Resistance of gibberellin-treated persimmon fruit to Alternaria alternata arises from the reduced ability of the fungus to produce endo-1,4-beta-glucanase

Black-spot symptoms, caused by Alternaria alternata, developed in persimmon fruits during prolonged storage at -1 degreesC. A preharvest treatment wit h gibberellic acid (GA(3)) extended the storage life of the fruit by delayi ng both black-spot development and fruit softening. Conversely, treatment o f persimmon fruits with paclobutrazol (PBZ), an inhibitor of gibberellin (G A) synthesis, enhanced black-spot development and fruit softening during st orage. Production of endo-1,4-beta -glucanase (EC 3.2.1.4, EG) by A. altern ata in culture and in the presence of cell walls from PBZ-treated: fruits a s the carbon source, was enhanced by 150% over production in the presence o f cell walls from control fruits, whereas endoglucanase (EG) production in the presence of cell walls from GA(3)-treated fruits was reduced by 49% rel ative to controls. To determine the importance of EG in symptom development , A. alternata EG was purified from a culture-inducing medium. It had a mol ecular mass of 41 kDa, its optimal pH and temperature for activity were 5.5 and 47 degreesC. respectively, and the pi was 4.3. Its K-m and V-max were 0.43 mg ml(-1) and 18 mu mol reducing groups minute per milligrams of prote in, respectively. The internal sequence of a 21-mer amino acid peptide from the purified EG showed 62% similarity and 38% identity to the EG-1 of Tric hoderma reesei and of T. longibrachiatum. Purified EG induced black-spot sy mptoms on the fruit, similar to those caused by A. alternata, whereas boile d enzyme caused only pricking signs. Our results suggest that the black-spo t symptoms caused by A. alternata, in persimmon, are related to the ability of the fungus to produce EG in developing lesions.