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Physiology and Biochemistry

Endopolygalacturonase from Valencia Oranges Infected with Diplodia natalensis. Charles R. Barmore, Associate professor, University of Florida, Institute of Food and Agricultural Sciences and Florida Department of Citrus, Citrus Research and Education Center, Lake Alfred 33850; Sandra E. Snowden(2), and G. Eldon Brown(3). (2)(3)Graduate research assistant, and research scientist III, respectively, University of Florida, Institute of Food and Agricultural Sciences and Florida Department of Citrus, Citrus Research and Education Center, Lake Alfred 33850. Phytopathology 74:735-737. Accepted for publication 15 February 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-735.

The decay of oranges (Citrus sinensis ' Valencia' ) by Diplodia natalensis was found to be associated with two endopolygalacturonase (endo-PG) isozymes with molecular weights of 64,000 and 45,000 daltons. The endo-PG isozymes were apparently of fungal origin, since endo-PG was not detected in injured-uninfected rind, but was produced by the fungus during growth in vitro. Pectin degradation was aided by endogenous pectinmethylesterase (PE) which caused demethylation of pectin in the host cell walls at the edge of the lesion. In vitro production of PE by D. natalensis was not observed. Endo-PG activity in rind decayed by D. natalensis was substantially less than in comparable tissue infected with Penicillium italicum. In vitro production of endo-PG by D. natalensis, but not by P. italicum, was repressed during growth of the fungus. In addition, endo-PG activity in the exudate from tissue decayed by D. natalensis was minimal. However, endo-PG activity was substantially increased by adjusting the pH of the exudate from its initial level of pH 3.6 to pH 8.5 prior to assay at pH 5.0, optimum pH for enzyme activity. Cell walls at the edge of the lesion were slightly swollen, and maceration of tissue at the site of hyphal penetration was more extensive in the mesocarp than in the exocarp.