METABOLISM OF FLAVONOIDS AND MYCOTOXINS IN CITRUS FRUITS INFECTED BY ALTERNARIA ALTERNATE

Fungi of the genus Alternaria are responsible for substantial pre-harvest losses in Citrus, although susceptibility to A. alternate depends on the Citrus species. In this work the metabolism of flavonoids and mycotoxins were analyzed in ‘Fortune’ fruits infected by A. alternate. HPLC-MS analysis revealed the presence of two A. alternate mycotoxins involved in the evolution of brown spot in fruits. These were identified as 3,7,9-trihydroxy-1-methyl-6H-dibenzo [b,d] pyran-6-one, known as alternariol (AOH), and 3,7-dihydroxy-9-methoxy-1-methyl-6H-dibenzo[b,d] pyran-6-one, known as alternariol monomethyl ether (AME). The flavanones hesperidin and isonaringin, and the polymethoxyflavone sinensetin increase the biosynthesis of these micotoxins. Inoculation of fruits with the above-mentioned toxins showed that both molecules caused necrosis in the inoculated citrus tissues, suggesting that the mycotoxins may be involved in the evolution of brown spot caused by A. alternate. The increased expression of the above mycotoxins was associated with: (i) the end of mycelial growth, high sporulation and an increase in hyphal melanisation; (ii) catabolism of the flavonoids present in these fruits.