequently in the plate xylem. The plate xylem is also accompanied by highly protoplasmic parenchyma cells. Hyphae of mycorrhizal fungi of the host root occasionally penetrate into the distal part of the xylem bridge. We combine structural observations and physiological facts into a hypothesis for translocation of water and nutrients between host and parasite. Some evolutionary aspects related to endogeny/exogeny of haustoria are discussed, and it is argued that the Triphysaria haustorium represents a greatly advanced and/or reduced condition within Scrophulariaceae.     

Regulation of K+ Uptake and Transport to the Xylem in Barley Roots; K+ Distribution Determined by Electron Probe X-ray Microanalysis of Frozen-Hydrated Cells

Intact roots of young barley plants (Hordeum vulgare L. cv.Proctor) were induced to transport K⁺ to the xylem at rapidor slow rates. Roots were then rapidly frozen in liquid nitrogenand fractured in the zone 70 mm behind the root tip to givetransverse faces for electron probe microanalysis. With SEMvisualization, analyses were made over the cytoplasm and vacuole(or lumen) of 14 cells types along the root radius between theouter cortex and stele, with particular emphasis on the xylem,xylem parenchyma, and phloem. Data were recorded in the formof colour-coded maps and also quantitatively. For both typesof roots, K⁺ concentration was lower over the xylem and phloemthan in the remainder of the root. The concentration of K⁺ wasgreater in the vacuole than in the cytoplasm, while for P itwas the reverse. Significantly, in roots induced to transportK⁺ rapidly the concentration of K⁺ was low in the early maturingmetaxylem and protoxylem, and in the sieve tubes of the metaphloemand protophloem. The concentrations of K⁺ in various cell typesare discussed in relation to regulation of K⁺ loading of thexylem in long-distance ion transport. Key words: Ion transport, nutrient deficiency     

Zostera capensis setchell: Root structure in relation to function

Root structure of the seagrass Zostera capensis Setchell was investigated by light and electron microscopy. Roots possess conspicuous root hairs which greatly increase the surface area available for absorption. Exodermal cells abutting root-hair bases possess transfer cell characteristics. The strategic location of these cells suggests that they participate in absorptive and/or transfer processes between the epidermis and cortex. Vascular parenchyma cells within the stele also possess transfer cell features. Wall ingrowths of these cells about xylem elements, sieve tubes, companion cells and other vascular parenchyma cells, suggesting that they play a role in absorptive and/or transfer processes between the stele and cortex. Apoplastic barriers in the form of suberin lamellae and Casparian bands occur in walls of both the exodermis and endodermis. However, plasmodesmata perforate the suberin lamellae in these walls, and a symplastic pathway can be traced from the root hairs to vascular parenchyma transfer cells contiguous with conducting elements of the stele. The occurrence of wall ingrowths adjacent to xylem elements implies that transfer processes occur between vascular transfer cells and xylem. Although reduced, xylem could therefore play a role in transport. Structural evidence obtained in this study supports the role of the roots in absorptive processes and shows pathways available for transport from the water column to the conducting tissues of the root.     

Vascular tissue-specific gene expression of xylem sap glycine-rich proteins in root and their localization in the walls of metaxylem vessels in cucumber

Root-specific cDNAs of glycine-rich protein (cucumber root glycine rich protein-1 and -2; CRGRP-1 and CRGRP-2) were cloned previously by use of an antiserum raised against whole xylem sap of Cucumis sativus. The accumulation of the corresponding mRNA at high levels was detected in the root-hair zone of cucumber tap root [Sakuta et al. (1998) Plant Cell Physiol. 39: 1330]. The RNA gel blot analysis with the CRGRP-1- and -2-specific probes revealed that the CRGRP genes expressed only in root but not at all in aboveground organs. When the localization of these mRNAs were examined by in situ hybridization, CRGRP mRNAs were found only in the parenchyma cells in the central cylinder of young lateral roots and it was most abundant in the cells that surrounded xylem vessels in the root-hair zone of the tap root. In immunoblotting of xylem sap collected from cucumber stem with an antiserum raised against CRGRP-1 that had been produced in an E. coli expression system, the antibodies, which did not cross-react with GRP1.8 of kidney bean, reacted with two proteins, whose mobilities corresponded to those of proteins deduced from the CRGRP-1 and -2 cDNAs. Immunohistochemical staining revealed that the CRGRPs accumulated specifically in the lignified walls of metaxylem vessels in the root, stem and leaf and in the lignified cell walls of perivascular fibers in cucumber stems. Immunostaining was also detected in the walls of metaxylem vessels and in the cell walls of adjacent sclerenchyma in the hypocotyl of kidney bean. These data clearly indicate that the novel glycine-rich proteins were produced in the vascular tissue of the root, transported systemically over a long distance via the xylem sap and immobilized in the walls of metaxylem vessels and sclerechyma cells in aboveground organs.     

Mechanisms of Reduction of the Stelar Pattern along Barley Roots

The stelar pattern along the seminal and nodal roots of barley (Hordeum vulgare L.) is gradually simplified due to a decreasing frequency of longitudinal cell division in the apical meristem. The decrease involves the proportion of stelar parenchyma, the number of vascular strands on the periphery of the stele and, in nodal roots with a more complex structure, the number of central metaxylem files. In spite of the fact that the stelar parenchyma is reduced in distal parts of the roots to approximately one half, the discontinuity of central and peripheral metaxylem is preserved. Reduction of the number of central metaxylem files is due to fusion. In the reduction of peripheral xylem and phloem strands, the development of certain xylem strands is discontinued and they are terminated blindly. Two phloem strands that had alternated radially with them, approach each other, coalesce and a single phloem strand continues to develop. In this way the regular alternation of phloem and xylem is re-established. The importance of fusions ensuring reduction of the functional continuity in vascular tissue by formation of a network structure must be stressed. This reduction mechanism is involved not only in files of the wide central metaxylem but also in phloem strands which are thus preferred over blindly terminating peripheral xylem strands.     

植物钙素吸收和运转

近年来,钙素在植物体内的吸收和运输研究主要集中在细胞和分子水平,但整株水平上的研究也同样重要.整株水平上的钙吸收和运输包括根细胞的钙吸收、钙离子横向穿过根系并进入木质部、在木质部运输、从木质部移出并进入叶片或果实及在叶片或果实中运转分配等环节,既经过质外体也穿越共质体.钙离子通道、Ca2 -ATP酶和Ca2 /H 反向转运器等参与根细胞的钙吸收.在钙离子横向穿根进入木质部的过程中,需要穿越内皮层和木质部薄壁细胞组织.根系内皮层凯氏带阻挡了Ca2 沿质外体途径由内皮层外侧向内侧的移动,部分Ca2 由此通过离子通道流进内皮层细胞而转入共质体并到达木质部薄壁细胞组织,而由木质部薄壁细胞组织进入中柱质外体可能需要Ca2 -ATP酶驱动;还有一些Ca2 由内皮层细胞运出,沿内皮层内侧的质外体途径进入木质部导管,并通过导管运向枝干.钙离子以螯合态的形式在枝干导管运输;水流速率是影响钙离子沿导管运输的关键因子.钙离子在果实和叶片中的运输和分配不仅通过质外体途径也通过共质体途径.     

Perfusion of onion root xylem vessels: a method and some evidence of control of the pH of the xylem sap

We describe a method for perfusing the xylem in the stele of excised onion roots with solutions of known composition under a pressure gradient. Tracer studies using [¹⁴C] polyethylene glycol 4000 and the fluorescent dye, Tinopal CBSX, indicated that perfusing solutions passed exclusively through the xylem vessels. The conductance of the xylem was small over the apical 100 mm of the root axis but increased markedly between 100 and 200 mm. Unbuffered perfusion solutions supplied in the range pH 3.7–7.8 emerged after passage through the xylem adjusted to pH 5.2–6.0, indicating the presence of mechanisms for absorbing or releasing protons. This adjustment continued over many hours with net proton fluxes apparently determined by the disparity between the pH of the perfusion solution and the usual xylem sap pH of about 5.5. Mild acidification of the xylem sap by buffered perfusion solutions increased the release of ⁸⁶Rb (K⁺) and ³⁵SO₄ ^2- from the stelar tissue into the xylem stream. The ion-transporting properties of onion roots seemed little changed by excision from the bulbs, or by removal of the apical zones of the root axis. The pH of sap produced by root pressure resembles that found in the outflow solutions of perfused root segments.     

The importance of anatomical structural study of roots for the understanding of physiological processes

Abstract

Besides being species‐specific, the inner structure of the root is influenced by the place and time of origin during the growth period. From the root tip up to the base of a particular root, the zones of cell division, cell elongation, formation of root hairs and root branching can be distinguished. The root tip that is covered by a root cap and mucilage is protected against evaporation and water contact. From the end of the lateral parts of the root cap, the cells become exposed to the surrounding environment. The cells can elongate by water uptake or can shrink by water loss. All processes of geotropic growth take place there. In this study, some differences are illustrated using Zea mays plants. Radicle and roots emerging from several nodes of the shoot as well as lateral roots are compared. The distances from the tip and from the base of the root are also very important for characterization of particular root functions. Distinctive features such as root diameter, size of the stele and of the cortex, ratio of cortex to stele, number and width of the xylem vessels, size of cells, special thickenings and stage of lignification as well as symptoms of maturation are observed.