Polygalacturonase gene pgxB in Aspergillus niger is a virulence factor in apple fruit

Aspergillus niger, a saprophytic fungus, is widely distributed in soil, air and cereals, and can cause postharvest diseases in fruit. Polygalacturonase (PG) is one of the main enzymes in fungal pathogens to degrade plant cell wall. To evaluate whether the deletion of an exopolygalacturonase gene pgxB would influence fungal pathogenicity to fruit, pgxB gene was deleted in Aspergillus niger MA 70.15 (wild type) via homologous recombination. The Delta pgx13 mutant showed similar growth behavior compared with the wild type. Pectin medium induced significant higher expression of all pectinase genes in both wild type and Delta pgx13 in comparison to potato dextrose agar medium. However, the Delta pgxB mutant was less virulent on apple fruits as the necrosis diameter caused by Delta pgxB mutant was significantly smaller than that of wild type. Results of quantitive-PCR showed that, in the process of infection in apple fruit, gene expressions of polygalacturonase genes pgal, pgall, pgaA, pgaC, pgaD and pgaE were enhanced in Delta pgxB mutant in comparison to wild type. These results prove that, despite the increased gene expression of other polygalacturonase genes in Delta pgxB mutant, the lack of pgxB gene significantly reduced the virulence of A. niger on apple fruit, suggesting that pgxB plays an important role in the infection process on the apple fruit.