Search

Total and cell wall-bound cadmium and the major antioxidants were measured in thalli of the lichen Xanthoria parietina (L.) Th. Fr. exposed to two Cd concentrations, namely 4.5 or 9.0 μm, in liquid medium during exposure periods of either 24 or 48 h. Total Cd in the thalli was within the range of previous field measurements and was proportional to the exposure concentration, but less than proportional with respect to exposure duration. More than half of the total Cd was immobilised by the cell wall. The adopted conditions of Cd stress caused: (i) no changes in dry weight and protein concentration; (ii) an increase in the level of ascorbic acid and a decrease in that of reduced glutathione, as well as an increase in guaiacol peroxidase activity; (iii) no changes or moderate decreases in the activities of superoxide dismutase, catalase, dehydroascorbate-, NADPH-dependent glutathione disulfide-, and monodehydroascorbate reductases and of ascorbate peroxidase; (iv) an increase of the level of thiobarbituric acid-reactive substances, assumed to reflect malondialdehyde formation arising from membrane lipid peroxidation. Thus, X. parietina might withstand realistic levels of Cd stress by: (1) intercepting the heavy metal at cell wall level, (2) the intervention of antioxidant metabolites, and (3) a moderate increase in guaiacol peroxidase activity.

In this work, we tried to go deeper inside distribution and toxicity of cadmium (Cd) in the macrolichen Xanthoria parietina (L.) Th. Fr. Thalli of this species were treated with 0 (control), 4.5, 9, 18, or 36 muM Cd for 24 or 48 hours. Transmission electron microscopy, X-ray microanalysis, and electron energy loss spectroscopy were exploited to study distribution and ultrastructural effects of Cd in thalli; spectrophotometric techniques were utilized for measuring Cd effects on chlorophyll (Chl) content; light fluorescence microscopy was used to evaluate Chl autofluorescence. The highest Cd concentration caused ultrastructural alterations both in the mycobiont and in the photobiont, more severe in the latter, decreased total Chl content and progressively quenched Chl autofluorescence. Cell wall immobilization was observed in both bionts, and evidence pointing to a Cd-binding ability by the concentric bodies in the mycobiont was also obtained. Lower Cd concentrations led to slight or even no effects on thallus structures and on Chl content and autofluorescence. The results obtained suggest that: (1) among the two bionts, the algal partner appears to be more susceptible to Cd stress, probably because of the presence of delicate and sensitive components such as the chloroplast and photosynthetic pigments; (2) a concentration threshold exists for the occurrence of evident structural and functional damage in X. parietina thalli exposed to Cd., ((c) 2005 Wiley-Liss, Inc.)

Thalli of the lichen Xanthoria parietina (L.) Th. Fr. were soaked for either 24 or 48 h in a buffered medium in the presence of environmentally relevant concentrations (4.8 and 9.6 μM) of hexavalent chromium [Cr(VI)]. Treatment effects on the antioxidant status, differential distribution and fate of Cr(VI) among the mycobiont and the photobiont cells, and potential damage to cell ultrastructure in the two bionts, were evaluated. The adopted conditions of low Cr(VI) stress caused: (i) an increase in the level of ascorbic acid and a decrease in that of reduced glutathione, as well as a moderate increase in guaiacol peroxidase activity, only observed after treatment with 9.6 μM Cr(VI); (ii) no changes in malondialdehyde content; (iii) a remarkable Cr accumulation in the mycobiont cytosol and compartmentalisation in the mycobiont vacuoles;(iv) a modest apoplastic Cr immobilisation by the outer part of the cell walls, of both the mycobiont and the photobiont. The response of X.parietina to low concentrations of Cr(VI) appears to be a complex phenomenon, which might reflect maintenance of cellular homeostatic equilibria, rather than specific response pathways.

The results of several experiments concerning the presence and composition of alkaloids in different tissues (stems, leaves, roots) of Catharanthus roseus L. plants and explants, healthy and infected by clover phyllody phytoplasmas, are reported. The alkaloids extracted and determined by the reverse phase high-pressure liquid chromatography were vindoline, ajmalicine, serpentine, vinblastine, and vincristine. The total alkaloid concentration was higher in infected plants than in the controls, in particular the increase of vinblastine in infected roots was very significant. The ultrastructural observations of infected roots showed alterations of the cell walls and of the nuclei. These results demonstrate that phytoplasmas, detected in all infected tissues by light fluorescence and transmission electron microscopy, play an important role on secondary metabolism of the diseased plants, modifying both the total content of alkaloids and their ratio.

ABSTRACT Localization of hydrogen peroxide (H(2)O(2)) and the roles of peroxidases, malondialdehyde, and reduced glutathione in three apple cultivars were compared in healthy trees, trees infected with apple proliferation phytoplasma (APP), and trees that had recovered from the infection. In recovered apple trees, symptoms of the disease and the pathogen had disappeared from the canopy, but phytoplasmas remained in the roots. H(2)O(2) was detected cytochemically by its reaction with cerium chloride to produce electron-dense deposits of cerium perhydroxides.H(2)O(2) occurred in the plasmalemma of the phloem of leaves of recovered apple trees, but not in healthy or APP-infected leaves. In all cultivars, the peroxidase activity detected in tissue from APP-diseased trees was greater than or equal to that of tissue from recovered trees, which equaled or exceeded that of tissue from healthy trees, at two sampling times (May and September). In contrast, the glutathione content of leaves decreased in the reverse order. More malondialdehyde was observed in leaves from recovered trees than in leaves from healthy or APP-infected trees in three of six cultivar-date combinations; in the other three combinations, the malondialdehyde contents of leaves from healthy, infected, and recovered trees were not significantly different from one another. The results suggest that some components of the oxidant-scavenging system in recovered leaves are not very active, leading to an overproduction of H(2)O(2) and, possibly, to a membrane lipid peroxidation.The production of H(2)O(2) appears to be involved in counteracting pathogen virulence.

The potassium pyroantimonate (KPA) Ca(2+) precipitation technique, X-ray microanalysis and Electron Energy Loss Spectroscopy carried out by transmission electron microscopy were used to analyze the Ca(2+) distribution in Catharanthus roseus L. leaves infected with phytoplasmas belonging to different taxonomic groups, and in phytoplasma cells. The analysis revealed that the distribution of Ca(2+) was different in healthy and diseased plants (where the KPA deposits were numerous) and no differences were observed in the tissues of the three types of infected C. roseus L. Since no KPA precipitates were visible in the phloem and on phytoplasma cells, it is likely that Ca(2+) ions are not directly involved in phytoplasma replication, but, in infected cells is a response to the pathogen indicative of a higher Ca(2+) in the plasmalemma.

Three inbred lines of maize (33-16, MO17 and B73) differing in their susceptibility to Barley yellow dwarf virus and Maize dwarf mosaic virus were studied to compare the ultrastructural modifications induced by the two viruses in leaf tissues of different age. The results demonstrate that the alterations induced by the two viruses in the different maize lines could depend on the particular line tested.

The alterations of cell walls and the localization of several compounds such as polyphenols, suberin, lignin, in plum and apple plants infected with plum leptonecrosis (PLN) and apple proliferation (AP) phytoplasmas respectively, were investigated. Catharanthus roseus plants, infected with AP or PLN were also studied. The 4,6-diamidino-2-phenylindole (DAPI) test and transmission electron microscopy showed the presence of phytoplasmas in all infected plants. Specific histological stainings for cutinized/suberinized cell walls, tannin deposits and vacuolar polyphenol inclusions, performed on leaf and stem tissues, revealed an increase of these substances in infected plum and apple plants. No differences occurred in C. roseus. Total polyphenol analysis confirmed a strong increase (3-fold) in the polyphenol content in infected tissues, particularly in plum leaves. From the data obtained it appears that polyphenols can be considered as defence-related metabolites in plum and apple plants infected by phytoplasmas. Further investigations are necessary to determine whether these compounds play a specific role in the development of all phytoplasma/host interactions and in the defence-related processes.

X-Ray microanalysis, performed by both scanning and transmission electron microscopes equipped with energy-dispersive spectrometers (EDS), was used to analyze the element distribution in phloem tumors induced by maize rough dwarf virus (MRDV) in Zea mays L. The content of both Si and Ca was lower in tumors than in the nontumoral surrounding tissues. The Si content increased in old tumors where this element was mainly concentrated in the trichomes growing at their surface. The peaks of other detectable elements, P, K, and S, were higher in tumor than in nontumor tissues. The distribution of inorganic cations in the cells was also studied by the potassium pyroantimonate (KPA) precipitation technique. The KPA precipitates have proved to be primarily of calcium salts and they occurred quite frequently in the cells at the tumor periphery.