NOXA Is Important for Verticillium dahliae's Penetration Ability and Virulence

NADPH oxidase (Nox) genes are responsible for Reactive Oxygen Species (ROS) production in living organisms such as plants, animals, and fungi, where ROS exert different functions. ROS are critical for sexual development and cellular differentiation in fungi. In previous publications, two genes encoding thioredoxin and NADH-ubiquinone oxidoreductase involved in maintaining ROS balance were shown to be remarkably induced in a highly versus a weakly aggressive Verticillium dahliae isolate. This suggested a role of these genes in the virulence of this pathogen. NoxA (NADPH oxidase A) was identified in the V. dahliae genome. We compared in vitro expression of NoxA in highly and weakly aggressive isolates of V. dahliae after elicitation with extracts from different potato tissues. NoxA expression was induced more in the weakly than highly aggressive isolate in response to leaf and stem extracts. After inoculation of potato detached leaves with these two V. dahliae isolates, NoxA was drastically up-regulated in the highly versus the weakly aggressive isolate. We generated single gene disruption mutants for NoxA genes. noxa mutants had significantly reduced virulence, indicating important roles in V. dahliae pathogenesis on the potato. This is consistent with a significant reduction of cellophane penetration ability of the mutants compared to the wild type. However, the cell wall integrity was not impaired in the noxa mutants when compared with the wild type. The resistance of noxa mutants to oxidative stress were also similar to the wild type. Complementation of noxa mutants with a full length NoxA clones restored penetration and pathogenic ability of the fungus. Our data showed that NoxA is essential for both penetration peg formation and virulence in V. dahliae.