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Resistance of Gibberellin-Treated Persimmon Fruit to Alternaria alternata Arises from the Reduced Ability of the Fungus to Produce Endo-1,4-β-Glucanase

First, second, and fourth authors: Department of Postharvest Science of Fresh Produce, Institute for Technology and Storage of Agricultural Products, The Volcani Center, Agricultural Research Organization, P.O. Box 6, Bet Dagan 50250, Israel; and third author: Department of Plant Pathology and Microbiology, The Hebrew University of Jerusalem, Faculty of Agricultural, Food and Environmental Quality Sciences, Rehovot 76100, Israel

Black-spot symptoms, caused by Alternaria alternata, developed in persimmon fruits during prolonged storage at -1°C. A preharvest treatment with gibberellic acid (GA₃) extended the storage life of the fruit by delaying both black-spot development and fruit softening. Conversely, treatment of persimmon fruits with paclobutrazol (PBZ), an inhibitor of gibberellin (GA) synthesis, enhanced black-spot development and fruit softening during storage. Production of endo-1,4-β-glucanase (EC 3.2.1.4, EG) by A. alternata in culture and in the presence of cell walls from PBZ-treated fruits as the carbon source, was enhanced by 150% over production in the presence of cell walls from control fruits, whereas endoglucanase (EG) production in the presence of cell walls from GA₃-treated fruits was reduced by 49% relative to controls. To determine the importance of EG in symptom development, A. alternata EG was purified from a culture-inducing medium. It had a molecular mass of 41 kDa, its optimal pH and temperature for activity were 5.5 and 47°C, respectively, and the pI was 4.3. Its K_m and V_max were 0.43 mg ml^-1 and 18 μmol reducing groups minute per milligrams of protein, 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 Trichoderma reesei and of T. longibrachiatum. Purified EG induced black-spot symptoms on the fruit, similar to those caused by A. alternata, whereas boiled enzyme caused only pricking signs. Our results suggest that the black-spot symptoms caused by A. alternata, in persimmon, are related to the ability of the fungus to produce EG in developing lesions.

Additional keywords: postharvest diseases, quiescent infection.