Mode of Action of Hymexazol in <i>Pythium aphanidermatum</i>

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  • <i>Pythium aphanidermatum</i> に対するヒメキサゾールの作用機構
  • Pythium aphanidermatumに対するヒメキサゾールの作用機構〔英文〕
  • Pythium aphanidermatum ニ タイスル ヒメキサゾール ノ

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Hymexazol (3-hydroxy-5-methylisoxazole) at 3μg/ml did not inhibit dry weight increase of hyphae of P. aphanidermatum during the initial 5hr exposure but almost completely inhibited growth thereafter. The fungicide did not inhibit respiration or respiratory expansion of growing hyphae during the initial 3hr, but prevented subsequent respiratory expansion. This indicates that the toxic action is triggered within 3hr. Hymexazol did not appreciably inhibit incorporation of 14C-acetate into fatty acids, triglycerides and phospholipids, or incorporation of the 14C-labeled methyl group of methionine into individual phospholipid classes during a 4hr or 3hr incubation period, respectively. Incorporation of 14C-phenyl-alanine into protein was not inhibited during a 4hr incubation period. Incorporation of 3H-uridine into RNA was inhibited approximately 50% after a 2hr incubation, but incorporation of 14C-aspartic acid into RNA was not inhibited until after 3hr. This suggests that the earlier effect on uridine incorporation may have resulted from inhibition of exogenous uridine transport or its conversion to a nucleotide. Hymexazol did not interfere with nuclear division or nuclear movement in germinating zoospores. Uptake of hymexazol by P. aphanidermatum from an ambient solution was extremely small and reached a maximum within 2hr; therefore the lag period associated with toxicity is apparently not due to time required for uptake. Differences in sensitivity of P. aphanidermatum, Pellicularia filamentosa f. sp. sasakii and Phytophthora drechsleri to hymexazol were not correlated with differences in uptake of the toxicant. The delay in toxicity of hymexazol suggests that it may require conversion to an active derivative which could inhibit major pathways such as RNA synthesis or that it directly inhibits an obscure pathway which leads to effects on growth only after an appreciable delay.

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