Structural cell-wall proteins in protoxylem development: evidence for a repair process mediated by a glycine-rich protein

Polyclonal antibodies were used to localize structural cell-wall proteins in differentiating protoxylem elements in etiolated bean and soybean hypocotyls at the light- and electron-microscopic level. A proline-rich protein was localized in the lignified secondary walls, but not in the primary walls of protoxylem elements, which remain unlignified, as shown with lignin-specific antibodies. Secretion of the proline-rich protein was observed during lignification in different cell types. A glycine-rich protein (GRP1.8) was specifically localized in the modified primary walls of mature protoxylem elements and in cell corners between xylem elements and xylem parenchyma cells. The protein was secreted by Golgi bodies both in protoxylem cells after the lignification of their secondary walls and in the surrounding xylem parenchyma cells. The modified primary walls of protoxylem elements were visualized under the light microscope as filaments or sheets staining distinctly with the protein stain Coomassie blue. Electron micrographs of these walls show that they are composed of an amorphous material of moderate electron-density and of polysaccharide microfibrils. These materials form a three-dimensional network, interconnecting the ring- or spiral-shaped secondary wall thickenings of protoxylem elements and xylem parenchyma cells. The results demonstrate that the modified primary walls of protoxylem cells are not simply breakdown products due to partial hydrolysis and passive elongation, as believed until now. Extensive repair processes produce cell walls with unique staining properties. It is concluded that these walls are unusually rich in protein and therefore have special chemical and physical properties.     

Anatomical Characteristics of the Stem of Sigillaria cf. brardii from Coal Balls in Guizhou Province,China

The stem specimens of Sigillaria cf. brardii were collected from the coal balls of Upper Permian in Shuicheng Coal Mines in Guizhou Province. The main anatomical characteristics of Sigillaria cf. brardii are described as follows: The stem is siphonostelic, with pith composed entirely of polygonal parenchyma cells, there are secondary walls in some pith cell cavities these secondary walls show the characters of cell division. Surrounding the pith is the continuous cylindrical primary xylem which consists entirely of tracheids. The outermost, and part are the protoxylem elements show spiral secondary thickenings. In cross section, the outer edge of exarch primary xylem appears regularly sinuous, with trace of mesarch leaf originating from the furrows. The centripetal metaxylem is characterized by scalariform wall thickenings on the tracheids, and delicated strands of secondary wall materials extending between abjacent bars, these structures are called fimbris, or williamson striations, and are characteristic in lepidodendrids. The secondary xylem consists of tracheids and vascular rays. The tracheids, too, have scalariform wall thickenings and fimbris. The rays are one-to twocell width and several to more than ten cells in height.     

扁圆封印木（相似种）茎干的解剖特征

贵州省水城矿区晚二叠世煤核中扁圆封印木（相似种Ｓｉｇｉｌｌａｒｉａ ｃｆ．ｂｒａｒｄｉｉＢｒｏｎｇｎ．）茎干的主要解剖特征如下：管状中柱,具多边形薄壁细胞组成的髓。初生木质部成环带状,外缘呈规则的齿槽状,向心式发育。次生木质部显束状特征,横切面管胞为方圆至长方形,纵切面为梯状壁增厚,并具流苏纹。射线１—２列细胞宽,数个至十余个细胞高。叶迹起源于初生木质部外缘的槽中,中始式,但以向心发育为主。     

Ultrastructural Localization of a Bean Glycine-Rich Protein in Unlignified Primary Walls of Protoxylem Cells

A polyclonal antibody was used to localize a glycine-rich cell wall protein (GRP 1.8) in French bean hypocotyls with the indirect immunogold method. GRP 1.8 could be localized mainly in the unlignified primary cell walls of the oldest protoxylem elements and also in cell corners of both proto- and metaxylem elements. In addition, GRP 1.8 was detected in phloem using tissue printing. The labeled primary walls of dead protoxylem cells showed a characteristically dispersed ultrastructure, resulting from the action of hydrolases during the final steps of cell maturation and from mechanical stress due to hypocotyl growth. Primary walls of living protoxylem and adjacent parenchyma cells were only weakly labeled. This was true also for the secondary walls of proto- and metaxylem cells, which in addition showed high background labeling. Inhibition of lignification with a specific and potent inhibitor of phenylalanine ammonia-lyase did not lead to enhanced labeling of secondary walls, showing that lignin does not mask the presence of GRP 1.8 in these walls. Dictyosomes of living proto- and metaxylem cells were not labeled, but dictyosomes of xylem parenchyma cells without secondary walls, adjacent to strongly labeled protoxylem elements, were clearly labeled. These observations suggest that GRP 1.8 is not produced by xylem vessels but by xylem parenchyma cells that export the protein to the wall of protoxylem vessels.     

Comparative localization of three classes of cell wall proteins.

The localization of the cell wall proline-rich proteins (PRPs), and the gene expression of the cell wall glycine-rich proteins (GRPs) and the hydroxyproline-rich glycoproteins (HRGPs) were examined in several dicot species. The PRPs are accumulated in the corner walls of the cortex where several cells are joined together and in the protoxylem cell walls of 3-day-old soybean root. In 1-month-old soybean plants, the PRPs are specifically deposited in xylem vessel elements of the young stem, and they are accumulated in both phloem fibers and xylem vessel elements and fibers of the older stem. Likewise, the PRPs are localized in xylem vessel elements and fibers in tomato, petunia, potato and tobacco stems. They are also found in outer and inner phloem fiber cell walls of tomato stem and in outer phloem fiber cell walls of petunia stem. The gene expression of the HRGPs and the GRPs is developmentally regulated in tomato, petunia and tobacco stems. HRGP mRNAs are abundant in outer and inner phloem regions, while GRP mRNAs are present mostly in primary xylem and in the cambium region. Immunocytochemical localization showed that the GRPs have a localization pattern similar to that of the PRPs in tomato, petunia and tobacco stems.     

Spatial relation between xylem and phloem in the stem of Hibiscus cannabinus L. (Malvaceae)

The distribution of the phloem in relation to the xylem was examined in the stem of Hibiscus cannabinus L. with reference to a report in the literature that this species has internal (intraxylary) phloem, a feature not previously observed in the Malvaceae. In the present study, the stem was found to have phloem only outside the xylem (external or extraxylary phloem). In the protophloem, the sieve tubes are obliterated while the internode elongates and the associated cells become fibres with thick secondary walls. Fibres occur in the secondary phloem also. As seen in transections of stems, the secondary xylem forms a continuous ring. The primary xylem extends in the form of arcs into the pith. The tracheary cells of the protoxylem become crushed or completely obliterated in elongating internodes. The associated parenchyma cells either retain thin walls or develop secondary thickenings.     

Water transport in xylem conduits with ring thickenings

Helical or annular wall thickenings are not only present in protoxylem, but may also he a feature of the tracheids or vessel elements of secondary wood. The frequency of their occurrence tends to be a function of climatic factors and conduit diameter. In order to obtain a functional explanation for these structures, the hydrodynamic behaviour of xylem conduits with various patterns of annular wall thickenings was investigated numerically using a commercial CFD (Computational Fluid Dynamics) package. The fluid flow phenomena are presented in detail. The calculations show that the developing pressure gradient of the structures with corrugated walls is in each case lower than that of a smooth pipe with a diameter corresponding to the distance between two opposite thickenings. Furthermore, complex flow patterns with circulation zones between the thickenings develop which are dependent on the geometry of the wall. It may be hypothesized that these circulation zones influence the lateral water flow. The results are discussed with regard to the relationships between the water conduction function of the xylem and ecological factors.     

ONTOGENY AND STRUCTURE OF THE SECONDARY PHLOEM IN PYRUS MALUS

Evert , Ray F. (U. Wisconsin, Madison.) Ontogeny and structure of the secondary phloem in Pyrus malus. Amer. Jour. Bot. 50(1): 8–37. Illus. 1963.—The secondary phloem of apple consists of sieve-tube elements, companion cells, phloem parenchyma cells, fiber-sclereids, and ray parenchyma cells. The sieve-tube elements are generally long, slender cells with very oblique end walls and much-compounded sieve plates. All sieve-tube elements initially possess nacreous thickenings. Similar wall thickenings were observed in the differentiating fiber-sclereids and xylem elements. Of the 245 sieve-tube elements critically examined, 242 were associated with companion cells. All of the companion cells were shorter than their associated sieve-tube elements. Young companion cells possess slime bodies which later become dispersed. Callose is often found on the sieve-tube element side of the common wall between sieve-tube element and companion cell. In several collections, callose was found on both sides of that wall. The parenchyma cells are of 3 types: crystal-containing cells; tannin-and/or starch-containing cells; and those with little or no tannins or starch. Any type parenchyma cell may be on to genetically related to a sieve-tube element, that is, may be derived from the same phloem initial as the sieve-tube element. Morphologically, the phloem parenchyma cells intergrade with the companion cells, the tannin- and starch-free parenchyma cells often being difficult to distinguish from companion cells. Most of the tannin- and starch-free parenchyma cells collapse when the contiguous sieve-tube elements become nonfunctional. The fiber-sclereids arise from parenchyma cells which overwinter on the margin of the cambial zone and differentiate in nonfunctional phloem.     

PHLOEM OF SPHENOPHYLLUM

The phloem of most fossil plants, including that of Sphenophyllum, is very poorly known. Sphenophyllum was a relatively small type of fossil arthrophyte with jointed stems bearing whorls of leaves ranging in form from wedge or fan-shaped to bifid, to linear. The aerial stem systems of the plant exhibited determinate growth involving progressive reduction in the dimensions of the stem primary bodies, fewer leaves per whorl, and smaller and simpler leaves distally. The primary phloem occurs in three areas alternating in position with the arms of the triarch centrally placed primary xylem. Cells of the primary phloem, presumably sieve elements, are axially elongate with horizontal to slightly tapered end walls. In larger stems with abundant secondary xylem and secondary cortex or periderm, a zone of secondary phloem occurs whose structure varies in the three areas opposite the arms of the primary xylem, as opposed to the three areas lying opposite the concave sides of the primary xylem. The axial system of the secondary phloem consists of vertical series of sieve elements with horizontal end walls. In the areas opposite the protoxylem the parenchyma is present as a prominent ray system showing dilation peripherally. Sieve elements in the areas opposite the protoxylem arms have relatively small diameters. In the areas between the protoxylem poles the secondary phloem sieve elements have large diameters and are less obviously in radial files, while the parenchyma resembles that of the secondary xylem in these areas in that it consists of strands of cells extending both radially and tangentially. An actively meristematic vascular cambium has not been found, indicating that this layer changed histologically after the cessation of growth in the determinate aerial stem systems and was replaced by a post-meristematic parenchyma sheath made up of axially elongate parenchyma lacking cells indicative of being either fusiform or ray initials. A phellogen arose early in development in a tissue believed to represent pericycle and produced tissue comparable to phellem externally. Normally, derivatives of the phellogen underwent one division prior to the maturation of the cells. Concentric bands of cells with dark contents apparently represent secretory tissue in the periderm and cell arrangements indicate that a single persistent phellogen was present. Sphenophyllum is compared with other arthrophytes as to phloem structure and is at present the best documented example of a plant with a functionally bifacial vascular cambium in any exclusively non-seed group of vascular plants.     

A fasciclin-domain containing gene, ZeFLA11, is expressed exclusively in xylem elements that have reticulate wall thickenings in the stem vascular system of Zinnia elegans cv Envy

The vascular cylinder of the mature stem of Zinnia elegans cv Envy contains two anatomically distinct sets of vascular bundles, stem bundles and leaf-trace bundles. We isolated a full-length cDNA of ZeFLA11, a fasciclin-domain-containing gene, from a zinnia cDNA library derived from in vitro cultures of mesophyll cells induced to form tracheary elements. Using RNA in situ hybridization, we show that ZeFLA11 is expressed in the differentiating xylem vessels with reticulate type wall thickenings and adjacent parenchyma cells of zinnia stem bundles, but not in the leaf-trace bundles that deposit spiral thickenings. Our results suggest a function for this cell-surface GPI-anchored glycoprotein in secondary wall deposition during differentiation of metaxylem tissue with reticulate vessels.     

Tissue-Specific Expression of Cell Wall Proteins in Developing Soybean Tissues

Cell wall hydroxyproline-rich glycoproteins (HRGPs) and glycine-rich proteins (GRPs) were examined at the protein and at the mRNA levels in developing soybean tissues by tissue print immunoblots and RNA blots. In young soybean stems, HRGPs are expressed most heavily in cambium cells, in a few layers of cortex cells surrounding primary phloem, and in some parenchyma cells around the primary xylem, whereas GRPs are highly expressed in the primary xylem and also in the primary phloem. In older soybean stems, HRGP genes are expressed exclusively in cambium cells and GRP genes are most heavily expressed in newly differentiated secondary xylem cells. Similar expression patterns of HRGPs and of GRPs were found in soybean petioles, seedcoats, and young hypocotyls, and also in bean petioles and stems. HRGPs and GRPs become insolubilized in soybean stem cell walls. Three major HRGP mRNAs and two major GRP mRNAs accumulate in soybean stems. Soluble HRGPs are abundant in young hypocotyl apical regions and young root apical regions, whereas in hypocotyl and root mature regions, soluble HRGPs are found only in a few layers of cortex cells surrounding the vascular bundles. GRPs are specifically localized in primary xylem cell walls of young root. These results show that the gene expression of HRGPs and GRPs is developmentally regulated in a tissue-specific manner. In soybean tissues, HRGPs are most heavily expressed in meristematic cells and in some of those cells that may be under stress, whereas GRPs are expressed in all cells that are or are going to be lignified.     

Hypergravity stimulus enhances primary xylem development and decreases mechanical properties of secondary cell walls in inflorescence stems of Arabidopsis thaliana

BACKGROUND AND AIMS: The xylem plays an important role in strengthening plant bodies. Past studies on xylem formation in tension woods in poplar and also in clinorotated Prunus tree stems lead to the suggestion that changes in the gravitational conditions affect morphology and mechanical properties of xylem vessels. The aim of this study was to examine effects of hypergravity stimulus on morphology and development of primary xylem vessels and on mechanical properties of isolated secondary wall preparations in inflorescence stems of arabidopsis. METHODS: Morphology of primary xylem was examined under a light microscope on cross-sections of inflorescence stems of arabidopsis plants, which had been grown for 3-5 d after exposure to hypergravity at 300 g for 24 h. Extensibility of secondary cell wall preparation, isolated from inflorescence stems by enzyme digestion of primary cell wall components (mainly composed of metaxylem elements), was examined. Plants were treated with gadolinium chloride, a blocker of mechanoreceptors, to test the involvement of mechanoreceptors in the responses to hypergravity. KEY RESULTS: Number of metaxylem elements per xylem, apparent thickness of the secondary thickenings, and cross-section area of metaxylem elements in inflorescence stems increased in response to hypergravity. Gadolinium chloride suppressed the effect of hypergravity on the increase both in the thickness of secondary thickenings and in the cross-section area of metaxylem elements, while it did not suppress the effect of hypergravity on the increase in the number of metaxylem elements. Extensibility of secondary cell wall preparation decreased in response to hypergravity. Gadolinium chloride suppressed the effect of hypergravity on cell wall extensibility. CONCLUSIONS: Hypergravity stimulus promotes metaxylem development and decreases extensibility of secondary cell walls, and mechanoreceptors were suggested to be involved in these processes.     

Unusual metaxylem tracheids in petioles of Amorphophallus (Araceae) giant leaves

BACKGROUND AND AIMS: Petioles of huge solitary leaves of mature plants of Amorphophallus resemble tree trunks supporting an umbrella-like crown. Since they may be 4 m tall, adaptations to water transport in the petioles are as important as adaptations to mechanical support of lamina. The petiole is a cylindrical shell composed of compact unlignified tissue with a honeycomb aerenchymatous core. In both parts numerous vascular bundles occur, which are unique because of the scarcity of lignified elements. In the xylemic part of each bundle there is a characteristic canal with unlignified walls. The xylem pecularities are described and interpreted. MATERIAL: Vascular bundles in mature petioles of Amorphophallus titanum and A. gigas plants were studied using light and scanning electron microscopy. KEY RESULTS: The xylemic canal represents a file of huge metaxylem tracheids (diameter 55-200 microm, length >30 mm) with unlignified lateral walls surrounded by turgid parenchyma cells. Only their end walls, orientated steeply, have lignified secondary thickenings. The file is accompanied by a strand of narrow tracheids with lignified bar-type secondary walls, which come into direct contact with the wide tracheid in many places along its length. CONCLUSIONS: The metaxylem tracheids in A. petioles are probably the longest and widest tracheids known. Only their end walls have lignified secondary thickenings. Tracheids are long due to enormous intercalary elongation and wide due to a transverse growth mechanism similar to that underlying formation of aerenchyma cavities. The lack of lignin in lateral walls shifts the function of 'pipe walls' to the turgid parenchyma paving the tracheid. The analogy to carinal canals of Equisetum, as well as other protoxylem lacunas is discussed. The stiff partitions between the long and wide tracheids are interpreted as structures similar to the end walls in vessels.     

Flange-type parenchyma cells: occurrence and structure in the haustorium of the dwarf mistletoeKorthalsella (Viscaceae)

Summary Flange cells are an unusual type of parenchyma cells with an open reticulate pattern of secondary wall thickenings. The cells superficially resemble tracheary elements but are otherwise fundamentally different. Flange cells were found in haustorial sucker tissue of the dwarf mistletoeKorthalsella. Such cells were previously unknown for a mistletoe, or other parasitic angiosperm. Flange cells are confined to the xylem of the sucker and occur as either diffuse aggregates amongst the ordinary parenchyma tissue lying between the tracts of vessels, or abut the vessels. Typical flange cells are absent at the parasite/host xylem interface. The cells contain a well differentiated protoplast, including chloroplasts with extensive granal stacks. Histochemical staining and fluorescence microscopy indicate lignification of the flange wall. In thin section, the flange wall is often stratified into dark and light staining layers. Flange cells inKorthalsella resemble contact cells, vessel associated cells and certain types of transfer cells reported in the literature. Based on morphological considerations, it is suggested that flange cells inKorthalsella are involved in absorption and transport between host and parasite. As host sap moves through the sucker apoplasm, substance might be selectively absorbed by the flange cell, before the remaining the sap passes into the vessels for long distance transport in the mistletoe.Dedicated to Prof. Dr. Rainer Kollmann on the occasion of his 65th birthday     

Structural and ultrastructural characteristics of the vascular apparatus of the sensitive plant (Mimosa pudica L.)

Summary 1. In motor organs ofMimosa pudica xylem contains living fibriform elements limited by a thick lignified highly pitted wall, whereas in other parts of the plant (stem, petiole, rachis), xylem and protoxylem vessels are closely associated with parenchyma cells which possess wall ingrowths. These ingrowths, at the apex of which the plasmalemma and the tonoplast touch, are localized like those of transfer cells of C type described byGunning andPate. Nevertheless, xylem parenchyma cells differ from cells of C type in several characteristics. Moreover, in motor organs, phloem contains cells characterized by wall ingrowths, less abundant on the parts adjacent to the sieve tubes; these cells which are localized near collenchyma cells of primary phloem, look like transfer cells of A type defined byGunning andPate; they are absent from internodes, petioles and rachides. 2. In motor organs, three types of vascular cells (companion cells, living xylem fibriform elements and protoxylem parenchyma cells) are characterized by reduced vacuolar volumes and well developed membrane systems, as compared with homologuous cells belonging to other parts of the plant. 3. A symplastic continuity holds from the middle of motor organs to their cortex: it is provided by the presence, in xylem and phloem respectively, of living fibriform elements and collenchyma cells bearing numerous pit fields containing large numbers of plasmodesmata. Several ultrastructural features suggest that the vascular apparatus ofMimosa pudica would be the site of intensive lateral transfer at different levels, specially in motor organs. Possible functions of certain structures observed are discussed in relation to some hypotheses relative to excitatory conduction pathways.     

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.     

Absorptive cells in protoxylem: Association between mitochondria and the plasmalemma

Summary In parenchyma cells grouped around the stem protoxylem in Pinus pinea characteristic wall ingrowths occur. The numerous mitochondria of such cells are very closely associated with the plasmalemma bounding the ingrowths. A functional relationship between this association and salt absorption from the xylem transpiration stream is suggested.