Elicitation of Preformed Antifungal Compounds by Non-Pathogenic Fungus Mutants and their Use for the Prevention of Postharvest Decay in Avocado Fruits

Abstract

C. gloeosporioides attacks unripe avocado fruits in the orchard. Germinated spores produce appressoria that germinate and breach the cuticle, but the resultant subcuticular hyphae become quiescent and do not develop further until fruit is harvested and ripens. Resistance of unripe avocado to attach by C. gloeosporioides is correlated with the presence of fungitoxic concentrations of the preformed antifungal compound, 1-acetoxy-2-hydroxy-4-oxoheneicosa-12, 15 diene in the pericarp of unripe fruits. The objective of this proposal was to study the signal transduction process by which elicitors induce resistance in avocado. It was found that abiotic elicitors, infection of avocado fruit with C. gloeosporioides or treatment of avocado cell suspension with cell-wall elicitor induced a significant production of reactive oxygen species (ROS). Ripe and unripe fruit tissue differ with regard to the ROS production. The unripe, resistant fruit are physiologically able to react and to produce high levels of ROS and increased activity of H+ATPase that can enhance the phenylpropanoid pathway ad regulate the levels of the antifungal compound-diene, inhibit fungal development, resulting in its quiescence. Interestingly, it was also found that growth regulators like cytokinin could do activation of the mechanism of resistance. Postharvest treatments of cytokinins strongly activated the phenylpropanoid pathway and induce resistance. We have developed non-pathogenic strains of C. gloeosporioides by Random Enzyme Mediated Integration and selected a hygromycin resistance, non-pathogenic strain Cg-142 out of 3500 transformants. This non-pathogenic isolate activates H+ATPase and induces resistance against Colletotrichum attack. As a basis for studying the importance of PL in pathogenicity, we have carried out heterologous expression of pel from C. gloeosporioides in the non-pathogenic C. magna and determine the significant increase in pathogenicity of the non-pathogenic strain. Based on these results we can state that pectate lyase is an important pathogenicity factor of C. gloeosporioides and found that fungal pathogenicity is affected not by pel but by PL secretion. Our results suggest that PH regulates the secretion of pectate lyase, and support its importance as a pathogenicity factor during the attack of avocado fruit by C. gloeosporioides . This implicates that if these findings are of universal importance in fungi, control of disease development could be done by regulation of secretion of pathogenicity factors.