Collapse Of Epidermal Cells Research Articles

Microscopic Analyses of Latent and Visible Monilinia fructicola Infections in Nectarines.

Little is known about the histologic features of a latent Monilinia fructicola infection and brown rot in infected fruit. This report informs on the results of an investigation whose aim was to analyze the microanatomy of nectarines with a latent and visible M. fructicola infection. Mature nectarines were inoculated with an M. fructicola isolate and incubated at 25°C for 0, 24, 48, 72, or 96 hours in the dark. For investigating the latent infection process, the inoculated nectarines were first incubated at 25°C for 24 hours in the dark and then incubated at 4°C for 72, 144, 216, and 288 hours in the dark. At the end of the incubation, samples of nectarine tissue were excised from the inoculation points and prepared for light and transmission electron microscopic examinations. No signs of disease were seen on the surface of nectarines with a latent infection over the 288-hour incubation period. When the tissue samples were microscopically examined, M. fructicola colonized the stomata and this stomatal colonization progressively increased over time and was associated with gradual collapse of the epidermal cells and colonization of the subepidermis. In nectarines with visible brown rot, the disease usually appeared after 24 hours on the surface and in the uppermost layers of epidermal cells, which began to collapse after 48 hours. Subsequently, the diseased tissues of the nectarines displayed (a) colonization of the epidermis and mesocarp by M. fructicola with thin and thick hyphae, (b) collapse and disruption of epidermal and mesocarpic cells, (c) lysogenic cavities in the subepidermis and mesocarp, (d) degradation of the cuticle and epidermis, and (e) M. fructicola sporulation. M. fructicola is active during latent infections because slow and progressive colonization of nectarine subcuticular cells by the fungus occurs.

(397) Atypical Disease Response to Hop Powdery Mildew

Open-pollinated hop seeds (Humulus lupulus var. lupuloides E. Small, H. l. var. pubescensE. Small, H.l. var. neomexicanusNelson and Cockerell, and H.l. var. lupulusL.) were collected from the United States, Canada, and Kazakhstan and screened for resistance to hop powdery mildew (Podosphaeramacularis Braun & Takamatus). A total of 2108 seedlings were repeatedly inoculated with high levels of P. macularis. Under greenhouse conditions, most seedlings exhibited a compatible disease response typical of susceptible hosts with sporulating colonies covering both leaf surfaces. Three genotypes from hop seed native to Emba, Kazakhstan, remained uninfected in greenhouse assays. Further experimentation of those genotypes in growth chambers at 18 °C revealed that they demonstrated an atypical disease response. Six days after inoculation, necrotic lesions on the adaxial leaf surfaces were visually apparent. Microscopic examination showed areas of collapsed epidermal cells, collapsed hyphae, and golden-brown discolorations extending out from the center of the infection. These symptoms contrasted those of a hypersensitive response in which fungal growth is restricted by the collapse of epidermal cells in advance of the fungus. The infection frequency of Kazakhstani genotypes was intermediate compared to H.l. cultivars Symphony (susceptible) and Nugget (resistant) after exposure to three preinoculation temperature regimes. These genotypes could represent a new source of polygenic resistance to hop powdery mildew, and could broaden hop germplasm available to global breeding programs.

The plant pathogen Rhodococcus fascians colonizes the exterior and interior of the aerial parts of plants.

Rhodococcus fascians is a plant-pathogenic bacterium that causes malformations on aerial plant parts, whereby leafy galls occur at axillary meristems. The colonization behavior on Nicotiana tabacum and Arabidopsis thaliana plants was examined. Independent of the infection methods, R. fascians extensively colonized the plant surface where the bacteria were surrounded by a slime layer. R. fascians caused the collapse of epidermal cells and penetrated intercellularly into the plant tissues. The onset of symptom development preceded the extensive colonization of the interior. The meristematic regions induced by pathogenic strain D188 were surrounded by bacteria. The nonpathogenic strain, D188-5, colonized the exterior of the plant equally well, but the linear plasmid (pFiD188) seemed to be involved in the penetration efficiency and colonization of tobacco tissues.

Rind distortion of lemon caused by Botrytis cinerea Pers

The infection process by Botrytis cinerea Pers. on lemon fruits (Citrus Union (L.) Burm.), and development of rind distortion are described. Infections occur on juvenile fruitlets soon after petal fall from hyphae growing over the surface of the fruit from adjacent colonised flower debris. The hyphae form compact infection cushions at their tips. There is a collapse of epidermal cells and several layers of underlying cells in the vicinity of the infection cushion, leading to the formation of small necrotic pits on the surface of the fruit. As the fruitlets grow, there is generalised hyperplasia in a zone up to 20 cell layers deep in the region of the necrotic pits, leading to conspicuous outgrowths on the fruit surface. The initial wound becomes stretched, flattened, and corky as the fruit enlarges. A cambial layer is located below the area of damaged tissue, fluorescence in the vicinity of the cork cambium indicates the presence of suberin in the cambium. There was no evidence of suberin or lignin deposition in the region of the original necrotic pit. The necrotic pits are characteristic of a non‐pathogenic reaction between the fungus and the host, with fungal hyphae failing to become established in the necrotic tissues of the pit. The hyperplasia leading to the rind distortions is considered to be a generalised reaction to the initial wound and is commonly observed in lemon following other types of injury to the rind. The principal sources of inoculum in orchards are Botrytis‐colonised floral debris caught up in flower clusters or adhering directly to the fruit surface. The etiology of the disorder helps explain why control by conventional fungicides is seldom successful. Not only are there physical limitations to providing a protective cover on very small fruit, but also the extended period over which flowering occurs in lemon imposes limits on the number of applications able to be economically applied. In the absence of effective control systems, growers are advised to modify husbandry practices such as lowering the density of shelter belts, opening lemon canopies to assist drying out of the flowers, and use of air blast orchard sprayers to physically remove floral debris from the vicinity of young fruit.

UV-band and acid rain effects on beach ( Fagus sylvatica L.) and holm oak ( Quercus ilex L.) leaves

Beech ( Fagus sylvatica L.) and holm oak ( Quercus ilex L.) leaves were subjected to simulated acid rain and UV-B radiation for 10 days. The almost glabrous beech leaves revealed a greater predisposition to developing macroscopic necrotic lesions than the pubescent holm oak leaves. This response is due primarily to their different morpho-anatomical structure, rather than to a difference in the synsthesis of UV-B absorbing pigments (flavonoids); the response was also accompanied by microscopically observable alterations (collapse of epidermal cells, fusion of surface wax structures, structural damage to the stomata) and by the reduction of stomatal conductance.

Early stages of infection of rapeseed petals and leaves by Sclerotinia sclerotiorum revealed by scanning electron microscopy

The primary ascospore inoculum of Sclerotinia sclerotiorum initially infects rapeseed (Brassica napus var oleifera) via petals. Infected petals fall onto leaf surfaces, resulting in infection of those organs. A scanning electron microscopy (SEM) study of this process was undertaken to elucidate the host‐parasite relationship and to determine the best plant organ for detection by serology of early field infection as an aid to disease forecasting and cost‐effective disease control. The behaviour of ascospores deposited on young petals and on leaves was compared. Ascospores were deposited by inverting a mature apothecium above either a leaf disc, a young petal or young petal placed on a leaf surface. Spore germination, host penetration and colonization were examined by SEM. On young petals, the following steps in pathogenesis were observed: ascospore adhesion and germination, penetration of the host from short germ tubes and collapse of epidermal cells. Petals were then covered with extensive mycelium. From these sites, the mycelium invaded leaf tissues and infection proceeded. In contrast, ascospores landing directly on leaf surfaces failed to germinate. The role of petals as sites of pre‐election in the aetiology of the disease is discussed in relation to the published literature.

Effects of O3 on the stomatal behaviour of Egyptian varieties of radish (Raphanus sativus L. cv. Baladey) and turnip (Brassica rapa L. cv. Sultani).

The effects of O2 (80 nl 1-1 ) on the photosynthetic rate (A) and stormatal conductance (g) of Egyptian varieties of radish (Raphanus sativus L. ev. Baladey) and turnip (Brassica rapa L. ev. Sultani) were examined. Exposure to O3 decreased A in both species, while gs was increased in radish and decreased in turnip. The effect of different concentrations of O3 , (50, 100, 150 nl l-1 on radish was examined and it was found that A decreased, while g, increased, with increasing concentrations of O3 . Scanning electron microscopy (SEM) of fumigated leaves showed that the increase in gs in radish resulted from the collapse of epidermal cells. Counts of the percentage of stomata which were fully open demonstrated that 80 nl 1-1 O3 caused an increase of 18% in radish and a decrease of 14% in turnip. The proportion of opened stomata was found to increase with increasing O3 concentration in radish.

Ultrastructure, autofluorescence, callose deposition and lignification in susceptible and resistant muskmelon leaves infected with the powdery mildew fungus Sphaerotheca fuliginea

Leaves of the susceptible cv. Ananas-Yokneam (AY) and of resistant cvs PI 124111F, PI 124112, PMR-45, PMR-6 of muskmelon ( Cucumis melo) were inoculated with either race 1 or race 2 of Sphaerotheca fuliginea and examined microscopically after staining with Calcofluor, basic aniline blue or phloroglucinol. At 20–24 h post inoculation of the susceptible AY at 23 °C, the fungal spores (both race 1 or race 2) developed one or two germtubes which penetrated into one or two epidermal cells. The penetration zones were surrounded with callose-like material but no autofluorescence nor lignin-like materials were observed in the penetrated epidermal cells. A similar response was observed in PMR-45 inoculated with race 2 (compatible). In contrast, the fungus developed a single germtube on the resistant PI 124111F, PI 124112, and PMR-6 inoculated with either race, as well as in PMR-45 inoculated with race 1, which induced autofluorescence, callose accumulation and lignification in the penetrated epidermal cells. Electron microscopical studies revealed that the rapid collapse of epidermal cells in the resistant cultivars was accompanied by the accumulation of callose-like deposits in cell walls and around haustoria, electron-opaque deposits in the plasma membrane and electron-opaque deposits between the cell wall and the plasma membrane. Occasionally, callose also appeared in epidermal and mesophyll cells adjacent to the penetrated cells. By 96–120 h post inoculation, abundant sporulation was observed in the compatible interactions whereas only 1–3 germtubes with no sporulation were seen in the incompatible interactions. Heat shock or chemical inhibitors (cycloheximide, blasticidin-S, cordycepin, 2-deoxy-D-glucose, α-aminooxyacetic acid, 2,4-dinitrophenol and sodium azide) failed to induce susceptibility in the resistant cultivars. The results suggest a similar structural response to powdery mildew in C. melo cultivars carrying the resistance genes Pm-1 (PMR-45), Pm-2 (PMR-6), Pm-3 (PI 124111F), Pm-4 (PI 124112), Pm-5 (PI 124112) and Pm-6 (PI 124111F).

Morphological and Anatomical Characterization of Leaf Burn in Corn Induced from Foliar‐Applied Nitrogen1

AbstractFoliar applied nutrients have been used to overcome nutrient deficiencies on a variety of crops; however, leaf burn is often a problem. Corn (Zea mays L.) Plants were treated with 120 g N L−1 from urea to study the leaf burn phenomenon. A 4‐µL drop applied to the adaxial leaf surface was observed at 2‐h intervals for 8 h. Leaf samples were studied with dissecting, compound, and scanning electron microscopes for changes resulting from the application of foliar applied fertilizer salts. Damage observed under the dissecting microscope after 2 h consisted of a darkening in the epidermal cells. After 8 h, the epidermis was desiccated, sunken, and discolored, forming a lesion on the leaf surface. Slides of leaf sections showed wrinkling and collapse of epidermal cells at 4, 6, and 8 h. Disorganization of mesophyll was observed after 8 h. Scanning electron microscope (SEM) micrographs revealed collapsed and wrinkled epidermal cells with sunken stomates after 2 h. Events associated with visual damage appear to be related to water loss since epidermal and mesophyll cells become desiccated.

Scanning electron microscopy of plant surfaces using low temperature dehydration prior to critical point drying

Low temperature dehydration prior to critical point drying of the delicate leaves of Phaseolus coccineus L. and the tougher waxy leaves of Pisum sativum L. gave better preservation of their surfaces than ambient temperature dehydration when examined in a SEM. Preservation of surface structures was best when the final stages of dehydration by ethanol were at 240 K (-33°C) or below. Low temperature dehydration failed to reduce the collapse of epidermal cells on the petals of Pelargonium zonal hybrid.

Influence of polishing and abrasion on the diffusive conductance of leaf surface of Festuca arundinacea Schreb.

Abstract Leaf surfaces of Festuca arundinacea Schreb. were subjected to controlled polishing and abrasion on a test-bed designed to simulate components of abrasive wind damage. Both treatments substantially increased the leaf surface conductance, particularly the polishing treatment. Scanning electron micrographs of cryo-fixed leaf surfaces showed displacement and smoothing of the epicuticular waxes, damage to the cuticle, collapse of epidermal cells and fracture of trichomes. The importance of the epicuticular waxes in determining leaf diffusivity and permeability is discussed.

Correlation between the deposition of fluorescent compounds in papillae and resistance in barley against Erysiphe graminis hordei

Events in the development of powdery mildew fungi and responses of barley epidermal cells were compared not only among combinations of barley and among races of mildew differing in virulence but also between epidermal cells adjacent and nonadjacent to stomata. Differences in fluorescence intensity and in the size of papillae and haloes were not observed until 12 h after inoculation; however, the fluorescence intensity and size of papillae increased rapidly and the differences became obvious 15 h after inoculation when the parasite penetrated into host cells. Failure of fungal penetration in incompatible combinations was mostly associated with papillae either with or without collapse of epidermal cells. The greater the resistance of barley, the greater was the rate of fungal cessation of growth at the papilla stage and the stronger was the fluorescence intensity of papillae. Haloes were larger in susceptible than in resistant hosts, but they usually did not form at sites where haustoria were produced.On the other hand, in incompatible combinations (with the [Formula: see text] genes), host epidermal cells adjacent to stomata (A cells) were less resistant than the nonadjacent ones (NA cells). More than 80% of haustoria formed were restricted to A cells, and about half of the conidia located in A cells formed haustoria. Many parasites of the remaining population in A cells ceased growth, followed by host cell collapse. In NA cells, most penetration pegs were terminated at the stage of papilla formation. The fluorescence intensity of papillae in NA cells was much stronger than that in A cells.It was suggested that cytologically specific events of compatibility or incompatibility are expressed at the papilla stage and that the fluorescent compounds deposited at the infection sites might represent chemical factors which are associated with resistance expressed at the stage of penetration, although it is not known whether the compounds as well as papillae themselves are a primary cause of resistance or a result of some unknown resistance mechanism.On compare des aspects du développement des champignons responsables du blanc de l'orge et les réactions des cellules épidermiques de l'orge, non seulement entre combinaisons d'orge et entre races de blanc différant par leur virulence, mais aussi entre cellules épidermiques adjacentes ou non aux stomates.