Alternariolide, a host-specific toxin responsible for Alternaria blotch of apple, has been characterized as a depsipeptide of the planar formula 1 on the basis of spectroscopic evidence.
Valsa ceratosperma, whichis the pathogenic fungus of apple canker, was grownin a synthetic medium. The neutral extract from the culture filtrate was chromatographedon a silica gel column to give five isocoumarins. Their structures were determined by MS, UV, IR, *H and 13C NMR,and CD spectra. Three of them were known compounds; (-)-5-methylmellein (1), (-)-5-carT boxylmellein (2) and (-)-5-hydroxylmethylmellein (3). Since the absolute configurations at C-3 in 2 and 3 were not known until now, both were determined to be R by chemical correlations. The two were new compounds; (+)-(37?,4S)-/ra/M-4-hydroxy-5-methylmellein (4) and (-)-(3R,4R)-cis-4-hydroxy-5-methylmellein (5). All the five compounds showed phytotoxicity in a bioassay using detached apple shoots and lettuce seedlings. The fungus, Valsa ceratosperma, is the pathogen of apple canker, which is one of the most harmful diseases to apple growing in the northern parts of Japan. Wetook an interest in investigating secondary metabolites of the fungus and their influence on phytotoxicity in the host plant. Now, we wish to report the isolation and structural determination of new isocoumarins (4 and 5) and the known ones (1, 2 and 3) produced by the title fungus, and their phytotoxicities against detached apple shoots and lettuce seedlings.
Endopolygalacturonase I [EC 3.2.1.15], the major component of endopolygalacturonases causing silver-leaf symptoms, was purified from culture liquids of Stereum purpureum by column chromatographies on CM-52and Sephadex G-100. The purified enzyme was homogeneouson polyacrylamide gel electrophoresis and ultracentrifugation. The sedimentation coefficient (520)W) was determined to be 3.21 S, and the molecular weight was estimated to be 40,000 by gel filtration, 41,000 by SDS-polyacrylamide gel electrophoresis and 44,000 by sedimentation equilibrium. The enzyme had an isoelectric point of pH 8.5. The optimal pH of the enzyme was 3.5 for trigalacturonic acid, 4.0 for tetragalacturonic acid, and 4.5 for pectic acid. The enzymewas stable in the range of pH 4.0 to 9.0 and up to 70°C for 30min. The amount of the enzyme which was required to induce silver-leaf symptomson apple trees was 20/ig/tree.
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