Xylem Tracheary Elements Research Articles

Localisation of hydrogen peroxide accumulation during Solanum tuberosum cv. Rywal hypersensitive response to Potato virus Y

The reactive oxygen species hydrogen peroxide (H 2O 2) was detected cytochemically in Solanum tuberosum cv. Rywal tissues as a hypersensitive response (HR) 24 and 48 h after a Potato virus Y (PVY) infection. Hydrogen peroxide was detected in vivo by its reaction with 3.3-diaminobenzidine, producing a reddish-brown staining in contact with H 2O 2. Hydrogen peroxide was detected in the necrotic area of the epidermal and mesophyll cells 24 and 48 h after the PVY infection. Highly localised accumulations of H 2O 2 were found within xylem tracheary elements, and this was much more intensive than in non-infected leaves. Hydrogen peroxide was detected cytochemically in HR also by its reaction with cerium chloride, producing electron-dense deposits of cerium perhydroxides. Inoculation with PVY NTN and also PVY N Wi induced a rapid hypersensitive response during which highly localised accumulations of H 2O 2 was detected in plant cell walls. The most intensive accumulation was present in the bordering cell walls of necrotic mesophyll cells and the adjacent non-necrotic mesophyll cells. Intensive electron-dense deposits of cerium perhydroxide were found along ER cistrenae and chloroplast envelopes connected with PVY particles. The precipitates of hydrogen peroxide were detected in the nuclear envelope and along tracheary elements, especially when virus particles were present inside. The intensive accumulation of H 2O 2 at the early stages of potato–PVY interaction is consistent with its role as an antimicrobial agent and for this reason it has been regarded as a signalling molecule.

The shoot and root growth of Brachypodium and its potential as a model for wheat and other cereal crops.

The grass genetic model Brachypodium (Brachypodium distachyon (L.) Beauv., sequenced line Bd 21) was studied from germination to seed production to assess its potential as a phenotypic model for wheat (Triticum aestivum L.) and other cereal crops. Brachypodium and wheat shoot and root development and anatomy were highly similar. Main stem leaves and tillers (side shoots) emerged at the same time in both grasses in four temperature and light environments. Both developed primary and nodal axile roots at similar leaf stages with the same number and arrangement of vascular xylem tracheary elements (XTEs). Brachypodium, unlike wheat, had an elongated a mesocotyl above the seed and developed only one fine primary axile root from the base of the embryo, while wheat generally has three to five. Roots of both grasses could develop first, second and third order branches that emerged from phloem poles. Both developed up to two nodal axile roots from the coleoptile node at leaf 3, more than eight nodal axile roots from stem nodes after leaf 4, and most (97%) of the deepest roots at flowering were branches. In long days Brachypodium flowered 30 days after emergence, and root systems ceased descent 42 cm from the soil surface, such that mature roots can be studied readily in much smaller soil volumes than wheat. Brachypodium has the overwhelming advantage of a small size, fast life cycle and small genome, and is an excellent model to study cereal root system genetics and function, as well as genes for resource partitioning in whole plants.

Cysteine proteases XCP1 and XCP2 aid micro‐autolysis within the intact central vacuole during xylogenesis in Arabidopsis roots

Establishing the mechanisms regulating the autolysis of xylem tracheary elements (TEs) is important for understanding this programmed cell death process. These data demonstrate that two paralogous Arabidopsis thaliana proteases, XYLEM CYSTEINE PROTEASE1 (XCP1) and XCP2, participated in micro-autolysis within the intact central vacuole before mega-autolysis was initiated by tonoplast implosion. The data acquisition was aided by the predictable pattern of seedling root xylogenesis, the availability of single and double total knock-out T-DNA lines, anti-sera that recognized XCP1 and XCP2, and the microwave-assisted processing of whole seedlings prior to immunolabeling and observation in the transmission electron microscope. During secondary wall thickening, XCP1 and XCP2 (in wild type), XCP1 (in xcp2 seedlings) or XCP2 (in xcp1 seedlings) were imported into the TE central vacuole. Both XCP1 and XCP2 heavily labeled dense aggregates of material within the vacuole. However, because of XCP1 deficiency in xcp1 and xcp1 xcp2 TEs, non-degraded cellular remnants first accumulated in the vacuole and then persisted in the TE lumen (longer than in the wild type) after the final mega-autolysis was otherwise complete. This delayed TE clearing phenotype in xcp1 was rescued by complementation with wild-type XCP1. Although TEs in the xcp2 single knock-out cleared comparably with wild type, the non-degraded remnants in xcp1 xcp2 TEs were more densely packed than in xcp1 TEs. Therefore, XCP2 has a minor but distinct role in micro-autolysis. After tonoplast implosion, XCP1 and XCP2 remained associated with disintegrating cellular material as mega-autolysis, aided by additional lytic enzymes, destroyed the bulk of the cellular contents.

Types, structure and potential for axial water flow in the deepest roots of field‐grown cereals

Deep root systems that extend into moist soil can significantly increase plant productivity. Here, the components of soil-grown root systems of wheat, barley and triticale are characterized, and types and water conducting potential of deep roots in the field are assessed. Root system components were characterized in plants grown in soil in PVC tubes, based on their origin and number and the arrangement of xylem tracheary elements (XTE) viewed using fluorescence microscopy. A new nomenclature is proposed. Deep roots were harvested in the field, and root types of the current crop and remnant roots from previous crops were identified by fluorescence and cryo-scanning electron microscopy. Four types of axile (framework) and five types of branch root were distinguished in the three cereals. Six per cent of deep roots were axile roots that originated from the base of the embryo; 94% were branch roots, of which 48% had only two XTE (10 microm diameter), and thus potentially low axial flow. Only 30% of roots in the cores were from the current crop, the remainder being remnants. Selection for more deep-penetrating axile roots and increased vascular capacity of deep branches is of potential benefit. Conventional root-length density measurements should be interpreted and applied cautiously.

Arabinogalactan-proteins in roots of Echinacea purpurea: Structural investigations and microscopic localization by immunofluorescent labelling

Arabinogalactan-proteins (AGPs) are glycoproteins which are supposed to be cell wall associated and belong to the putative active compounds of Echinacea preparations (1). (β-D-Glc)3Yariv phenylglycoside (Yariv) specifically binds to most plant-AGPs and was used to isolate AGPs from the high molecular weight fraction (>30.000Da) of an aqueous extract of root material from Echinacea purpurea (L.) Moench (Asteraceae). These AGPs with a carbohydrate-moiety consisting predominantly of Gal and Ara (2:1, >90%) have been quantitatively and structurally characterized. Methylation analysis detected as main components 1,6-Galp, 1,3-Galp, 1,3,6-Galp and in the side chains 1,5-Araf and terminal 1-Araf. Determination of the acidic compounds showed the presence of 1-GlcAp. For microscopic localization, a polyclonal antibody (generated in rabbit directed against Echinacea AGP) (2) in combination with a secondary FITC-labelled anti-rabbit antibody was used for an immunofluorescent labelling of AGPs on thin sections (16µm) of root material of Echinacea purpurea. Furthermore, a Yariv-antibody (generated in rabbit directed against albumin-coupled Yariv) was used to label a Yariv-treated thin section of the same roots. After addition of the FITC-conjugated secondary antibody, the sections were observed with a Confocal Laser Scanning Microscope (CLSM). In both cases the labelled AGP was located in the cell walls, often in contact with the cytoplasma membrane. Confirming results on tomato roots (3), especially xylem tracheary elements showed very strong immunolabelling of the secondary wall.

A short note on starch and xylem of Colocasia esculenta (taro) in archaeological deposits from Pitcairn Island, southeast Polynesia

We present results of starch analysis of archaeological deposits from Pitcairn Island. High concentrations of starch grains preserved in cell membranes, and xylem tracheary elements, consistent with introduced Colocasia esculenta (taro) were found. Because of limited age control, we are uncertain if the microfossils are prehistoric. Problems associated with identifying taxa with small starch grains in extractions from weathered deposits are highlighted.

(1→3),(1→4)‐ß‐d‐Glucans in the cell walls of the Poales (sensu lato): an immunogold labeling study using a monoclonal antibody

(1→3),(1→4)-ß-Glucans had previously been detected in nonlignified cell wall preparations of only the Poaceae and five other families in the graminoid clade of the Poales (s.l.). Cell walls of vegetative organs of 12 species in nine families of the Poales (s.l.) were examined by immunogold labeling using a monoclonal antibody to (1→3),(1→4)-ß-glucans. Three types of wall-labeling patterns were identified depending on the density of labeling of the nonlignified walls of epidermal and parenchyma cells and the lignified walls of sclerenchyma fibers and xylem tracheary elements: type 1 in Poaceae and Flagellariaceae, type 2 in Restionaceae and Xyridaceae, and type 3 in Cyperaceae and Juncaceae. Type 1 had the heaviest labeling of nonlignified walls and type 2 the heaviest labeling of lignified walls. Type 3 had the least wall labeling, with only very light labeling of nonlignified and lignified walls. No labeling was found over walls of Typhaceae, Sparganiaceae, or Bromeliaceae. The results are discussed in relation to Poales phylogeny.

Polysaccharide compositions of leaf cell walls of forage chicory ( Cichorium intybus L.)

The polysaccharide compositions of cell walls isolated from the laminae and midribs of chicory (cv. Grasslands Puna II) leaves were investigated. The walls, which, except for the walls of xylem tracheary elements in vascular bundles, were non-lignified, were fractionated progressively with 50 mM CDTA, 50 mM Na 2CO 3, 1 M KOH, 4 M KOH, 4 M KOH + 3.5% H 3BO 3, and hot water. Monosaccharide and linkage analyses of the polysaccharides in the walls and wall fractions indicated that the polysaccharides were similar to those in the non-lignified walls of other dicotyledons. However, the proportions of pectic polysaccharides were particularly high (67% of the total wall polysaccharides in the laminae). These included homogalacturonans (50% of the total wall polysaccharides in the laminae) and rhamnogalacturonan-I (RG-I). In contrast, the proportions of cellulose, xyloglucans, heteroxylans and glucomannans were particularly low. The xyloglucans were probably fucogalactoxyloglucans. The wall polysaccharides from the leaf laminae differed from those from the midribs in having higher proportions of homogalacturonans, lower proportions of non-pectic polysaccharides, and RG-I with a higher ratio of galactan to arabinan.

Hydrogen peroxide increases potato tuber and stem starch content, stem diameter, and stem lignin content

Field-grown potato plants were sprayed twice weekly, from 21 to 90 days after planting, with 5 or 50 mM hydrogen peroxide (H2O2) solutions. Relative to water-sprayed controls, the H2O2 treatments significantly enhanced tuber starch accumulation by between 6.7% and 30%, as determined by specific gravity or the anthrone spectrophotometric method. Pronounced effects of similar H2O2 treatments on aerial stem anatomy and starch content were also found in glasshouse experiments. H2O2 treated stems were up to 27% thicker than controls, mainly due to enlarged medullar parenchyma cells. Histochemical observations indicated that there were more starch grains in cortex and pith tissue of H2O2-treated stems. H2O2 also increased the number and size of xylem tracheary elements in the vascular bundles and the number of interfascicular fibers. Quantification using image analysis confirmed that stems of H2O2 treated plants contained up to 3.4-fold more starch and 62% more lignin. This new chemical treatment to promote starch accumulation has potential utility in potato crop production and research.

Seven Enzymes of Carbon Metabolism, Including Three Calvin Cycle Isozymes, Are Present in the Secondary Cell Wall Thickenings of the Developing Xylem Tracheary Elements in Pea Leaves

Antibodies directed against chloroplast aldolase and chloroplast fructose bisphosphatase and against cytosolic fructose bisphosphatase, cytosolic P‐glycerate kinase, and cytosolic glyceraldehyde‐3‐P dehydrogenase recognize antigens in the secondary cell wall thickenings in the developing xylem tracheary elements in pea (Pisum sativum L.) leaf mesophyll cells. The isozyme specificity indicates sorting and selection during cell death and xylem formation. Antibodies directed against ADP‐glucose pyrophosphorylase, sedoheptulose bisphosphatase, triose‐P isomerase, and the photosynthetic electron transport component plastocyanin also recognize secondary cell wall components. It seems possible that these proteins are being recycled and put to some alternate use in the developing xylem.

Immunolocalization of LeAGP-1, a modular arabinogalactan-protein, reveals its developmentally regulated expression in tomato.

Arabinogalactan-proteins (AGPs) are highly glycosylated cell surface proteins that are thought to function in plant growth and development. The developmentally regulated expression of LeAGP-1, a novel and major AGP in tomato, was examined in different organs and tissues of tomato (Lycopersicon esculentum Mill. cv. UC82B) plants with an anti-peptide antibody (i.e. the PAP antibody) directed specifically against the lysine-rich subdomain of the LeAGP-1 core protein. During cell differentiation in tomato plants, LeAGP-1 was associated with cell wall thickening and lignification of particular cell types. Specifically, LeAGP-1 was detected in secondary wall thickenings of maturing metaxylem and secondary xylem tracheary elements in roots and stems, and in thickened cell walls of phloem sieve elements. However, LeAGP-1 was also present in thin-walled, cortical parenchyma cells of seedling roots as well as thick-walled collenchyma cells in young stems, both of which are not lignified. Based on these observed patterns, possible roles for LeAGP-1 in plant growth and development are discussed.

Cloning and characterization of a novel Mg(2+)/H(+) exchanger.

Cellular functions require adequate homeostasis of several divalent metal cations, including Mg(2+) and Zn(2+). Mg(2+), the most abundant free divalent cytoplasmic cation, is essential for many enzymatic reactions, while Zn(2+) is a structural constituent of various enzymes. Multicellular organisms have to balance not only the intake of Mg(2+) and Zn(2+), but also the distribution of these ions to various organs. To date, genes encoding Mg(2+) transport proteins have not been cloned from any multicellular organism. We report here the cloning and characterization of an Arabidopsis thaliana transporter, designated AtMHX, which is localized in the vacuolar membrane and functions as an electrogenic exchanger of protons with Mg(2+) and Zn(2+) ions. Functional homologs of AtMHX have not been cloned from any organism. Ectopic overexpression of AtMHX in transgenic tobacco plants render them sensitive to growth on media containing elevated levels of Mg(2+) or Zn(2+), but does not affect the total amounts of these minerals in shoots of the transgenic plants. AtMHX mRNA is mainly found at the vascular cylinder, and a large proportion of the mRNA is localized in close association with the xylem tracheary elements. This localization suggests that AtMHX may control the partitioning of Mg(2+) and Zn(2+) between the various plant organs.

Wood, Bark, and Stem Anatomy of Gnetales: A Summary

Data from a series of eight papers, representing a survey of Gnetales at the species level, are summarized in order to develop concepts on the relationship between the anatomical data and ecology, phylogeny, and systematics. Most of the characters and character states newly reported have phylogenetic and ecological correlations for Gnetales as a whole rather than systematic significance within the genera. Wood features are sensitively related to habit, although strategies in lianoid species of Ephedra differ from those in lianoid Gnetum species. Data also bear close relationships to organography. Vessel details are given special attention because workers have questioned whether vessels originated in Gnetales independently of those in angiosperms, and thus whether or not vessel origin in the two groups is a synapomorphy. The evidence cited here favors origin of vessels in Gnetales independent of vessel origins in angiosperms Hitherto unappreciated contrasts between vessels of Gnetales and those of angiosperms are detailed: the torusmargo structure of pits in Gnetales (both Ephedra and Gnetum), and the aspiration ability of gnetalean pits represent modes of conduction and promotion of conductive safety different from those in angiosperms, and the shape difference (circular vs scalariform), cited in data matrices of cladograms, is secondary to the different functional syndromes. The intercalation of circular bordered pits into helical secondary wall thickenings of primary xylem tracheary elements of Ephedra and Gnetum (a feature found also in conifers and Ginkgo but not in angiosperms) is given new functional interpretations. Ephedra and Gnetum contain both tracheids and fiber-tracheids that co-occur in a mode different from that in angiosperms which contain both cell types together. Ray structure like that of Gnetales occurs widely within seed plants and likely offers little conclusive information about relationships. Wood data on Gnetales are compatible with the hypothesis that a vesselless group of gymnosperms, such as Pentoxylales or Bennettitales, is the closest sister group of angiosperms. Formation of successive cambia in Gnetum differs from that in Welwitschia: the latter has phellem but no other bark. Because Welwitschia has only phellem, there are more numerous bark features in common between Ephedra and Gnetum than one might have expected. The three genera appear monophyletic on the basis of wood and bark. Infrageneric anatomical features of systematic importance are few, although the arboreal Gnetum gnemon differs from the lianoid Gnetum species in significant ways

Secondary cell-wall-specific glycoprotein(s) from French bean hypocotyls.

Specific labeling of secondary cell walls of tracheary elements and of xylary and phloem fibers has been observed when wheat germ agglutinin (WGA) and anti-WGA antibodies were used during ultrastructural studies of French bean (Phaseolus vulgaris L.) hypocotyls. In this report we demonstrate that at least part of this labeling is due to the presence of secondary cell-wall-specific glycoproteins. Three major novel glycoproteins with relative molecular weights of 55,000, 86,000, and 90,000, purified by means of WGA-Sepharose affinity chromatography, have been characterized. Their amino acid composition indicates that they are not the members of known classes of structural cell-wall proteins, since they contain no hydroxyproline, a lower level of glycine than seen in glycine-rich proteins, and very little proline. N-terminal sequences of all three proteins show no significant homology with other proteins. Antibodies were raised against electrophoretically pure 90-kD glycoprotein. These were used to localize this protein in secondary cell walls of xylem tracheary elements and in xylary and phloem fibers, i.e. in the same compartments where labeling with WGA has been observed. To our knowledge this is one of the first biochemical and ultrastructural demonstrations of secondary cell-wall-specific glycoproteins.

Xylem changes in the correlative inhibition of lateral bud growth in flax (Linum usitatissimum L.)

Xylem or tracheary changes at the base of the cotyledonary buds of flax seedlings (Linum usitatissimum L.), released from inhibition by decapitation of the main apex were studied. The differentiation of xylem strands and/or tracheary elements was correlated with the growth in length of the lateral buds, especially 48–72 hr after the removal of the main apex. The xylem strands, connected to the hypocotylary stele or not, and the tracheary elements increased with age within and outside the strands of both non-decapitated and decapitated seedlings. In the latter, the differentiation of these structures, however, occurred much earlier and in greater abundance in the same regions. The early growth in length of lateral buds, 1 or 2 hr after decapitation, was correlated with the early development of tracheary perforations in the xylem strands. The xylary strands with perforated elements are known to be more efficient than those without them. Therefore, it is suggested that the inhibition of lateral-bud growth was due, in fact, to a lack of appropriate tracheary perforations in the bud xylem strands that were connected with the hypocotylary stele of flax seedlings.

Cell-Wall Histochemistry and Anatomy of Taro (Colocasia esculenta.)

The cell-wall histochemistry and anatomy of the corm, petiole and leaf blade of taro [Colocasia esculenta (L.) Schott], an important South Pacific food plant, are described. The results are discussed in relation to the possible protective action of taro cell walls (dietary fibre) against a range of diseases, including colorectal cancer. Cell walls may protect against colorectal cancer by adsorbing carcinogens, and lignified, suberised, and possibly cutinised walls adsorb hydrophobic carcinogens more than walls lacking these substances. The only cells in taro with lignified walls are xylem tracheary elements. The corm is covered with a thick periderm, and the histochemistry of the walls of the phellem indicates the presence of suberin and associated waxes. The leaf blade and petiole are covered by a cuticle, and the presence of cutin and associated waxes is also shown histochemically. Parenchyma is the most abundant cell type with unlignified walls and occurs as aerenchyma in the cortex of the corm and in the petiole and midrib of the leaf blade. Mucilage ducts occur throughout the corm and their contents stain with a reagent (β-glucosyl Yariv reagent) specific for arabinogalactan-proteins (a soluble dietary fibre). Large numbers of starch granules occur in the cortex and central cylinder of the corm, but few occur in the leaf.

Tissue response induced by Heterodera schachtii (Nematoda) in susceptible and resistant white mustard cultivars

The anatomy and ultrastructure of syncytia induced in white mustard (Sinapis alba L.) by the beet cyst nematode (Heterodera schachtii Schmidt) were studied in the nematode-susceptible cultivar Trico and the nematode-resistant cultivar Maxi. Although syncytia in both cultivars could reach similar volumes, they differed considerably in their ultrastructure. In the susceptible cultivar, xylem parenchyma cells responded with extensive hypertrophy, an increase in plastids and tubular smooth endoplasmic reticulum, and formation of elaborate ingrowths of the cell walls abutting the xylem tracheary elements. In the resistant cultivar, xylem parenchyma cells, occasionally incorporated into the syncytium, necrotized. Normally, however, these cells resisted incorporation and stopped the syncytia from reaching the xylem tracheary elements. This resulted in the differentiation of cell wall ingrowths on the inner tangential walls of neighbouring stelar parenchyma cells. The improper coordination of syncytium differentiation with xylem morphogenesis, the malfunction in the symplast–apoplast interaction with an abnormal deposition of protoplasmic constituents into the apoplast, the small number of plastids, the lack of tubular smooth ER and degradation processes, further demonstrated the incompatible nature of the host–parasite interaction in the resistant cultivar Maxi. Key words: Sinapis alba, Heterodera schachtii, ultrastructure, resistance.

Ultrastructural relationships of the developing syncytium induced by Heterodera schachtii (Nematoda) in root tissues of rape

The growth and development of the syncytium induced by the beet cyst nematode (Heterodera schachtii Schmidt) in rape (Brassica napus var. oleifera) and its relationships to adjacent root tissues were studied by serial sectioning and electron microscopy. In a 5-day-old syncytium, xylem parenchyma cells formed the largest part of the syncytium volume, while pericycle cells constituted the smallest fraction. Xylem parenchyma cells outside the confluent part of the syncytium were the first cells to differentiate into syncytium-component cells at the leading edges of syncytia. Early stages of differentiation were characterized by an increase in ribosomes and rough endoplasmic reticulum. Later stages involved formation of tubular structures and cell wall ingrowths. In lateral roots, into which main root syncytia extended, xylem differentiation was disturbed by strong hypertrophy of xylem parenchyma cells. Here, xylem tracheary elements similar to graniferous tracheary elements occurred. The parasitism of H. schachtii in rape induced dramatic changes in root morphogenesis, and the response of xylem parenchyma cells was of fundamental importance in this host–parasite system. Key words: Brassica napus, Heterodera schachtii, syncytium, ultrastructure.

Development of endodermal Casparian bands and xylem in lateral roots of broad bean

A clearing and fluorochrome staining procedure for broad bean (Vicia faba L. cv. Windsor) roots with laterals was devised which allowed observation of their tracheary elements and Casparian bands in situ. The Casparian band of the main root overlying the primordium disappeared early in its development and the margins of the remaining band were displaced outward during subsequent growth of the lateral. The Casparian band matured centripetally in parenchyma across the base of the lateral as far as the endodermis of the lateral and then acropetally into the lateral itself. Casparian bands of the laterals were always connected to those of the main roots. In actively growing roots, the primordium may have emerged as far as 2.7 mm from the main root and still contain no differentiating xylem or Casparian band. Longer laterals contained differentiating xylem 3 mm from the tip and maturing Casparian bands 5 mm from the tip. In nongrowing roots, some primordia not yet emerged from the main root had mature xylem tracheary elements 0.4 mm from the tip and mature Casparian bands 0.25 mm from the tip. Key words: Casparian band, primary xylem, lateral roots, endodermis.

DISTRIBUTION AND RELATIONSHIP OF CELL DIVISION AND MATURATION EVENTS IN PISUM SATIVUM (FABACEAE) SEEDLING ROOTS

Pea roots have open apical organization, where discrete initial cells do not exist. Differentiation of all tissues occurs in cylinders and vascular sectors that blend gradually with each other. This study reports the distribution of dividing cells and their relationship to maturation events in the 2 mm root tip, and in the 8–10 and 18–20 mm segments. Up to 200 μm from the root body/cap junction, cell division is uniformly distributed throughout all meristem regions. By 350 to 500 μ, xylem tracheary elements and cells of the pith parenchyma and middle cortex have stopped dividing. At this level cell division is almost entirely restricted to two cylinders, one composed of the inner root cap, the epidermis, and the outer cortex (outer cortex cylinder) and another composed of cells of the inner cortex, the pericycle and vascular tissue (inner cortex cylinder). When the protophloem matures, all cells in the phloem sector of the inner cortex cylinder, including the 1 layered pericycle, the endodermis and the phloem parenchyma, stop dividing. The 3–4 layered pericycle in the xylem sectors continues dividing until about 10 mm from the body/cap junction following the maturation of the protoxylem tracheary elements.