Stress Response and Pathogenicity of the Necrotrophic Fungal PathogenAlternaria alternata

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.

Resistance to oxidative stress via regulating siderophore-mediated iron acquisition by the citrus fungal pathogen Alternaria alternata

“…Fungal strains lacking NPS6, YAP1, HOG1, SKN7, or NOX all show increased sensitivity to ROS and reduced virulence in citrus. Thus we conclude that the reduced virulence of the mutants is likely attributable to their inability to deal with toxic ROS in planta (Chung, 2012). Our results support the important role of ROS detoxification during A. alternata pathogenesis in citrus.…”

“…Although the early stages of Alternaria brown spot in citrus require the production of host-selective toxin, our previous studies showed that the colonization of A. alternata in its host plants relies also on the ability to detoxify reactive oxygen species (ROS) Chung, 2012). During the course of host colonization, fungal pathogens of plants often need to overcome an oxidative burst resulting from the production and accumulation of highly toxic ROS (Apel & Hirt, 2004).…”

The Thioredoxin GbNRX1 Plays a Crucial Role in Homeostasis of Apoplastic Reactive Oxygen Species in Response to Verticillium dahliae Infection in Cotton

“…S8). The increased fungal colonization may have resulted from the high levels of extracellular ROS, which were reported to be important for the development and pathogenicity of some necrotrophic pathogens, such as Botrytis cinerea, Sclerotinia sclerotiorum, and Alternaria alternate (Govrin and Levine, 2000;Kim et al, 2011;Chung, 2012). These findings indicate that GbNRX1 functions as an important regulator of apoplastic ROS homeostasis after its rapid production in cotton in response to V. dahliae invasion.…”

Antifungal potential of Bacillus vallismortis R2 against different phytopathogenic fungi

“…The other cellular responses observed in bioautographs viz. inhibition of mycelial growth and early sporulation around the inhibition zone may be attributed to the induction and cross interaction between different signalling pathways viz "two component" histidine kinase, HOG1-mitogen activated protein kinase and SKN7-mediated ROS detoxification in fungal strains, which play an important role in regulating sporulation and other metabolic events in fungi (Kuang-Ren Chung, 2012).…”