Sieve elements rapidly develop ‘nacreous walls’ following injury − a common wounding response?

Thick glistening cell walls occur in sieve tubes of all major land plant taxa. Historically, these ‘nacreous walls’ have been considered a diagnostic feature of sieve elements; they represent a conundrum, though, in the context of the widely accepted pressure–flow theory as they severely constrict sieve tubes. We employed the cucurbit Gerrardanthus macrorhizus as a model to study nacreous walls in sieve elements by standard and in situ confocal microscopy and electron microscopy, focusing on changes in functional sieve tubes that occur when prepared for microscopic observation. Over 90% of sieve elements in tissue sections processed for microscopy by standard methods exhibit nacreous walls. Sieve elements in whole, live plants that were actively transporting as shown by phloem‐mobile tracers, lacked nacreous walls and exhibited open lumina of circular cross‐sections instead, an appropriate structure for Münch‐type mass flow of the cell contents. Puncturing of transporting sieve elements with micropipettes triggered the rapid (<1 min) development of nacreous walls that occluded the cell lumen almost completely. We conclude that nacreous walls are preparation artefacts rather than structural features of transporting sieve elements. Nacreous walls in land plants resemble the reversibly swellable walls found in various algae, suggesting that they may function in turgor buffering, the amelioration of osmotic stress, wounding‐induced sieve tube occlusion, and possibly local defence responses of the phloem.     

Sieve Elements of Minor Veins in the Leaves of Beta vulgaris L.

End walls of sieve elements of minor veins of the leaves ofBeta vulgaris L. do not contain the multi-perforate sieve plateswhich typically occur on the end walls of sieve-tube membersof major veins. Instead, both end and side walls of the sieveelements of minor veins contain scattered pores which may occursingly or in small numbers. These pores are similar to thosewhich are grouped in sieve plates of major veins in size, possessionof callose and plugs of filaments. In addition to these pores,there are tubular connections 0.1 µ in diameter throughcharacteristically thickened parts of the cell wall betweensieve cells and companion cells. Sieve elements of minor veinsdiffer from those of major veins in structure as well as infunction.     

Structure and identification of root bark of Quercus robur L.

The root bark structure of Quercus robur L. was analysed at different stages of root development and compared to the structure of stem bark. Root bark thickness varied considerably between different roots. Sclereid quantity decreased with increasing distance from the stem, which means it increased with age. Visible growth increments diminished with increasing distance from the stem. In lateral roots crystal quantity decreased with increasing distance from the stem. In lateral roots secondary phloem fibre length, sieve tube member length, and sieve tube diameter showed no regular trend. There were only a few basic structural differences between root and stem bark. The zone of cell differentiation (cell expansion, lignification) was wider in root bark; sieve tube collapse was delayed. In lateral root bark fewer sclereids were formed. The first-formed periderm often originated from deeper cell layers. Thus, primary elements were lacking after periderm formation. In root bark the phellem cell walls were of equal thickness. Thus, phellem lacked visible growth increments. Root bark phellem cells were slightly larger. The root phelloderm was more distinct. The secondary phloem fibres were slightly shorter than those in stem bark. Sieve tube members of stem and root bark were of similar length and diameter. The qualitative bark anatomical characters of oak root bark are suitable for root identifications. Due to minor structural differences between root and stem bark the characters must be used with care.     

SIEVE-ELEMENT ULTRASTRUCTURE IN PLATYCERIUM BIFURCATUM AND SOME OTHER POLYPODIACEOUS FERNS: THE NACREOUS WALL THICKENING AND MATURATION OF THE PROTOPLAST

Sieve elements of various ages were examined in petioles and midribs of Platycerium bifurcatum (Cav.) C. Chr. and Phlebodium aureum (L.) J. Sm., only older ones in similar parts of leaves of Polypodium schraderi Mett. and Microgramma lycopodioides (L.) Copel. Nacreous walls apparently are formed by most, if not all, protophloem and metaphloem sieve elements in all four species. In Platycerium and Phlebodium nacreous wall formation is closely correlated with the appearance of numerous membranes or vesicles in the region of the wall. These extracytoplasmic membranes apparently are derived from protrusions of the plasmalemma. After the nacreous layer is fully thickened, many endoplasmic reticulum (ER) membranes apparently end up outside the plasmalemma of Platycerium, where they degenerate and gradually intergrade in appearance with the fibrillar material comprising the nacreous thickening. In Phlebodium, Polypodium, and Microgramma the ER forms multivesicular bodies. As the cells approach maturity, the membranes delimiting the multivesicular bodies fuse with the plasmalemma and their vesicular contents, which are not discharged into the region of the wall, disappear. Gradually, the nacreous layer decreases in thickness and disappears. At maturity the enucleate sieve-element protoplasts of all four species are essentially similar. They are lined by a plasmalemma and a parietal, anastomosing network of ER and contain both plastids and mitochondria. The plastids in Polypodium and Microgramma are chloroplasts, but those in Platycerium and Phlebodium lack grana and intergrana lamellae.     

OBSERVATIONS ON METAPHLOEM IN THE VEGETATIVE PARTS OF PALMS

Metaphloem was studied in available vegetative parts of 374 species in 164 genera of palms. Sieve elements usually have compound sieve plates except in the subfamilies Lepidocaryoideae and Nypoideae. Sieve elements in roots usually have oblique to very oblique end walls, whereas in stems and leaves they have transverse to oblique walls. Within a phloem strand the degree of compounding of a sieve plate is directly correlated with element diameter. Plastids are normally present in functioning, enucleate sieve elements. Small quantities of “slime” substances have been detected in young sieve elements in stems and petioles of a few species. Many sieve plates in functioning sieve elements lacked callose in materials quick-killed in liquid nitrogen or chilled acetic-alcohol. Definitive callose is confined to sieve elements just before their obliteration. Sieve tubes in leaf and stem are usually ensheathed by contiguous parenchyma cells while those in root have very few contiguous parenchyma cells. Two types of contiguous parenchyma cells can be distinguished by difference in cytoplasmic density, especially with the electron microscope. Cells with denser cytoplasm are interpreted as companion cells. Lignified contiguous parenchyma cells are occasionally present in metaphloem of petioles. The possible diagnostic and taxonomic features of metaphloem are discussed.     

Transport of assimilates in the developing caryopsis of rice (Oryza sativa L.)

Assimilates entering the developing rice caryopsis traverse a short-distance pathway between the terminal sieve elements of the pericarp vascular bundle and the aleurone layer. The ultrastructure of this pathway has been studied. Sieve elements in the pericarp vascular bundle are smaller than their companion cells.The sieve elements show few connections with surrounding vascular parenchyma elements but are connected to companion cells by compound plasmodesmata. Companion cells, in turn, are connected to vascular parenchyma elements by numerous compound plasmodesmata present in wall thickenings. Assimilates leaving the sieve element — companion cell complex must laterally traverse cells of the pigment strand before they come into contact with the aleurone layer. The pigment strand cells have modified inner walls made up of a suberin-like material. This material may act as a permeability barrier isolating the apoplast from the symplast of the pigment strand. The walls of the pigment strand cells are traversed by numerous plasmodesmata. Water may be conducted to the endosperm through the isolated cell-wall system of the pigment strand while assimilates possibly move via plasmodesmata. High frequencies of plasmodesmata occur at the junction between the pigment strand and the nucellus and also between adjacent cells of the nucellus. By contrast, plasmodesmata are absent between the nucellus and the aleurone layer and also between the nucellus and the seed coat. A predominantly circumferential and symplastic transport pathway is likely between the pigment strand and nucellus. In view of the total absence of plasmodesmata between the nucellus and the aleurone layer assimilates entering the endosperm may have to cross the plasmalemma of the nucellus. It is possible that constraints to the flow of assimilates may occur in the short-distance pathway between the terminal sieve element — companion cell complexes and the endosperm, and this is discussed.     

Leaf structure in relation to solute transport and phloem loading in Zea mays L.

Small and intermediate (longitudinal) vascular bundles of the Zea mays leaf are surrounded by chlorenchymatous bundle sheaths and consist of one or two vessels, variable numbers of vascular parenchyma cells, and two or more sieve tubes some of which are associated with companion cells. Sieve tubes not associated with companion cells have relatively thick walls and commonly are in direct contact with the vessels. The thick-walled sieve tubes have abundant cytoplasmic connections with contiguous vascular parenchyma cells; in contrast, connections between vascular parenchyma cells and thin-walled sieve tubes are rare. Connections are abundant, however, between the thin-walled sieve tubes and their companion cells; the latter have few connections with the vascular parenchyma cells. Plasmolytic studies on leaves of plants taken directly from lighted growth chambers gave osmotic potential values of about-18 bars for the companion cells and thin-walled sieve tubes (the companion cell-sieve tube complexes) and about-11 bars for the vascular parenchyma cells. Judging from the distribution of connections between various cell types of the vascular bundles and from the osmotic potential values of those cell types, it appears that sugar is actively accumulated from the apoplast by the companion cell-sieve tube complex, probably across the plasmalemma of the companion cell. The thick-walled sieve tubes, with their close spatial association with the vessels and possession of plasmalemma tubules, may play a role in retrieval of solutes entering the leaf apoplast in the transpiration stream. The transverse veins have chlorenchymatous bundle sheaths and commonly contain a single vessel and sieve tube. Parenchymatic elements may or may not be present. Like the thick-walled sieve tubes of the longitudinal bundles, the sieve tubes of the transverse veins have plasmalemma tubules, indicating that they too may play a role in retrieval of solutes entering the leaf apoplast in the transpiration stream.     

THE THICKENED LEPTOID (SIEVE ELEMENT) WALL OF DENDROLIGOTRICHUM (BRYOPHYTA): CYTOCHEMISTRY AND FINE STRUCTURE

Leptoids (sieve elements) of Dendroligotrichum exhibit unevenly thickened lateral walls. The thickened wall areas are predominantly confined to the radial walls. With the light microscope the thickened wall cannot be resolved into distinct layers, but rather is optically homogeneous. Standard histochemical tests reveal that these walls are rich in cellulose (birefringent; IKI-H₂SO₄-positive) with small amounts of polyuronides (toluidine blue-positive) and pectins (hydroxylamine-positive) and are non-lignified. They also contain abundant natural aldehydes as revealed by the Schiff, silver hexamine, and silver proteinate reagents. Aldehyde blockades (sodium borohydrate, sodium chlorite) confirmed the presence of aldehyde groups in the cell wall. At the ultrastructural level, the lateral walls of sieve elements react strongly with uranyl and lead salts and yield little fine structural information. Electron cytochemical localization of aldehydes with silver proteinate revealed three distinct wall regions: outer, middle and inner. The outer and middle regions appeared polylamellate while the inner region contained no reaction product. The nacreous sieve elements of vascular plants are compared to the thickened sieve elements in bryophytes.     

ULTRASTRUCTURE OF THE NACREOUS LEPTOIDS (SIEVE ELEMENTS) IN THE POLYTRICHACEOUS MOSS ATRICHUM UNDULATUM

The physiological phloem equivalents, leptoids, of the polytrichaceous moss Atrichum undulatum appear to be similar to the nacreous sieve elements that occur in many higher plants. These leptoids are elongated cells with nacreous thickenings on their radial and tangential walls. Their oblique end walls, which lack such thickenings, are traversed by numerous pores through which the plasmalemma, endoplasmic reticulum, and cytoplasm are continuous between adjacent leptoids of a longitudinal file. These end walls closely resemble the simple sieve areas of the sieve elements found in Polypodium vulgare. The leptoid sieve pores have a median expanded area and frequently are occluded by small amorphous protein plugs at each end. Also, callose was observed as electron-luscent areas both on the faces of the end walls and as a thin cylinder surrounding the lateral area of each pore. Amorphous and granular cytoplasmic contents of the leptoids appear to be morphologically similar to the slime (P-protein) found in the sieve-tube elements of many angiosperms. Differentiating leptoids are characterized by the formation of numerous membrane-bound protein bodies in close association with polysomes and endoplasmic reticulum. As the leptoid matures, the contents of the protein bodies become dispersed in the cytoplasm. Ultrastructurally and ontogenetically the leptoids in the gametophores of A. undulatum appear almost identical to the sieve elements of P. vulgare and therefore should be considered sieve elements rather than phloem-like equivalents.     

Le phloème deSelaginella willdenowii: histophysiologie comparée

Metaphloem sieve elements ofSelaginella willdenowii are elongated cells with slightly oblique or transverse end walls. Pores are seen on both lateral and end walls, although they are more numerous on the latter. Parenchyma cells exhibiting strong enzyme activities (acid phosphatase, non specific esterase, succinate dehydrogenase, cytochrome oxidase, peroxidase) are present between sieve elements and tracheids in each vascular bundle. A functional association thus appears to exist between these parenchyma cells and the conducting elements.—The occurrence of transverse to slightly oblique end walls in sieve elements seems to characterize the ligulate Lycopsids (as opposed to the aligulateLycopodium where sieve elements possess slanting, very oblique, end walls).

     

THE NACREOUS WALLS OF SIEVE ELEMENTS IN SEAGRASSES

The nacreous walls of sieve elements occur in seagrasses in all three genera of the family Zosteraceae and the genus Halodule of the family Cymodoceaceae but are absent from another eight seagrass genera belonging to the families Hydrocharitaceae, Cymodoceaceae, and Posidoniaceae. They occur in leaf blades, leaf sheaths, rhizomes, and erect stems but are not present in root tissues. The nacreous wall is uneven along the inner limits reflecting irregular thickness. The wall consists of hemicellulose or pectin and cellulose, but no protein, lignin, or lipid. Ultrastructurally, the wall contains parallel microfibrils or loose fibrils embedded in an amorphous matrix. Open pores occur in sieve plates and branching plasmodesmata are present in enlarged sieve areas. Mitochondria, endoplasmic reticulum, and plastids are also present in these sieve elements.     

Xyloglucan endotransglycosylases have a function during the formation of secondary cell walls of vascular tissues

Xyloglucan transglycosylases (XETs) have been implicated in many aspects of cell wall biosynthesis, but their function in vascular tissues, in general, and in the formation of secondary walls, in particular, is less well understood. Using an in situ XET activity assay in poplar stems, we have demonstrated XET activity in xylem and phloem fibers at the stage of secondary wall formation. Immunolocalization of fucosylated xylogucan with CCRC-M1 antibodies showed that levels of this species increased at the border between the primary and secondary wall layers at the time of secondary wall deposition. Furthermore, one of the most abundant XET isoforms in secondary vascular tissues (PttXET16A) was cloned and immunolocalized to fibers at the stage of secondary wall formation. Together, these data strongly suggest that XET has a previously unreported role in restructuring primary walls at the time when secondary wall layers are deposited, probably creating and reinforcing the connections between the primary and secondary wall layers. We also observed that xylogucan is incorporated at a high level in the inner layer of nacreous walls of mature sieve tube elements.     

Ultrastructure of minor veins inCucurbita pepo leaves

Summary The minor veins ofCucurbita pepo leaves were examined as part of a continuing study of leaf development and phloem transport in this species. The minor veins are bicollateral along their entire length. Mature sieve elements are enucleate and lack ribosomes. There is no tonoplast. The sieve elements, which are joined to each other by sieve plates, contain mitochondria, plastids and endoplasmic reticulum as well as fibrillar and tubular (190–195 diameter) P-protein. Fibrillar P-protein is dispersed in mature abaxial sieve elements but remains aggregated as discrete bodies in mature adaxial sieve elements. In both abaxial and adaxial mature sieve elements tubular P-protein remains undispersed. Sieve pores in abaxial sieve elements are narrow, lined with callose and are filled with P-protein. In adaxial sieve elements they are wide, contain little callose and are unobstructed. The intermediary cells (companion cells) of the abaxial phloem are large and dwarf the diminutive sieve elements. Intermediary cells are densely filled with ribosomes and contain numerous small vacuoles and many mitochondria which lie close to the plasmalemma. An unusually large number of plasmodesmata traverse the common wall between intermediary cells and bundle sheath cells suggesting that the pathway for the transport of photosynthate from the mesophyll to the sieve elements is at least partially symplastic. Adaxial companion cells are of approximately the same diameter as the adaxial sieve elements. They are densely packed with ribosomes and have a large central vacuole. They are not conspicuously connected by plasmodesmata to the bundle sheath.     

The cell wall-plasmalemma interface in sieve tubes of barley

Both thick- and thin-walled sieve tubes in leaf-blade veins of Hordeum vulgare L. exhibit a distinct, electron-opaque inner wall layer after fixation in glutaraldehyde-osmium tetroxide and staining with uranyl acetate and lead citrate. This inner wall layer is thickest at the sieve plates and lateral sieve areas where it is permeated by a labyrinth of tubules formed by the plasmalemma. Along the lateral walls between sieve areas the inner wall layer apparently is penetrated by numerous microvilli-like evaginations of the plasmalemma, giving the cell wall-plasmalemma interface the appearance of a brush border. It is suggested that a similar brush-border-like structure may occur at the cell wall-plasmalemma interface of sieve elements in a wide variety of vascular plants.Abbreviation ER endoplasmic reticulum     

Sieve-element characters of Myristicaceae: Nuclear crystals, S-and P-type plastids, nacreous walls

The phloem of the Myristicaceae is composed of sieve elements, parenchymatous cells, and fibers. Within the metaphloem and secondary phloem parenchymatic layers including prominent secretory elements alternate with tangential bands of fibers and layers composed of sieve elements, companion cells and phloem-parenchyma cells. among the latter the sieve elements are most abundant and easily identified by the presence of thick (nacreous) walls. The most characteristic feature of the sieve elements of Myristicaceae (and found nowhere else among the Magnoliiflorae) are nuclear crystals, which are released into the lumen during nuclear degeneration and persist in the mature cell. P-and S-type sieve-element plastids were recorded for the 18 species investigated. Both types of the plastid are characterized by large diameters and many medium-sized starch grains. The sizes and contents (small protein crystals only) of the P-type plastids of the Myristicaceae do not conform to the tiny P-type plastids (with large protein crystals) of the Annonaceae, a family to which the Myristicaceae is traditionally allied.     

莲胚子叶中维管束的发育和传递细胞

莲子叶组织中的维管束均等的分布于两瓣子叶里,形成１周。筛分子由２个维管束母细胞分裂而来。维管束无形成层,无导管,也不具有螺纹管胞,呈有筛分子。子叶维管不好育成熟约在受一的２０天,成熟时细胞核消失,次生壁向内延伸,筛管分子的细胞壁形成网纹结构。子叶组织中传递细胞在物质分布广泛,传递细胞首选发生于维管束的末和边缘,其细胞壁内突,显极性生长。表面积与体积的比值有利于昨管的装载,同时在物质运输的受体和供体     

Ethylene-induced microtubule reorientations: mediation by helical arrays

Pruned source-sink transport systems from predarkened plants of Amaranthus caudatus L. and Gomphrena globosa L. were used to study the localization of ¹⁴C-labeled photosynthate imported into experimentally induced sink leaves by microautoradiography. During a 6-h (Amaranthus) or a 4-h (Gomphrena) transport period, ¹⁴C-assimilates were translocated acropetally from a mature source leaf provided with ¹⁴CO₂, into a younger induced sink leaf (dark/-CO₂). In addition, a young still-expanding source leaf exposed to ¹⁴CO₂ exported ¹⁴C-assimilates basipetally into a mature induced sink leaf (dark/-CO₂). Microautoradiographs showed that imported ¹⁴C-photosynthate was strongly accumulated in the sieve element/companion cell complexes of midveins, secondary veins, and minor veins of both the mature and the expanding sink leaf. Some label was also present in the vascular parenchyma and bundlesheath cells. In petioles, ¹⁴C-label was concentrated in the sieve element/companion cell complexes of all bundles indicating that assimilates were imported and distributed via the phloem. Moreover, a considerable amount of radioactivity unloaded from the sieve element/companion cell complexes of petiolar bundles, was densely located at sites of secondary wall thickenings of differen-tiating metaxylem vessels, and at sites of chloroplasts of the vascular parenchyma and bundle-sheath cells. These observations were more striking in petioles of Gomphrena than Amaranthus.Abbreviation se/cc sieve element/companion cell     

Proliferating sieve elements present in bud phloem anastomoses connect sieve tubes of axillary bud traces to stelar vascular bundles in the aquatic monocotyledonPotamogeton natans L. (Potamogetonaceae)

Summary The stem ofPotamogeton natans is characterized by a central stelar vascular system with reduced xylem and abundant phloem. Wide sieve tubes composed of short sieve-tube members joined by simple sieve plates and associated with companion cells establish an effective conduit for assimilates. At each node the phloem forms a network of parallel sieve elements connecting the stem phloem to leaf and bud traces. InP. natans an axillary bud rarely develops into a side branch, its procambial vascular bundles are each connected to the nodal complex via separate anastomoses. Their most unusual components are the anastomosai sieve elements (ANSE), characterized by thin cell walls pitted all over by tiny callose-lined pores resembling plasmodesmata, which can be detected as bright areas by fluorescence microscopy after staining with aniline blue. Several layers of ANSE make up the centre of an anastomosis and link to both the nodal and bud stelar sieve tubes via mediating (MSE) and connecting sieve elements (CSE). The ultrastructural differentiation of ANSE, MSE, and CSE corresponds to that of normal sieve elements, i.e., in the mature stage they are enucleate, evacuolate, and have lost most of their cytoplasm. Their plastids are of form-P2c, containing many cuneate protein crystals, typical of monocotyledonous sieve elements. Quantitative aspects of the pore areas are discussed in relation to the functional significance of bud anastomoses.Abbreviations ANSE anastomosai sieve elements - CSE connecting sieve elements - FM fluorescence microscopy - LM light microscopy - MSE mediating sieve elements - TEM transmission electron microscopy Dedicated to Professor Dr. Rainer Kollmann on the occasion of his retirement     

Cytochemical localization of ATPase activity in phloem tissues ofRicinus communis

Summary Standard lead precipitation procedures have been used to examine the localization of ATPase activity in phloem tissues ofRicinus communis. Reaction product was localized on the plasma membrane of the companion cells associated with sieve elements and of parenchyma cells in phloem tissues from the leaf, petiole, stem and root. ATPase activity was also present on the plasma membrane and dispersed P-protein of sieve elements in petiole, stem and root tissue, but was absent from the plasma membrane of these cells in the leaf minor veins. Substitution of-glycerophosphate for ATP produced no change in the localization of reaction product in leaf tissue. These findings are discussed in relation to current theories on the mechanism of sugar transport and phloem loading.     

The development of the endoder mis andPhi layer of apple roots

Summary Suberin lamellae and a tertiary cellulose wall in endodermal cells are deposited much closer to the tip of apple roots than of annual roots. Casparian strips and lignified thickenings differentiate in the anticlinal walls of all endodermal andphi layer cells respectively, 4–5 mm from the root tip. 16 mm from the root tip and only in the endodermis opposite the phloem poles, suberin lamellae are laid down on the inner surface of the cell walls, followed 35 mm from the root tip by an additional cellulosic layer. Coincidentally with this last development, the suberin and cellulose layers detach from the outer tangential walls and the cytoplasm fragments. 85 mm from the root tip the xylem pole endodermis (50% of the endodermis) develops similarly, but does not collapse. 100–150 mm from the root tip, the surface colour of the root changes from white to brown, a phellogen develops from the pericycle and sloughing of the cortex begins. A few secondary xylem elements are visible at this stage.Plasmodesmata traverse the suberin and cellulose layers of the endodermis, but their greater frequency in the outer tangential and radial walls of thephi layer when compared with the endodermis suggests that this layer may regulate the inflow of water and nutrients to the stele.