Cell Wall Appositions Research Articles

Ultrastructure of eastern cottonwood clones susceptible or resistant to leaf rust

The infection process in four clones of eastern cottonwood susceptible or resistant to leaf rust caused by Melampsora medusae was studied by light and electron microscopy. Infection was initiated by stomatal rather than direct entry. Typical dikaryotic haustoria were observed in all clones within 1 day of inoculation. Some healthy-appearing haustoria were observed in susceptible clones throughout the duration of the study, which was terminated during the initiation of uredial production. Incompatibility was expressed differently in the two resistant clones. In clone St 75, most haustoria and invaded host cells that were observed appeared necrotic within 2 days of inoculation. Cell wall appositions appeared during this time in cells adjoining necrotic host cells. Some infected cells disintegrated within 4 days of inoculation. Affected host cells of clone St 92, on the other hand, plasmolyzed during the first 2 to 3 days after inoculation. Necrotic host cells were not observed in this clone until the 4th day after inoculation. Hyphal ramification and host plasmolysis were extensive at 6 days after inoculation.

Interaction between Venturia inaequalis and Apple Callus Tissue Cultures: an Electron Microscopic Study

AbstractUntrastructural interactions between Venturia inaequalis and callus cultures from scab susceptible and resistant apple varieties, were similar. Host cell wall changes, appositions, and invagination of host plasmamembrane at sites of close contact with fungal hyphae were regularly observed. The ultrastructural observations are described and discussed. The host cell alterations as well as many fungal structures corresponded to those known in young leaves of susceptible apple varieties.

Relationship of autofluorescent and ultraviolet-absorbing components in cell walls and wall appositions to disease resistance in kohlrabi roots

It has been shown that wound plugs induced mechanically by bending of kohlrabi root hairs are more resistant to subsequent penetration attempts by Olpidium brassicae than are papillae induced by the fungus. A histochemical, autofluorescence, and ultraviolet-microspectrophotometric study was undertaken to determine if a differential composition of phenolic derivatives could explain this difference in resistance. Phenolics in the secondary wall thickenings of xylem vessel elements of roots were distinct from those in wound plugs, papillae, and root hair cell walls. But phenolics in wound plugs and papillae could not be distinguished from each other by the methods employed. Thus, the results do not support the hypothesis that wound plugs are more resistant to penetration than are papillae because they contain phenolic derivatives not found in the latter. However, the possibility that an earlier deposition of phenolics in wound plugs accounts for their greater resistance to penetration has not been ruled out.

Ultrastructure of susceptible, host resistant, and nonhost resistant interactions of alfalfa with Phytophthora megasperma

Zoospores of Phytophthora megasperma Drechs. f. sp. medicaginis (Pmm), an alfalfa (Medicago sativa L.) pathogen, and P. megasperma f. sp. glycinea (Pmg), a pathogen of soybean (Glycine max (L.) Merr.), were used to inoculate alfalfa seedlings from the Pmm-susceptible cultivar 'Vernal' and the Pmm-resistant breeding line M193. The susceptible interaction ('Vernal'–Pmm) was characterized by rapid disruption of host mitochondria in cells adjacent to intercellular hyphae, dissolution of host cell walls, and death of host cells 12–24 h after inoculation. Colonization of roots was extensive, and hyphae usually appeared healthy. Both host resistant (M193–Pmm) and nonhost resistant (M193 or 'Vernal' – Pmg) interactions were characterized by hyphal growth inhibition, production of cell wall appositions in root cells in contact with hyphae, and rapid necrosis of root cells. Wall appositions were more numerous in root cells in contact with Pmg than with Pmm, and rapid cell necrosis was more extensive in the nonhost resistant interaction than in the host resistant interaction. Numerous abnormalities were observed in Pmg hyphae. Nonhost resistance was expressed more rapidly and appeared to be more effective than host resistance in limiting hyphal growth. An examination of a compatible interaction of Pmg with soybean indicated that its mode of pathogenesis may be fundamentally different from that of Pmm in alfalfa.

An ultrastructural study of the late-blight fungus Phytophthora infestans and its interaction with the foliage of two potato cultivars possessing different levels of general (field) resistance

In the susceptible reaction of foliage of the cultivar Majestic there was extensive colonization of the host tissue prior to the onset of necrosis. The development of intracellular hyphae was confined to the initially parasitized palisade cells. Thereafter the fungus grew as intercellular hyphae, which penetrated host cells and formed haustoria. Haustorial morphology was highly variable, ranging in type from small spherical to much larger digitlike structures. Haustorial formation was preceded by the laying down of a moderately electron-dense penetration matrix bounded by the host plasmalemma. This material probably constituted the extrahaustorial matrix once haustorial development had taken place. The matrical material stained with silver proteinate reagent, and this reaction was blocked by dimedone, indicating that it was carbohydrate in nature. In the resistant reaction of foliage of the cultivar Shamrock, the epidermal cells rapidly became necrotic. Quantities of exceedingly electron-dense granules appeared in the necrotic host cells. Host organelles were no longer recognizable, but the fungal cytoplasm remained intact. In adjoining host cells, thin cell wall appositions were formed, which had a heterogeneous composition. On occasions when the fungus attempted to invade an underlying mesophyll cell, papillae usually formed in that cell at the site of incipient penetration.

Cell wall appositions and plant disease resistance: Acoustic microscopy of papillae that block fungal ingress

Plant cells react to localized stress by forming wall appositions outside their protoplasts on the inner surface of their cellulose walls. For many years it has been inferred that appositions elicited by encroaching fungi, termed "papillae," may subsequently also deter them and thus represent a disease-resistance mechanism. Recently, it has been shown that preformed, oversized papillae, experimentally produced in coleoptile cells of compatible barley, Hordeum vulgare, can completely prevent direct entry of Erysiphe graminis f. sp. hordei that ordinarily penetrates and causes disease. To discover how these papillae may function, acoustic microscopy was used to contrast their in vivo elastic properties with those of ineffective normal papillae and contiguous cell wall. Raster and line scans showed intense acoustic activity at sites of preformed papillae; scans in selected focal planes identified this activity with the papillae, not with subtending cell wall. Minimal acoustic activity was found in normal papillae. It is suggested that some wall appositions could serve in disease resistance as viscoelastic barriers to mechanical forces exerted by the special penetration structures of advancing pathogenic fungi.

Ultrastructure of lesions produced by Cercospora beticola in leaves of Beta vulgaris

The fungus Cercospora beticola Sacc. incites a leaf spot disease of sugarbeet ( Beta vulgaris L.). Lesion formation was studied by electron microscopy. One week after inoculation, the fungus was established in the intercellular spaces of the mesophyll and degenerative changes had occurred in cells near infection sites. Visible necrotic spots were produced as these lesions enlarged. During the degenerative sequence cell membrane systems and organelles were disrupted; affected cells quickly collapsed and became necrotic. After 3 weeks a boundary zone with two regions separated the central necrotic tissue from apparently unaffected, healthy tissue outside the lesion. Two types of cell wall apposition occurred, one a localized callose-type in the lesion center, and the second a generalized wall thickening in the inner boundary zone. Electron-dense material occluded intercellular spaces in the outer boundary zone. During most phases of the disease, hyphae bridged the intercellular spaces or were attached to and followed the contours of host cell walls. Only late in the disease, long after host cells were necrotic, were hyphae observed within largely disintegrated host cell walls.

Microsurgical isolation of intact plant cell wall appositions for microanalyses

Little is known about the chemical components of the plant cell wall apposition as they relate to its structure and function. The small sizes (5–15 μm diameter) of the appositions, their low frequencies in the cells, and their intimate connections to the cell wall have almost precluded meaningful cytochemical and (or) biochemical analyses. With the development of analytical techniques using the electron microprobe it is now feasible to discover the elemental composition of minute cellular structures, such as the wall apposition, but problems in specimen preparation remain. As a necessary and critical first step in microprobe analysis we have found it best to microsurgically remove fresh wall appositions from their mother cells and then admit them directly into the scanning electron microscope (SEM) after air drying and gold coating. This paper describes the requisite technologies of specimen preparation, microtool fabrication, specimen selection and isolation which are involved. The technique described could find ready application in the microanalyses of many other subcellular structures.