RELATIONSHIP BETWEEN AMBIENT pH MODULATION AND OCHRATOXIN A ACCUMULATION BY ASPERGILLUS CARBONARIUS

Many fungi are epiphytic on grapes in the vineyard, but the main concern is the group of black mycotoxigenic Aspergillus species. These species are the causal fungal pathogens of black rot diseases in the field, and may cause severe postharvest fruit decay and accumulation of secondary metabolites such as the mycotoxin ochratoxin A (OTA) in the colonized tissue. Aspergillus carbonarius is considered as the major sourse of OTA contamination in grapes and derived products . OTA contamination of grapes constitutes a serious health and economic problem, primarily in the areas of the world with Mediterranean-type climates. Unlike other important mycotoxins, the mechanism of OTA accumulation and its contribution to A. carbonarius pathogenicity has not yet been completely elucidated in detail. Host pH modulation by pathogens has been suggested as an important factor in enhancing the virulence of postharvest pathogens. The organic acids production, such as D-gluconic acid (GLA) and citric acid, by A. carbonarius in the growth medium or in the decayed fruit tissue was directly related to ambient pH reduction. Under these conditions, induced transcript expression of genes involved in OTA biosynthesis concurrently with the mycotoxin accumulation has been observed. The high accumulation of OTA in decaying tissue during the acidification process raised the question of its of its biological role in A. carbonarius’ pathogenicity. Treatment of colonized grapes with sodium bicarbonate minimized OTA secretion with a concomitant reduction in decay development, suggesting that tissue acidification is a significant pathogenicity factor of A. carbonarius. The present data suggest that ambient pH is a regulatory signal for induction of mycotoxin production caused by A. carbonarius under the dynamic nutritional growth conditions occuring in culture. The molecular mechanisms inducing OTA biosynthesis during colonization in the host acidic environment are under investigation.