Acidification of apple and orange hosts by Penicillium digitatum and Penicillium expansum.

New information about virulence mechanisms of Penicillium digitatum and Penicillium expansum could be an important avenue to control fungal diseases. In this study, the ability of P. digitatum and P. expansum to enhance their virulence by locally modulating the pH of oranges and apples was evaluated. For each host, pH changes with a compatible pathogen and a non-host pathogen were recorded, and the levels of different organic acids were evaluated to establish possible relationships with host pH modifications. Moreover, fruits were harvested at three maturity stages to determine whether fruit maturity could affect the pathogens' virulence. The pH of oranges and apples decreased when the compatible pathogens (P. digitatum and P. expansum, respectively) decayed the fruit. The main organic acid detected in P. digitatum-decayed oranges was galacturonic acid produced as a consequence of host maceration in the rot development process. However, the obtained results showed that this acid was not responsible for the pH decrease in decayed orange tissue. The mixture of malic and citric acids could at least contribute to the acidification of P. digitatum-decayed oranges. The pH decrease in P. expansum decayed apples is related to the accumulation of gluconic and fumaric acids. The pH of oranges and apples was not affected when the non-host pathogen was not able to macerate the tissues. However, different organic acid contents were detected in comparison to healthy tissues. The main organic acids detected in P. expansum-oranges were oxalic and gluconic and in P. digitatum-apples were citric, gluconic and galacturonic. Further research is needed to identify the pathogenicity factors of both fungi because the contribution of organic acids has profound implications.

Use of GFP-tagged strains of Penicillium digitatum and Penicillium expansum to study host-pathogen interactions in oranges and apples.

Penicillium digitatum and Penicillium expansum are responsible for green and blue molds in citrus and pome fruits, respectively, which result in major monetary losses worldwide. In order to study their infection process in fruits, we successfully introduced a green fluorescent protein (GFP) encoding gene into wild type P. digitatum and P. expansum isolates, using Agrobacterium tumefaciens-mediated transformation (ATMT), with hygromycin B resistance as the selectable marker. To our knowledge, this is the first report describing the transformation of these two important postharvest pathogens with GFP and the use of transformed strains to study compatible and non-host pathogen interactions. Transformation did not affect the pathogenicity or the ecophysiology of either species compared to their respective wild type strains. The GFP-tagged strains were used for in situ analysis of compatible and non-host pathogen interactions on oranges and apples. Knowledge of the infection process of apples and oranges by these pathogens will facilitate the design of novel strategies to control these postharvest diseases and the use of the GFP-tagged strains will help to determine the response of P. digitatum and P. expansum on/in plant surface and tissues to different postharvest treatments.