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 enhancing host resistance to green mold. Oranges (Citrus sinensis cv. 'Valencia') treated with inducing agents and challenged by the green mold pathogen (Penicillium digitatum) showed a delay in the onset and progression of disease symptoms compared with inoculated fruits not treated with the elicitors. Chitosan (a preparation of ground crab shells), Margosan-O (an oil-based plant-derived product from neem seed) + Aspire (a water dispersible granule containing an antagonistic yeast), Aspire, and chitosan + Aspire reduced fruit decay 38, 41, 42, and 44%, respectively. The inducing agents reduced disease incidence but not disease severity. Application of elicitors followed by inoculation with P. digitatum and P. digitatum infection alone increased total soluble proteins in the flavedo (the tissue that forms the outer colored rind) twofold relative to the untreated control. The flavedo is an important tissue that is vulnerable to postharvest diseases especially at storage and transport of the harvested crop. No apparent qualitative differences were visualized in protein patterns analyzed by SDS-PAGE of all treatments across all days of incubation. A temporal differential induction 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 immunodetection. Lysozyme and a polygalacturonase-inhibiting protein were detected at low activity levels. However, the defensive proteins appeared to be constitutive and slightly induced but did not involve the de novo synthesis of novel proteins.