Antibiotics as protectant fungicides against rice blast

Abstract

Effect of an antibiotic, blasticidin S, on respiration of spores, mycelial pellets, and isolated mitochondria of Piricularia oryzae was studied. Blasticidin S was previously reported to be effective as a therapeutic fungicide against the rice blast.The spores were obtained by culturing the fungus on a medium consisted of 1% soluble starch, 0.2% yeast extract powder, 10% rice stem juice, and 1.5% agar, at 27°C for 10-14 days. The mycelial pellets were prepared by seeding 70ml of liquid medium (2% glucose, yeast extract powder 0.5%, rice stem juice 10%) in 500ml flasks with appropriate amount of spores, and culturing in a reciprocal shaking machine operating at about 120 strokes per minute at 27°C, for 3 days. The mitochondrial fraction was prepared from the above mentioned mycelial pellets by the Schneider's method. The respiration was measured in the usual way in a 12-unit rotary Warburg respirometer at 30°C.Blasticidin S partially inhibited the oxygen uptake of the spores in the presence of glucose, pyruvate, succinate, or glutamate at the minimum spore germination inhibitory concentration (1μg/ml). The inhibition was still not complete at a concentration of 10μm/ml. Endogenous respiratin of the spores was not inhibited at this concentration.Inhibition of respiration of the mycelial pellets caused by blasticidin S was almost the same as in the case of the spores. The oxygen uptake of the mycelial pellets in the presence of the abovementioned substrates was partially inhibited at the minimum mycelial growth inhibitory concentration (0.1μg/ml). Exogenous respiration was not completely inhibited at a concentration of 100μg/ml. Endogenous respiration of the pellets was not inhibited at this concentration.The oxygen uptake of the mitochondrial fraction of the mycelial pellets in the presence of succinate was inhibited by blastmycin (antimycin A) at a concentration of 5μg/ml, but not by blasticidin S at a concentration of 500μg/ml. Therefore, blastcidin S was not considered to be directly inhibiting the succinoxidase system in the fungus.