Fungistatic action of Aureobasidium pullulans on Penicillium expansum in “Rocha” pear: implications for oxidative stress during fruit storage

Abstract

In “Rocha” pear, postharvest diseases, blue and grey moulds caused by Penicillium expansum and Botrytis cinerea, respectively, are particularly important. The control of postharvest decay has been based on the use of synthetic fungicides. However, there are risks to consumer health and the environment. In previous work, the antagonistic activity of the yeast-like fungus Aureobasidium pullulans (isolated from the surfaces of fruit, leaves and branches of “Rocha” pear trees), when applied as a wound treatment, was able to significantly reduce postharvest decay caused by P. expansum under storage conditions, and during simulated shelf-life. For more effective prevention and reduction of postharvest diseases in fruits it is necessary to understand the complex interactions in biological control between host, pathogen and antagonist, for which information is currently lacking. Thus, the aims of this study were to evaluate the biocontrol activity of the yeast-like fungus A. pullulans against P. expansum and to elucidate the resistance mechanism involved in the antagonistic activity of A. pullulans to oxidative stress. Fruits of Pyrus communis L. cv. Rocha were stored at −0.5°C, 95% relative humidity for 5.5 months. Fruit maturity was measured in terms of pericarp colour, sugar analysis, Streif index and flesh firmness. Assays of carbon dioxide and ethylene, electrolyte leakage, ascorbate, glutathione and antioxidant enzymes were used to evaluate oxidative stress. Furthermore, malondialdehyde, considered as a final product of lipid peroxidation, and the levels of hydrogen peroxide and methanesulfonic acid were determined and linked to oxidative stress impact on the lipid phase of cell membranes. During a 5-month period, fruit firmness, value of hue angle and colour, and Streif index changed significantly. Sucrose content decreased whereas fructose, glucose and sorbitol levels increased. After inoculation with A. pullulans and infection with P. expansum, a fungistatic, rather than fungicidal, effect was detected from 2.5 months onwards. Three months after storage, the higher superoxide dismutase activity and hydrogen peroxide levels caused by A. pullulans plus P. expansum were possibly associated with additional unsaturated fatty acids, as reflected by increased acyl lipid peroxidation. After five months of storage, in tissues inoculated with A. pullulans followed by infection with P. expansum, oxidative stress was linked to a general increase in the antioxidant peroxidase system activity. Glutathione and ascorbate content also increased along with the loss of cellular membrane selectivity. The results revealed a fungistatic effect of A pullulans on P. expansum. The prevalence of oxidative stress on “Rocha” pear wounds during the storage period was characterized, taking into account the interactions between A. pullulans and P. expansum. The information gained forms the basis for a biocontrol strategy, and the development of successful formulations for application in the orchard and after harvest. This strategy could eradicate and protect fruit against postharvest diseases, and is thus a promising alternative to the use of synthetic pesticides, being safe for the environment and for human health.