Graduate Thesis Or Dissertation
 

Copper tolerance in four fungal species

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/z316q458z

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  • This study was initiated to determine the tolerance to copper of Aspergillus niger, Stemphylium sarcinaeforme, Monilinia fructicola and Fusarium oxysporum f. sp. lycopersici. Included were strains of the latter two fungi that were induced to tolerate relatively high levels of copper by continual culture on sublethal levels of the toxicant. Some of the effects of copper on the physiological activities of the copper-tolerant and parent strains were studied. The tolerances of the mycelia of Fusarium and Monilinia were increased 12- and 8-fold, respectively. The spores of the tolerant strains of Fusarium and Monilinia could withstand concentrations of copper 60 and 17 times higher, respectively, than the parent strains when measured by the inhibition of spore germination. Measured by the ability of spores to form colonies after a copper treatment, the tolerant strains of Fusarium and Monilinia were 50 and 5 times more tolerant, respectively, than their parent strains. Polymodal dosage-response curves were obtained for the inhibition of spore germination of Aspergillus, Stemphylium and both strains of Monilinia, following a two-hour treatment with copper sulfate. Nearly linear dosage-response curves were observed for the parent and tolerant strains of Fusarium, but complete inhibition of germination was not reached even at a concentration of 1M CuSO₄. The polymodal curves were not a characteristic of the particular treatment method nor were they influenced by pH or caused by differential uptake of copper as the concentration increased. The ED₅₀ concentration of copper for inhibition of spore germination increased when filtrates from spore suspensions of parent and tolerant strains of Monilinia and Fusarium growing on solid media were included in the spore treatment solutions. When spores were treated with copper sulfate in their own exudates, the ED₅₀ concentrations were increased 22-, 56-, and 375-fold for the tolerant and parent strains of Fusarium and the parent strain of Monilinia, respectively. However, exudates from the tolerant Fusarium strain were 5-fold more effective in protecting spores of the parent Fusarium strain from inhibition of spore germination by copper than the parent strain exudates. Exudates from the tolerant Fusarium strain protected spores of the parent Monilinia strain, but the protection was less than with exudates from either Monilinia strain. Nine different amino acids were present in exudates from the tolerant strain of Fusarium but only two were present in the parent strain exudates. The exudates from the tolerant and parent Monilinia strains contained 15 and 14 amino acids, respectively. The concentration of individual amino acids in the exudates varied between the tolerant and parent strains of both fungi, but generally, concentrations were higher in exudates from the tolerant strains. The Fusarium strains were grown in liquid culture and the amino acid content of the culture filtrates and the free and bound amino acids in the spores and mycelium were determined. As with the exudates, more kinds and higher concentrations of amino acids were present in culture filtrates, spores and mycelium of the tolerant strain than the parent strain. Addition of copper to the growth medium increased the concentration of amino acids with both strains, except that the concentration of free amino acids in the tolerant strain was decreased. The amounts of copper remaining in culture filtrates from the 1mM copper treatment of the two Fusarium strains were nearly equal, but there was almost twice as much copper with the free amino acids from the spores and mycelium of the tolerant strain as the parent strain. On the other hand, higher copper concentrations were found with the bound amino acids of the spores and mycelium of the parent strain than the tolerant strain. This suggests that copper is bound at different sites in the two strains. The amino acids in the culture filtrates and the free amino acids in the spores and mycelium can combine with copper and reduce its toxicity. Since more amino acids are produced by the tolerant strain than by the parent strain, this could explain the acquired tolerance.
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