Abstract
The production of host-selective toxins by the necrotrophic fungus Alternaria alternata is essential for the pathogenesis. A. alternata infection in citrus leaves induces rapid lipid peroxidation, accumulation of hydrogen peroxide (H2O2), and cell death. The mechanisms by which A. alternata avoids killing by reactive oxygen species (ROS) after invasion have begun to be elucidated. The ability to coordinate of signaling pathways is essential for the detoxification of cellular stresses induced by ROS and for pathogenicity in A. alternata. A low level of H2O2, produced by the NADPH oxidase (NOX) complex, modulates ROS resistance and triggers conidiation partially via regulating the redox-responsive regulators (YAP1 and SKN7) and the mitogen-activated protein (MAP) kinase (HOG1) mediated pathways, which subsequently regulate the genes required for the biosynthesis of siderophore, an iron-chelating compound. Siderophore-mediated iron acquisition plays a key role in ROS detoxification because of the requirement of iron for the activities of antioxidants (e.g., catalase and SOD). Fungal strains impaired for the ROS-detoxifying system severely reduce the virulence on susceptible citrus cultivars. This paper summarizes the current state of knowledge of signaling pathways associated with cellular responses to multidrugs, oxidative and osmotic stress, and fungicides, as well as the pathogenicity/virulence in the tangerine pathotype of A. alternata.
Highlights
Alternaria species have different lifestyles, ranging from saprophytes to endophytes to pathogens [1]
Our studies have demonstrated that cellular detoxi cation of reactive oxygen species (ROS) regulated by the redox-responsive YAP1 transcription regulator is important for pathogenesis of A. alternata to citrus [83, 84]
Our studies revealed that formation of conidia by A. alternata is closely regulated by the FUS3 and SLT2 mitogen-activated protein (MAP) kinases-mediated signaling pathways, as well as by the G-protein and the NADPH oxidase (NOX) complex [125, 165,166,167]
Summary
Alternaria species have different lifestyles, ranging from saprophytes to endophytes to pathogens [1]. Alternaria species are a highly successful group of fungal pathogens that cause diseases in a wide variety of economically important crops, including apple, broccoli, cauli ower, carrot, citrus, pear, rice, strawberry, tomato, potato, and tobacco, as well as many ornamental and weed species. Alternaria alternata (Fr.) Keissler has several pathogenic variants, each producing a unique HST and causing disease in different host plants [5, 9, 10, 14, 15]. The tangerine pathotype of A. alternata produces the host-selective ACT toxin with a core 9,10-epoxy-8-hydroxy-9-methyldecatrienoic acid structure [20] and causes brown spots on citrus leaves and fruit. Studies show that A. alternata has evolved a dramatic exibility and uniqueness in the signaling pathways in order to respond to diverse environmental stimuli and to thrive within host plants. is paper discusses signaling pathways related to oxidative and osmotic stress resistance, fungicide sensitivity, conidia formation, and pathogenesis of A. alternata
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