DIFFERENTIAL INDUCTION OF PROTEINS IN ORANGE FLAVEDO BY BIOLOGICALLY-BASED ELICITORS AND CHALLENGED BY PENICILLIUM-DIGITATUM SACC

The effects of biologically based inducing agents (elicitors) applied singly or in combination to harvested oranges were investigated for en hancing host resistance to green mold. Oranges (Citrus sinensis cv. 'V alencia') treated with inducing agents and challenged by the green mol d pathogen (Penicillium digitatum) showed a delay in the onset and pro gression of disease symptoms compared with inoculated fruits not treat ed with the elicitors. Chitosan (a preparation of ground crab shells), Margosan-O (an oil-based plant-derived product from neem seed) + Aspi re (a water dispersible granule containing an antagonistic yeast), Asp ire, and chitosan + Aspire reduced fruit decay 38, 41, 42, and 44%, re spectively. The inducing agents reduced disease incidence but not dise ase severity. Application of elicitors followed by inoculation with P. digitatum and P. digitatum infection alone increased total soluble pr oteins in the flavedo (the tissue that forms the outer colored rind) t wofold relative to the untreated control. The flavedo is an important tissue that is vulnerable to postharvest diseases especially at storag e and transport of the harvested crop. No apparent qualitative differe nces were visualized in protein patterns analyzed by SDS-PAGE of all t reatments across all days of incubation. A temporal differential induc tion of chitinase, beta-1,3-glucanase, and peroxidase was demonstrated as a result of elicitor application followed by challenge inoculation with P. digitatum. Induction of these enzymes was corroborated by imm unodetection. Lysozyme and a polygalacturonase-inhibiting protein were detected at low activity levels. However, the defensive proteins appe ared to be constitutive and slightly induced but did not involve the d e novo synthesis of novel proteins.