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Mycorrhizal fungi as bioindicators of heavy metal soil toxicity

Different methods were used to monitor heavy metal toxicity to mycorrhizal fungi. The germination test was shown to be a sensitive indicator of heavy metal toxicity. The germination level of control Gi. rosea spores was high. On the highest Pb concentration germination was strongly decreased. Also the appearance of the hyphae can indicate the presence of heavy metals. The problem with this test lies in the vulnerability to parasites. Gigaspora spores were more susceptible to parasitic attack than G. mosseae. Spores colonized with parasites showed decreased viability and no germination of infected spores was observed. Parasites can also alter the uptake and distribution of heavy metals.

Three plant species (Zea mays, Plantago lanceolata and Medicago sativa) were used to assess the tolerance of AMF to soil toxicity and to compare heavy metal uptake by nonmycorrhizal and mycorrhizal plants. Mycorrhizal parameters were high only in plants inoculated with G. mosseae. In the case of both Gigaspora species, the colonisation level was low, although the arbuscule richness was high. The alkaline phosphatase test showed the impact of soil toxicity on the vitality of the fungus. G. mosseae was demonstrated to be the most tolerant and Gi. margarita the most sensitive. Therefore, G. mosseae was most suitable as an indicator of the influence of soil toxicity on mycorrhizal colonisation level and morphological alterations.

The morphology of mycorrhizal structures was significantly altered. Arbuscules were often strongly septated or their branching was reduced. The accumulation of lipid like-bodies was observed. Arbuscule degeneration stages were different from those usually observed. The strongest morphological changes of arbuscules were observed in roots of plants grown on the Chrzanów waste, whereas in plants cultivated on the Trzebionka waste the arbuscules were the most healthy-looking. The same was observed in Gi. rosea. The extraradical mycelium often formed internal wall thickening, which were usually stained by rhodizonate.

G. mosseae reacts to industrial wastes by the formation of abundant crystals in vesicles and spores within plant roots. Most of the intraradical mycelium was filled with oil droplets at much higher abundance than in control roots. The SUDAN IV and Oil Red 0 staining, applied on roots stained with aniline blue, gave a positive reaction for both lipid droplets and crystals. The analysis of the extramatrical mycelium of several AMF species cultivated on industrial wastes showed that G. mosseae is much more sensitive than other strains of the genus Glomus studied. In this case, the reduction of the extramatrical mycelium was very high.

On this basis, we can conclude that morphological changes observed in mycorrhiza of G. mosseae are good indicators of soil pollution. This was supported by the experiments using substratum spiked with individual heavy metals. The strongest changes in arbuscule morphology were observed in the case of Zn. The arbuscules formed in roots of plants treated with Zn were often cauliflower-like, with poorly developed branches and an oily appearance. At the highest Zn concentrations the intraradical mycelium was poorly stained, but remnants of arbuscules were well recognizable. Arbuscules from Cd treated cultures were usually much smaller and not branched. Hyphal walls often formed darkly stained thickenings. No obvious modifications of arbuscule morphology were found in the case of Pb. Instead, dark precipitates were observed most commonly within the vacuole-like structures. With the increase of heavy metal concentration also the decrease of mycorrhizal colonisation level was noted. Few irregular vesicles were observed within roots of plants treated with the tested elements.

The above described differences show that observations done on mycorrhizal structures can clearly indicate not only the presence of the disturbance, but can also suggest the nature of this stress factor. This would lead to the conclusion that G. mosseae mycorrhiza could be used for the evaluation of soil toxicity, if soil samples are taken from a given area, sterilized and inoculated with the fungus of the known behaviour. Observations of native fungi in the soil are not appropriate, as their reaction patterns are mostly unknown. The second possibility involves spores of Gi. rosea, which is an excellent model for studying the impact of heavy metals on spore germination, also under laboratory conditions.

More information on the Genomyca -project can be found at: http://www.dijon.inra.fr/genomyca/

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