Relationship Between Host Acidification and Virulence of Penicillium spp. on Apple and Citrus Fruit

Abstract

Penicillium expansum, P. digitatum, and P. italicum acidify the ambient environments of apple and citrus fruit during decay development. They use two mechanisms for this: the production of organic acids, mainly citric and gluconic, and NH(4)(+) utilization associated with H(+) efflux. Exposure of P. expansum and P. digitatum hyphae to pH 5.0 increased their citric acid production, compared with the production of organic acids at acidic ambient pH. In decayed fruit, both pathogens produced significant amounts of citric and gluconic acids in the decayed tissue and reduced the host pH by 0.5 to 1.0 units. Ammonium depletion from the growth medium or from the fruit tissue was directly related to ambient pH reduction. Analysis of transcripts encoding the endopolygalacturonase gene, pepg1, from P. expansum accumulated under acidic culture conditions from pH 3.5 to 5.0, suggesting that the acidification process is a pathogenicity enhancing factor of Penicillium spp. This hypothesis was supported by the finding that cultivars with lower pH and citric acid treatments to reduce tissue pH increased P. expansum development, presumably by increasing local pH. However, organic acid treatment could not enhance decay development in naturally acidic apples. Conversely, local alkalinization with NaHCO(3) reduced decay development. The present results further suggest that ambient pH is a regulatory cue for processes linked to pathogenicity of postharvest pathogens, and that specific genes are expressed as a result of the modified host pH created by the pathogens.

A New Versatile Microarray-based Method for High Throughput Screening of Carbohydrate-active Enzymes

“…T. versicolor typically grows on dead or decaying wood, which contains a limited set of poly- Tables 1 and 2, respectively. mers, mostly cellulose, lignin, and xylan (39). C. acutatum and P. expansum are some of the most important fungal pathogens of fruits including apples (40,41) and are therefore well adapted to grow on apple pomace medium. Fruits contain a rich variety of polysaccharides in primary and secondary walls, and this is reflected in the diverse array of plant cell wall-degrading enzymes secreted by these two fungal species.…”

In Vitro and In Vivo Activity of Selected Plant Crude Extracts and Fractions Against Penicillium Italicum

“…The filamentous fungus Penicillium italicum Wehmer (blue mould) causes a universal post-harvest disease of almost all kinds of citrus fruit (Prusky et al 2004). Economic losses caused by post-harvest pathogens are greater than is often realized and the avoidable losses between the farm gate and the consumer are of considerable concern (Soylu et al 2005).…”

Virulence Regulation of Phytopathogenic Fungi by pH

“…The ability of the pathogen to alter the pH locally has been described initially for Sclerotinia sclerotiorum, but similar findings been have extended to some other pathogens, such as Colletotrichum spp., Alternaria alternata, Botrytis cinerea, Penicillium expansum, P. digitatum, P. italicum, Phomopsis mangiferae, and Fusarium oxysporum (39, 52, 53, 89,100,101,123,124,129). To simplify the terminology in the following discussion, fungi that alkalinize the environment will be referred to as ''alkaline fungi'' and those that acidify it will be called ''acidic fungi'' (Fig.…”

“…S. sclerotiorum and Botrytis decrease the host pH by secreting significant amounts of oxalic acid (OA) (89,129), gluconic acid is secreted by Phomopsis mangiferae (39), and combinations of gluconic and citric acids are mainly secreted by Penicillium (124) and Aspergillus (130). Penicillium expansum acidifies the tissue to pH levels of 3.5 to 4.0, at which polygalacturonase ( pg1) transcription was found to be significantly enhanced (124). Acidic pH-specific expression of other members of the PG family was found in S. sclerotiorum (129) and in B. cinerea (157).…”

“…This pH regulation modulates the arsenal of proteins secreted by the attacking organisms (42, 107,121). A change in the ambient pH during fungal attack has been suggested to be a critical factor in the expression of pathogenicity factors such as pectate lyase (PL) and endoglucanases in several host/pathogen interactions (53, 123,124,159). This finding has raised the question of how great is the importance of environmental pH regulation for fungal pathogenicity genes.…”