Cortical Strands Research Articles

Advances on the Visualization of the Internal Structures of the European Mistletoe: 3D Reconstruction Using Microtomography.

The European mistletoe (Viscum album) is a dioecious epiphytic evergreen hemiparasite that develops an extensive endophyte enabling the absorption of water and mineral salts from the host tree, whereas the exophytic leaves are photosynthetically active. The attachment mode and host penetration are well studied, but little information is available about the effects of mistletoe age and sex on haustorium-host interactions. We harvested 130 plants of Viscum album ssp. album growing on host branches of Aesculus flava for morphological and anatomical investigations. Morphometric analyses of the mistletoe and the (hypertrophied) host interaction site were correlated with mistletoe age and sex. We recorded the morphology of the endophytic systems of various ages by using X-ray microtomography scans and corresponding stereomicroscopic images. For detailed anatomical studies, we examined thin stained sections of the mistletoe-host interface by light microscopy. The diameter and length of the branch hypertrophy showed a positive linear correlation with the age of the mistletoe. Correlations with their sex were only found for ratios between host branch and hypertrophy size. A female bias of about 76% was found. In a 4-year-old mistletoe, several small, almost equally sized sinkers and the connected cortical strands extend over more than 5 cm within the host branch. In older mistletoes, one main sinker was predominant and occupied an increasingly large proportion of the stem cross-section. Bands of vessels ran along the axis of the wedge-shaped haustoria and sinkers and bent sideways toward the mistletoe-host interface. At the interface, the vascular elements of the host wood changed their direction and formed vortices near the haustorium.

The Aspergillus nidulans Peripheral ER: Disorganization by ER Stress and Persistence during Mitosis

The genetically amenable fungus Aspergillus nidulans is well suited for cell biology studies involving the secretory pathway and its relationship with hyphal tip growth by apical extension. We exploited live-cell epifluorescence microscopy of the ER labeled with the translocon component Sec63, endogenously tagged with GFP, to study the organization of ‘secretory’ ER domains. The Sec63 A. nidulans ER network includes brightly fluorescent peripheral strands and more faintly labeled nuclear envelopes. In hyphae, the most abundant peripheral ER structures correspond to plasma membrane-associated strands that are polarized, but do not invade the hyphal tip dome, at least in part because the subapical collar of endocytic actin patches constrict the cortical strands in this region. Thus the subapical endocytic ring might provide an attachment for ER strands, thereby ensuring that the growing tip remains ‘loaded’ with secretory ER. Acute disruption of secretory ER function by reductive stress-mediated induction of the unfolded protein response results in the reversible aggregation of ER strands, cessation of exocytosis and swelling of the hyphal tips. The secretory ER is insensitive to brefeldin A treatment and does not undergo changes during mitosis, in agreement with the reports that apical extension continues at normal rates during this period.

Capturing in vivo dynamics of the actin cytoskeleton stimulated by auxin or light.

We present here a transient expression system that allows the response of actin microfilaments to physiological stimuli (changes in auxin content, light) to be observed in single cells in vivo. Etiolated, intact rice seedlings are attached to glass slides, transfected biolistically with talin fused to yellow-fluorescent protein to visualize actin microfilaments, and either treated with auxin or irradiated. The talin marker labels distinct populations of actin that are differentially expressed depending on the physiological state of the coleoptile (active elongation versus ceased elongation). Whereas longitudinal transvacuolar bundles prevail in cells that have ceased to elongate, fine cortical strands are characteristic for elongating cells. The visualized actin structures remain dynamic and responsive to signals. Exogenous auxin triggers a loosening of the bundles and an extension of the cortical strands, whereas irradiation reorientates cortical strands into longitudinal arrays. These responses correspond in quality and timing to the signal responses inferred previously from fixed specimens and biochemical studies. In big advantage over those methods it is now possible to observe them directly at the single cell level. Thus, the rice coleoptile system can be used as a convenient model to study actin dynamics in vivo, in response to physiologically relevant stimuli.

Time and cell cycle dependent formation of heterogeneous tubulin arrays induced by colchicine in Triticum aestivum root meristem.

We have investigated the appearance and reorganization of tubulin-containing arrays induced by colchicine in the root meristem of wheat Triticum aestivum, using immunostaining and electron microscopy. Colchicine caused depolymerization of microtubules and formation of tubulin cortical strands composed of filamentous material only in C-mitotic cells. After prolonged exposure to the drug, both interphase and C-mitotic cells acquired needle-type bundles, arranged as different crystalloids and/or macrotubules. The unmodified tyrosinated form of alpha-tubulin was detected within microtubules in control cells, but was not found within cortical strands. It was identified, however, within needle-type bundles. The modified acetylated form of alpha-tubulin, which was absent in control cells, was detected within needle-type bundles. Thus, cortical strands were transitory arrays, transformed into needle-type bundles during prolonged exposure to colchicine. Cortical strands appeared in a cell cycle-dependent manner, whereas needle-type bundles were cell cycle stable arrays. The diverse morphological organization, intracellular distribution and stability of tubulin-containing arrays may be associated with heterogeneity of alpha-tubulin isoforms. We assume that non-microtubular arrays substitute for microtubules in conditions where normal tubulin polymerization is inhibited.

Seedling survival, post-harvest recovery and growth rates of the woodrose-producing mistletoe Erianthemum dregei (Loranthaceae) on Sclerocarya birrea

The mistletoe Erianthemum dregei (Eckl. & Zeyh.) V. Tieghem produces a woodrose type haustoriurn on host trees. In some rural areas of southern Africa woodroses are harvested for their unique ornateness and prepared for sale as curios. Concern for the sustainability of mistletoe populations led to this study. Sclerocarya birrea (A. Rich.) Hochst hosts were artificially inoculated with hand-excised mistletoe seeds and post-harvest recovery of mistletoes on 12 S. birrea trees were investigated a year after disinfection. E . dregei exhibited 25% germination after 5 days but no seedlings survived a month in the field. Seventy-eight living E . dregei removed in 1994 were replaced one year later by 68 ‘new’ plants probably through resprouting from internal, longitudinal cortical strands which allow for rapid vegetative recovery after removal. Poor seedling establishment and the occurrence of new recruits on branches between 0.0002 to 0.025 m 2 cross-sectional area could indicate resprouting rather than recruitment from seed. The growth rates of 22 E dregei woodroses growing on S. birrea were assessed over a 12 month period during 1994/5. These woodroses varied in size from seedlings to individuals with a diameter of 25 cm. Absolute growth rate increased linearly with woodrose size, with most growth occurring during the first 6 months (August–February) relative to the second (February–August). Relative growth rates declined (negative In function) from very high rates for very small woodroses to fairly similar slow rates of growth for all other sizes. Woodroses of diameter 10 cm and 25 cm are typical size classes sold as curios. Using the growth data, it is estimated that woodroses of these sizes are 40 ± 25 (S.D.) and 56 ± 35 years old respectively. These slow rates of growth must be taken into consideration when determining sustainable harvesting limits.

A cytochalasin-sensitive actin filament meshwork is a prerequisite for local wound wall deposition inNitella internodal cells

Reorganization of the actin cytoskeleton following cell wall puncturing of characean internodal cells was studied by immunofluorescence and confocal laser scanning microscopy. Injury locally destroyed the parallel subcortical actin filament bundles and cortical actin strands that are characteristic of unwounded regions. At wounds, a delicate three-dimensional interlaced structure of actin strands, with meshes up to 5 μm wide, formed by de novo assembly of isolated filaments and by the elongation of residual subcortical actin bundles and cortical actin strands. The actin meshwork persisted for up to 2 h, corresponding to the duration of intense wound wall secretion. Actin filament bundles continuous with the subcortical bundles outside the wound then regenerated, their parallel alignment probably assisted by endoplasmic flow. Cytochalasin D concentrations that arrested cytoplasmic streaming completely inhibited the formation of the actin meshwork, wound wall deposition and recovery of actin bundles. Concentrations that only reduced streaming velocity delayed meshwork formation and wound walls were thinner than in controls. The actual amount of F-actin within the meshwork, however, was clearly greater in the presence of low cytochalasin concentrations. In late stages of recovery, the actin bundles became very thick and intervening spaces became wider thereby forming a conspicuous, three-dimensional lattice that was continuous with interwebbing subcortical bundles and cortical actin around the periphery of the wound. Our experiments suggest that actin meshwork formation is a prerequisite for plasma membrane-directed transport of vesicles involved in wounding-induced exocytosis in characean internodes. Stabilization of the meshwork by subinhibitory concentrations of cytochalasin D is probably caused by actinbinding properties of the drug that either induce bundling or impede function of associated proteins.

Lipids trigger changes in the elasticity of the cytoskeleton in plant cells: a cell optical displacement assay for live cell measurements.

An assay has been developed to quantitatively measure the tension and elasticity of the cytoskeleton in living plant cells. The cell optical displacement assay (CODA) uses a focused laser beam to optically trap and displace transvacuolar and cortical strands through a defined distance within the cell. Results from these experiments provide evidence for the classification of at least two rheologically distinct cytoskeletal assemblies, cortical and transvacuolar, that differ in their tension and response to both signaling molecules and reagents that perturb the cytoskeleton. It is further demonstrated that the tension of the transvacuolar strands can be significantly decreased by the addition of either linoleic acid, 1,2 dioctanoyl-sn-glycerol, or 1,3 dioctanoylglycerol. These decreases in tension could also be induced by lowering the cytoplasmic pH. In contrast, addition of Ca2+, Mg2+, or the ionophore A23187 to the cells caused a considerable increase in the tension of the transvacuolar strands. The data provides evidence that: (a) linoleic acid may be a signaling molecule in plant cells; (b) diacylglycerol functions as a signaling molecule through a protein kinase C-independent pathway mediated by PLA2; and (c) Ca2+ and pH have regulatory roles for controlling cytoskeleton tension and organization.

Le système endophytique d'Arceuthobium oxycedri. II. Aspects ultrastructuraux des zones de contact entre les tissus de l'hôte et du parasite

Ultrastructural study of the contact areas between the endophytic system of Arceuthobium oxycedri and the tissues of its host, Juniperus oxycedrus, shows that a crushed zone often occurs around the large cortical strands and along the primary sinkers in the secondary phloem of the host; such a structure seems rather unfavourable to the transfer of substances. On the other hand, the presence in the secondary xylem of half-bordered pits in direct contact with the cells of the primary and especially secondary sinkers certainly makes it easier for exchanges to take place. At the tip of the sinkers, the cells of the parasite, in the course of their growth, damage the cells of the host and sometimes penetrate into the lumen of its tracheids. Electron microscopy shows important acid phosphatase and ATPase activities at the level of the plasmalemma, plasmodesmata, and also some small endo- or exo-cytotic vesicles in the primary and secondary sinkers. Such activities are a sign of intense active transfer processes in these parts of the endophytic system. Around the contact zones, the parenchymatous cells of the host become very similar in aspect to those of the parasite.

Le système endophytique d'Arceuthobium oxycedri. I. Organisation, étude cytologique et cytochimique

In the endophytic system or haustorial apparatus that Arceuthobium oxycedri develops inside the parasitized branch of Juniperus, the present work shows that the cortical strands which connect the haustoria are either longitudinal or circular, that they always occur in the host secondary phloem, and that the smallest ones do not contain tracheids. Contrary to the secondary haustoria, in which parenchyma cells and tracheids are irregularly distributed, the primary haustoria have a more elaborate structure. At the ultrastructural level, the endophytic system is characterized by the presence of chloroplasts in all parts, including the tip of the haustoria, where these are, however, less differentiated. These plastids contain starch, but the cells are mainly rich in lipidic inclusions, especially in the deeper parts of the haustoria. Cell walls are always thick and endoplasmic reticulum profiles are locally abundant. In comparison with the corresponding healthy tissues, the host tissues neighbouring the endophytic system also become richer in starch and lipid inclusions.

Anatomy and ultrastructure of the endophytic system of Pilostyles thurberi (Rafflesiaceae)

The endophytic system of Pilostyles thurberi Gray consists of initially uniseriate filaments which develop into an anastomosing complex of larger cortical strands and radial sinkers. In the cortical strands three cell types are recognized, two of which differ largely in the density of the cytoplasm, the shape of the nucleus, and the degree to which the cytoplasm becomes plasmolyzed during fixation. The nuclei of both cell types contain two nucleoli which are physically connected by a nucleolar bridge. The third cell type demonstrates sieve plates, including a calloselike substance in the sieve pores and is consequently considered to be a sieve element. The sieve elements appear to form a discontinuous system and are regarded as a vestigial cell type. Plasmodesmal connections across the host–parasite interface have not been observed.

Anatomy of the endophyte of Viscum album L. (Loranthaceae)

Anatomy of the endophyte of Viscum album L. (Loranthaceae). An anatomical investigation into the nature of the host-parasite interaction of V. album and several of its phanerogamic hosts using SEM and light microscopy was conducted. Three kinds of parasite cell (haustorial parenchyma cells, cells resembling transfer cells and haustorial tracheids) were identified at the host-parasite interface. The terms haustorial parenchyma and haustorial tracheid are defined. Haustorial tracheids were seen to have penetrated the walls of host vessel elements and it is suggested that V. album is able to establish on a wide range of hosts because of the anatomically plastic nature of its haustorium. The development of the haustorium depends to a large extent on the nature of the surrounding host tissues. Parasite-induced host abnormalities including hypertrophy, distorted xylem elements, vessel-wall penetration and tylosis-occluded vessels were observed. The macroanatomical features observed are discussed and interpreted by-proposing a new theory for the ontogenesis of the V. album haustorium. Cortical strands with ‘chisel’ and ‘pencil’ shaped apices were both found to be present at the same time on one plant and thus were not seasonally separated.

Nuclear DNA Metabolism in the Tip of the Cortical Strands of Viscum album L

Nuclear DNA Metabolism in the Tip of the Cortical Strands of Viscum album L

Origin and early growth of the sinkers ofViscum album L.

Sinker initiation in the speciesViscum album was investigated throughout the year. A cytological study shows that the cortical strands give rise to sinkers from March to August, with a maximum of activity during the first half of summer. Thus, sinker initiation is a cyclic phenomenon.

Resin disease: a new disease of lodgepole pine dwarf mistletoe

A disease syndrome (termed resin disease) of Arceuthobiumamericanum on lodgepole pine is reported in Colorado, Idaho, Montana, Utah, and Wyoming. The disease effectively kills shoots from established dwarf mistletoe plants. In one area in Colorado, all of the dwarf mistletoe plants were diseased, and most of the aerial shoots were killed. Isolations from naturally infected resin disease cankers yielded 11 fungi; Alternariaalternata and Aureobasidiumpullulans were isolated most commonly. In inoculated dwarf mistletoe plants, Alternariaalternata was the most pathogenic, and it was followed closely by Aureobasidiumpullulans. Cladosporiumherbarum and Epicoccumnigrum may also play some role in disease development. Apparently, several weakly parasitic fungi can cause resin disease symptoms. A necrophylactic periderm was found in all naturally and artificially induced resin disease cankers examined and is presumably the mechanism by which dwarf mistletoe shoots are killed. The necrophylactic periderm isolates the shoots and cortical strands from the sinkers. Periderm formation apparently results from the invasion of the pine bark by the resin disease fungi.

The formation of branching strands at the apex of radicles of Arceuthobium pusillum cultured in vitro

The obligate parasite eastern dwarf mistletoe cultured on a modified White's medium produced thin, branching strands at the apex of the radicles. These strands were similar to young cortical strands in vivo.

Studies on the Ontogeny of the Dwarf Mistletoes, Arceuthobium. III. Development of the Inflorescence

In vascular plants, the apical meristem of the shoot normally represents a continuation of growth in the apical meristem of the embryo itself. This is not the case in Arceuthobium. Here the shoot apex of the embryo is rudimentary and eventually dies after infection of the host occurs. The inflorescence of Arceuthobium is, therefore, an adventitious structure originating in the endophytic system rather than from the shoot apex of the seedling. Inflorescence buds arise in either of 2 ways. In some species (A. douglasii and A. americanum), buds first appear as small meristematic protuberances on the outer surface of cortical strands. In other species (A. campylopodum), the buds arise at the ends of short branches. The former, or diffuse, type gives rise to inflorescences along the entire surface of the host branch; in the latter, or condensed, type inflorescences are formed in clusters. Early ontogeny of the inflorescence apex of both types is described. Studies of subsequent growth of the inflorescence apex show 5 well-defined plastochronic stages: (1) maximal area stage; (2) minimal area stage; (3) early post-minimal stage; (4) late post-minimal stage; and (5) pre-maximal stage.

STUDIES ON THE ONTOGENY OF THE DWARF MISTLETOES, ARCEUTHOBIUM. III. DEVELOPMENT OF THE INFLORESCENCE

In vascular plants, the apical meristem of the shoot normally represents a continuation of growth in the apical meristem of the embryo itself. This is not the case in Arceuthobium. Here the shoot apex of the embryo is rudimentary and eventually dies after infection of the host occurs. The inflorescence of Arceuthobium is, therefore, an adventitious structure originating in the endophytic system rather than from the shoot apex of the seedling. Inflorescence buds arise in either of 2 ways. In some species (A. douglasii and A. americanum), buds first appear as small meristematic protuberances on the outer surface of cortical strands. In other species (A. campylopodum), the buds arise at the ends of short branches. The former, or diffuse, type gives rise to inflorescences along the entire surface of the host branch; in the latter, or condensed, type inflorescences are formed in clusters. Early ontogeny of the inflorescence apex of both types is described. Studies of subsequent growth of the inflorescence apex show 5 well‐defined plastochronic stages: (1) maximal area stage; (2) minimal area stage; (3) early post‐minimal stage; (4) late post‐minimal stage; and (5) pre‐maximal stage.

Spencerites moorei comb. nov. From Southeastern Kansas

A fragment of a conelike structure, referable to the genus Spencerites, from a Bevier coal ball collected in southeastern Kansas is described. It is shown to be conspecific with Lepidodendron moorei. The axis is badly crushed with only the sclerified cortical strands and occasional vascular fragments being preserved. Sporangia and sporophylls are also poorly preserved but possibly similar in detail to Spencerites insignis. The spore is conspecific with Spencerisporites gracilis; there is no evidence of heterospory. It is suggested that Spencerites may actually represent only a fertile portion of a branch system rather than an apical strobilus.

Modes of union and interaction between parasite and host in the Loranthaceae. II. Phoradendreae

The large genus Phoradendron includes not only species like those of North America already described and Ph. fanshawei from Trinidad, with cortical strands and sinkers comparable especially with those of Viscum capense , but also others with very different modes of attachment. Some have much enlarged primary haustoria, with subordinate cortical strands and sinkers ( Phoradendron trinervium on some hosts) or without them ( Ph. hartii, Ph. flavens ). Ph. trinervium on Calophyllum lucidum , in contrast with its behaviour on other hosts, forms no major wedge but the cortical strands show an exaggerated development with anastomosis of the strands and the formation of numerous sinkers. In Phoradendron crassifolium and Ph. tetrapteron the haustorium simply spreads against the host wood to form a saddle-like attachment. In Ph. perrottetii also the haustorium spreads against the host wood, occasionally girdling it completely, forming downward branches, sometimes twenty or more, which run parallel and in their older parts fuse. Dendrophthora inaequidentata forms a very large primary haustorium, with slender cortical strands. The growth of the haustorium in width is accompanied by conspicuous hypertrophy of host wood on the lower side of it, the vessels and other elements of which abut on the haustorium, linking up with the parasite vascular system and without continuity with host wood above the haustorium. Tangential enlargement of the haustorium with stimulation of the adjoining cambium and annexation of the wood it forms occurs also though less conspicuously in some species of Phoradendron , particularly Phoradendron trinervium (on Rollinia mucosa ) and Ph. flavens .

DEVELOPMENTAL STUDIES OF ELODEA CANADENSIS MICHX.: I. MORPHOLOGICAL DEVELOPMENT AT THE SHOOT APEX

The shoot apex consists of a few initial cells at the tip of a thimble devoid of leaf initials for at least 100 μ. Leaf primorida are initiated from the superficial layer of cells, whereas branch buds arising among the; very youngest leaf primordia are produced deeper in the apex. Chinks occur where three or more cell walls come together. The tissue of the stem for the first 200 μ has no internodes. Two squamulae intervaginales lie on the adaxial surface of each leaf with which their development is associated. Internodes are initiated by the longitudinal growth and division of cells from the bottom of the leaf insertion disks. Cells of the young node divide longitudinally to increase the diameter of the nodal disk and to split the intercalary meristem into segments. Internodes are thus initiated with lacunae. Cells destined to become wood vacuolate at the seventh leaf whorl. Scalariform thickenings are produced but quickly disintegrate along with the rest of the xylem cells leaving a lacuna in the center of the stem. The bast surrounding the central xylem differentiates only slightly, beginning at the 20th leaf whorl, whereas the leaf traces and vertical cortical strands are apparent in younger tissue.