Coordinate control of oxidative stress tolerance, vegetative growth, and fungal pathogenicity via the AP1 pathway in the rough lemon pathotype of Alternaria alternata

Successful pathogenesis by the necrotrophic pathogen Alternaria alternata in citrus has been thought to be dependent on the production of host-specific toxins. This study determined whether the fungal pathogen can modulate reactive oxygen species (ROS) in the host plant by cloning and characterizing an RLAP1 gene in the rough lemon pathotype of A. alternata, which encodes a polypeptide that resembles many YAP1-like transcription activators of yeasts and fungi. Expression of the RLAP1 gene in a wild-type strain was responsive to many ROS-generating compounds. Mutants disrupted at the RLAP1 locus were hypersensitive to H2O2 and ROS-generating oxidants, showed reduced vegetative growth, were less effective in detoxifying H2O2, were non-pathogenic to rough lemon, and yet were unaltered in conidial formation or toxin production. When inoculated through wound sites, the mutants failed to incite any lesions. However, co-inoculation of the mutants with an NADPH oxidase inhibitor apocynin or diphenylene iodonium slightly restored fungal pathogenicity; introduction of the RLAP1 gene to a null mutant fully restored all phenotypes to wild type. Upon activation by H2O2, RLAP1::sGFP fusion protein is localized to the nucleus. Overall, the results reveal the essential role of effective detoxification via an RLAP1-mediated pathway in oxidative stress response and pathogenicity in A. alternata. It is proposed that the function of RLAP1 in A. alternata is to protect the pathogen from the toxicity of ROS generated by the citrus host.