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THE SIGNALING PROCESS INVOLVED IN THE ACTIVATION OF THE MECHANISM OF DEFENSE OF FRUITS TO PATHOGEN ATTACK

D Beno Mohatem and D Prusky


Department of Postharvest Science of Fresh Produce, The Voicani Center POSS, Bet Dagan, Israel

Background and objectives
Post-harvest decay of avocado fruits by fungal pathogens, particularly Colletoffichurn gloeasporiaides is a major economic problem. Resistance of unripe avocado fruits to such postharvest pathogens is closely related to the presence in peel of the antifungal compound 1 -acetoxy-2-hydroxy-4-oxo-heneicosa-1 2,1 5 diene. Levels of this compound normally decrease during fruit ripening, corresponding to the appearance of decay by C. gloeospoiloide. Decrease of the antifungal diene has been attributed to the enzyme lipoxygenase, the activity of which is, in tum, inhibited by the flavan-3-ol, epicatechin. Thus, epicatechin appears to play a key role in fruit susceptibility during ripening by indirectly controlling levels of the antifungal diene.
It was suggested that fruit resistance could be modulated not only by delaying the normal decline of the antifungal diene during hpening but also by increased synthesis. Several biotic and abiotic elicitors in fact caused transient increases in levels of the antifungal diene. Challenge inoculation with the avocado pathogen.C. gloeosporioldes, or with a non-pathogenic mutant of C. magna also led to increased levels of the antifungal diene. Exposure of avocado fruits to 30% C02 for 24 h and short treatment with 30 ugli C2H4 similarly increased the level of the antifungal diene. Treatments that increased the diene level also led to high concentrations of epic-atec.hin in the fruit peel, suggesting that they might be affecting diene degradation rather than synthesis.
The expression of genes for phenylpropanoid pathway enzymes like phenylalanine ammonia lyase, chalcone synthase and flavanone 3 hydroxytase is often triggered by the abiotic or biotic elicitors described before.
The importance of epicatechin in post-harvest disease compelled us to test the singling process involved in the activation of the mechanism of resistance of avocado.


Results and conclusions
Plasma membranes of isolated pedcarp tissue inoculated with C. gloeosporioides , showed a rapid increase in NADPH dependent 02' generation. The generation of 02- coupled NADPH oxidase, increased by 3 fold within l h and remain higher than the control up to 4 hrs after inoculation. Treatment of isolated pericarp tissue with 1 mM H202 induced a rapid increase in the levels of epicatechin. PAL one of the enzymes involved in epicatechin synthesis also increased paraliely and its activity doubled 3 hrs after treatment with 1 mM H202. Preincubation of avocado peels in the presence of 1 0 $M staurosporine, a protein kinase inhibitor, inhibited the activation of PAL activity by H202 Present result indicate the presence of signal transduction events occurring in fruits at early stages during activation of defense genes involved in preformed resistance. The importance of signaling events by fungal elicitors and H202 will be discussed.


References
Prusky, D., 1996, Annual Review of Phytopathology-34:413-434