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  • 1
    ISSN: 1399-3054
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: In ectomycorrhizae auxins are proposed to attenuate elicitor-induced defence reactions in the host plant. To examine this hypothesis we compared the elicitor-induced accumulation of peroxidase isoforms between suspension-cultured spruce (Picea abies[L.] Karst.) cells incubated in media with and without auxins. In spruce cells changes in ionically and covalently wall-bound as well as symplasmic peroxidase (EC 1.11.1.7) activities were observed when elicitors from the following fungal species were applied: (1) Hebeloma crustuliniforme, an ectomycorrhizal partner of spruce; (2) Suillus variegatus, an ectomycorrhizal fungus incompatible with spruce; (3) Heterobasidion annosum, a spruce pathogen. Activity staining after SDS-PAGE and western blotting showed an accumulation of an ionically wall-bound 38-kDa peroxidase isoform. In addition, two covalently wall-bound isoforms (34 and 53 kDa) that could be released from spruce cell walls by cellulase and pectinase treatment were also induced by elicitors from these fungi. Moreover, in cells cultured without auxins all the elicitors triggered a rapid and transient accumulation of ionically wall-bound peroxidases, which reached a maximum activity 48 h after elicitor application. This early and transient peroxidase accumulation was diminished and delayed in cells cultured in the presence of auxins. In contrast, activity of peroxidases released into the culture medium of spruce cells or into the medium of protoplasts was suppressed by the elicitors of Hebeloma crustuliniforme. However, this suppression was attenuated by the action of auxins. It is suggested that under natural conditions, in infected spruce roots, the elicitors of the compatible fungus cause both suppression of the peroxidase (which is secreted to the free space of the roots), and induction of wall-bound and symplasmic peroxidases. On the other hand, auxins synthesized by the fungus could weaken these different elicitor-mediated effects.
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  • 2
    ISSN: 1399-3054
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: In culture, the ectomycorrhiza-forming fungi Amanita muscaria (Pers. ex Fries) Hock. and Hebeloma crustuliniforme (Bull. ex Fries) Quel. only grow on media with glucose or fructose but not with sucrose as sole carbohydrate source. This is due to their lack of wall-bound invertase activity. Therefore, utilization of sucrose by the fungi within a mycorrhizal association is believed to depend on the wall-bound invertase activity of the host. This enzyme activity was studied in the apoplast of suspension cultured cells of Picea abies (L.) Karst. An ionically and a tightly wall-bound isoform of acid invertase were found that function as β-d-fructofuranoside-fructohydrolases (EC 3.2.1.26). The ionically bound enzyme could be easily released from walls of intact cells with buffer of high ionic strength. In its native form, the ionically bound invertase isoform is a monomeric protein with a molecular mass of 61 kDa, as determined by gel filtration and SDS-PAGE. Glycoprotein nature of the enzyme was demonstrated with antibodies directed against the digoxigenin-labeled protein. The Km values of both enzymes for sucrose, their natural substrate, are relatively high (ionically bound invertase Km= 16 mM, tightly bound invertase Km= 8.6 mM). Activity of both wall-bound invertase isoforms strongly depends on the apoplastic pH. They have a narrow pH-optimum and exhibit highest activity at pH 4.5. with elevated activity between pH 4.5 and 6.0. Furthermore, fructose acts as competitive inhibitor of both isoforms, whereas glucose is not inhibitory. Unloading of sucrose from host cells to the apoplastic interface of the Hartig net in ectomycorrhizae appears to depend on the rate of hydrolysis by the wall-bound invertase of the host. Since the activity of the plant invertase depends on the actual pH value and the fructose concentration in the mycorrhizal interface, we suggest that the fungus can actively influence the activity of the plant invertase by acidification of the cell wall and by fructose uptake. Thus, the fungus itself can regulate its own supply of glucose and fructose.
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  • 3
    ISSN: 1432-2048
    Keywords: Ectomycorrhiza ; Elicitor inactivation ; Elicitor-induced reaction ; Hebeloma — Picea cells ; Signal transduction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Elicitors released from hyphae or cell walls of the ectomycorrhizal fungus Hebeloma crustuliniforme (Bull. ex Fries.) Quél. induced in suspension-cultured cells of Picea abies (L.) Karst. a set of fast reactions: (i) an immediate efflux of Cl− into the medium, followed by a K+ efflux; (ii) an influx of Ca2+ (measured as accumulation of 45Ca2+ in the cells); (iii) a phosphorylation of a 63-kDa protein and dephosphorylation of a 65-kDa protein (detectable by 4 min after elicitor application); (iv) an alkalinization of the medium, and (v) a transient synthesis of H2O2. The removal of extracellular Ca2+ by EGTA delayed the elicitor-induced alkalinization. A further reduction of this response could be achieved by TMB-8 an inhibitor of Ca2+ release from intracellular stores. Moreover, the inhibition of protein kinase activity by staurosporine prevented the extracellular alkalinization completely. However, the effectiveness of the elicitors in inducing the extracellular alkalinization was strongly impaired by constitutively secreted enzymes of spruce cells which cleaved the elicitors to inactive fragments. It is suggested that in ectomycorrhizae the efficacy of elicitors released from fungal cell walls is controlled by apoplastic enzymes of the host; the plant itself is able to reduce the activity of fungal elicitors on their way through the plant cell wall. But those elicitors which finally reach the plasma membrane of host cells induce reactions that are similar to the early defense reactions in plant-pathogen interactions.
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  • 4
    ISSN: 1432-2048
    Keywords: Key words: Arbuscular mycorrhiza ; Glomus ; H2O2 synthesis ; Medicago ; Plant defence ; Symbiosis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract. The diaminobenzidine (DAB) staining technique was used to examine the accumulation of H2O2 in parts of roots of Medicago truncatula Gaertn. colonized by the arbuscular mycorrhiza (AM)-forming fungus Glomus intraradices Schenk and Smith. At the cellular level, the combination of bright-field and fluorescence microscopy revealed that a brownish stain, indicative of H2O2 accumulation was present within cortical root cells in the space occupied by arbuscules. Accumulation of H2O2 was especially pronounced in cells containing arbuscules that were clumped and less branched. Moreover, H2O2 accumulated around hyphal tips attempting to penetrate a host cell. In contrast, no H2O2 accumulation was observed in hyphal tips growing along the middle lamella, or in appressoria or vesicles. On the basis of these findings we suggest that a locally restricted oxidative burst is involved in the temporal and spatial control of the intracellular colonization of M. truncatula cells by the AM-forming fungus G. intraradices.
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  • 5
    ISSN: 1432-2048
    Keywords: Key words: Chitinase ; Chitin elicitor ; Ectomycorrhiza ; Hebeloma ; Picea ; Plant defence (suppression)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract. Rapid reactions comprising efflux of K+ and Cl−, phosphorylation of a 63-kDa protein (pp63), extracellular alkalinization and synthesis of H2O2 are equally induced in cells of Picea abies (L.) Karst. by chitotetraose, colloidal chitin and cell wall elicitors from the ectomycorrhizal fungus Hebeloma crustuliniforme (Bull. ex Fries.) Quél. an ectomycorrhizal partner of spruce. Cleavage of fungal cell wall elicitors and of artificial chitin elicitors to monomeric and dimeric fragments by apoplasmic spruce chitinases (36-kDa class I chitinase, pI 8.0, and 28-kDa chitinase, pI 8.7; EC 3.2.1.14) equally prevented induction of these rapid reactions. Also, N-acetylglucosamine oligomers and elicitors from the fungal cell walls showed a similar dependence of their activity on the degree of polymerisation. From these results it is suggested that, during ectomycorrhiza formation, only some of the chitin-derived elicitors reach their receptors at the plant plasma membrane, initiating reactions of the hypersensitive response in the host cells. The remaining fungal elicitors will be degraded to varying extents by wall-localized chitinases of the host root, reducing the defence reactions of the plant and allowing symbiotic interactions of both organisms.
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  • 6
    ISSN: 1432-2048
    Keywords: Key words: Cantharidin ; Ectomycorrhiza ; Elicitor-induced reactions ; Mastoparan ; Picea ; Protein Phosphorylation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract. The first responses in spruce [Picea abies (L.) Karst.] cells induced by elicitors (N-acetylglucosamine oligomers) from ectomycorrhizal fungi have been described as follows: efflux of Cl− and K+, influx of Ca2+, extracellular alkalinization, phosphorylation of a 63-kDa protein (pp63), dephosphorylation of a 65-kDa protein (pp65) and synthesis of H2O2 (Salzer et al. 1996, Planta 198: 118–126). In order to obtain new insights into the triggering mechanism and the sequence of these rapid responses we used compounds which are known to activate or block specific steps within an elicitor-induced signal transduction cascade in plant cells. Comparable to elicitors the two protein phosphatase inhibitors, cantharidin and calyculin A, as well as mastoparan, an activator of trimeric G-proteins, were able to induce the release of Cl− and K+ from spruce cells and the alkalinization of the medium. Half-maximal activation of the alkalinization occurred at 133 nM calyculin A, 2.3 μM cantharidin and 1.6 μ mastoparan. The structural analogue of mastoparan, Mas 17, which has no G-protein-stimulating properties, was unable to trigger the above-mentioned reactions. In addition, cantharidin and calyculin A induced an increased synthesis of H2O2 in spruce cells which was prolonged in comparison to the elicitor-induced transient formation of H2O2. Also, the cantharidin-induced release of K+ was more pronounced and longer lasting than that caused by elicitors from the ectomycorrhizal fungus Hebeloma crustuliniforme (Bull. ex Fries.) and N-acetylglucosamine oligomers. Furthermore, cantharidin, calyculin A and mastoparan induced the phosphorylation of pp63. Remarkably, the protein kinase inhibitor, staurosporine, inhibited all the rapid responses described above, no matter whether they were triggered by fungal elicitors or by the protein phosphatase inhibitors. These results indicate that in the initial signalling events in spruce cells, essential protein phosphorylations occur either as an (auto) phosphorylation of a membrane-bound receptor kinase prior to the activation of a G-protein or (and) immediately downstream of the activated G-protein in a phosphorylation cascade and are the basic requirements for the ion fluxes following downstream.
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