The determinants of specificity from Mycosphaerella species

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The mechanism of plant host-parasite specificity was investigated with a system of Mycosphaerella blight of pea. Two types of resistance were induced in pea tissues by the treatment with the high molecular weight elicitor prepared from the spore germination fluid of M. pinodes. One of them was pisatin production, and the other was the formation of infection inhibitors. Pisatin accumulation was caused by the activation of pisatin synthetic pathway in wounded tissues within 6 h after the eliciter-treatment. The activation of phenylalanine ammonia-lyase(PAL), one of regulatory enzymes in the pathway, was induced within 3 h, and the expressions of PAL- and chalcone synthase-gene were detected within 1 h after the treatment. The concomitant presence of the low molecular weight suppressor, which was secreated into spore germination fluid of M. pinodes, resulted in delay of these defense reactions for at least 3 h. Another substance which inhibits the penetration of M. pinodes was detected in the elicitor solution placed on uninjured tissues 1-2 h after the treatment. Other fungal elicitors also induced the same type of rejection reaction in the treated tissues. This type of resistance was also negated by the concomitant presence of suppressor with elicitor. Some nonpathogenic fungi of pea were able to establish these infection on the suppressor-treated tissues. The tested fungi avirulent to pea were not able to produce the suppressor effective to pea plant. The suppressor of M. pinodes, therefore, seems to be responsible for the determination of specificity in pea plant-Mycosphaerella pinodes system.Mycosphaerella ligulicola and My. melonis also produce the determinants of specificity which may negate the defense reaction of respective host plant induced by elicitor. The purification of these substances containing sugar and peptide is under way.