Cellular Responses Required for Oxidative Stress Tolerance, Colonization, and Lesion Formation by the Necrotrophic Fungus Alternaria alternata in Citrus

Abstract

The pathogenic capability of the tangerine pathotype of Alternaria alternata relies on the production of host-selective ACT toxin. Inoculation of A. alternata in leaves of the citrus quickly induced rapid lipid peroxidation, accumulation of hydrogen peroxide (H(2)O(2)), and cell death, indicative of host defensive response. We previously demonstrated an essential role of the A. alternata AaAP1 gene, encoding a redox-responsive YAP1-like transcription factor, to contribute to fungal pathogenicity. The AaAP1 null mutant fails to incite necrotic lesions. In this study, we show further that the fungal mutant defective at the AaAP1 locus displayed reduced activities for glutathione-S-transferase, glutathione peroxidase, glutathione reductase, and ligninolytic peroxidase, yet retained normal production of ACT toxin. In contrast to the wild-type progenitor and the genetically reverted strain, the mutant strain was unable to detoxify H(2)O(2) effectively and was killed upon exposure to H(2)O(2). The mutant strain induced lower levels of H(2)O(2) accumulation in citrus leaves, compared to those induced by the wild-type or by the genetically reverted strain. Upon exposure to H(2)O(2), A. alternata apparently changed expression of a wide array of the genes regulated by AaAP1. Thus, the impairment of the AaAP1 null mutants to incite necrotic lesions is apparently a consequence of their inability to alleviate the toxicity of ROS, and circumvention of plant defenses is important for the disease process.

A Major Facilitator Superfamily Transporter-Mediated Resistance to Oxidative Stress and Fungicides Requires Yap1, Skn7, and MAP Kinases in the Citrus Fungal Pathogen Alternaria alternata

“…Northern blot hybridization revealed further that the accumulation of the AaMFS19 transcript was decreased in fungal strains defective for the stress-responsive transcription regulator Yap1 (Fig 4A), consistent with previous findings [10]. The AaMFS19 transcript was detected at similar levels in the wild-type and the Yap1 complementation strains.…”

“…alternata mutant impaired for the stress-responsive transcription regulator Yap1 increased hypersensitivity to a wide spectrum of oxidants as well as to both 2,3,5-triiodobenzoic acid (TIBA) and 2-chloro-5-hydroxypyridine (CHP) compared to its progenitor strain [10]. To test whether or not AaMFS19 plays a role in resistance to oxidants, CHP, and TIBA, A .…”

“…alternata after subtraction with cDNA from a Yap1 mutant [10]. In the present study, AaMFS19 containing 12 transmembrane helixes has been shown to be required for cellular resistance to cumyl H 2 O 2 , KO 2 , and many singlet oxygen-generating compounds including eosin Y, rose Bengal, hematoporphyrin, methylene blue, and cercosporin [25].…”

“…alternata pathogenesis [9]. Alternaria alternata infection in citrus leaves triggers rapid lipid peroxidation and accumulation of hydrogen peroxide (H 2 O 2 ), which eventually leads to cell death [10]. Experiments have demonstrated that the ability to detoxify toxic reactive oxygen species (ROS) is too required for A .…”

“…alternata [10]. In the present study we report a functional characterization of a 12-spanner MFS transporter to explore its roles in resistance to oxidants and fungicides, and in virulence.…”