Role of gluconic acid in pathogenicity of Aspergillus carbonarius in grapes

Many fungi are epiphytic on grapes in the vineyard, but the main concern is for the black mycotoxigenic Aspergillus species that causes severe losses. Aspergillus carbonarius causes severe postharvest decay of grapes in the orchard, and is considered as the major source of ochratoxin A (OTA) contamination of grapes and derived products. Our previous findings indicated that production of organic acids, such as D-gluconic acid (GLA), 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 takes place. The contribution of GLA secretion to the pathogenicity of A. carbonarius in grapes and its involvement in OTA biosynthesis remain unclear. Deletion of the gene encoding glucose oxidase (gox) in A. carbonarius was carried out to suppress the conversion of glucose to GLA with the aim to investigate the roles of GLA and OTA accumulation in A. carbonarius pathogenicity. The obtained results showed that the GLA accumulation was completely inhibited in grape berries infected with ∆gox knockout mutant, which was accompanied by a concomitant reduction in decay development compared to the wild-type strain, suggesting that tissue acidification significantly contribute to A. carbonarius pathogenicity. Interestingly, however an increased production of OTA was detected by ∆gox mutant both in vitro and in vivo, suggesting that not GLA production, but other environmental factors, such as high sugar content, may determine the accumulation of OTA. The present data suggest that GLA, but not OTA, is contributing to Aspergillus colonization in grapes.