Abstract
Reactive oxygen species (ROS) produced by hosts serve as a general defense mechanism against various pathogens. At the interaction site between the host and pathogen, host cells rapidly accumulate high concentrations of ROS, called the oxidative burst, that damage and kill the invading microbes. However, successful pathogens usually survive in a high ROS environment and have evolved strategies to overcome these detrimental effects. Here we characterized the biological function of the extracellular superoxide dismutase (SOD) FoSod5 from Fusarium oxysporum f. sp. vasinfectum. FoSOD5 is strongly up-regulated during infection of cotton, and a ΔFoSOD5 mutant was significantly reduced in virulence on cotton. Purified 6 × His-FoSod5 could significantly inhibit the reduction of NBT and WST-1, indicating that FoSod5 was a functional SOD protein. Based on CRISPR/Cas9 technology, several different FoSod5 variants were generated and used to assess the secretion, expression, and subcellular localization of FoSod5 in F. oxysporum. The subcellular localization of FoSod5 is altered under different environmental conditions. During normal growth conditions, FoSod5 was primarily localized to the phialides; however, in a nutrient-limited environment, FoSod5 was localized to a wide array of fungal structures including the septum and cell wall. FoSod5 is an alkaline-induced glycosylphosphatidylinositol (GPI) protein and the GPI anchor was required for proper protein subcellular localization. The multiple mechanisms fungi utilize to tolerate the oxidative burst is indicative of the importance of this plant defense response; however, the presence of a conserved extracellular SOD in many phytopathogenic fungi suggests tolerance to ROS is initiated prior to the ROS entering the fungal cell.
Highlights
The ascomycete fungus Fusarium oxysporum is an important pathogen that can infect and cause disease on a wide range of hosts including plants, animals, and humans (Gordon, 2017)
Several Sod proteins from various fungal species were used as a query and identified five superoxide dismutases (SOD) proteins in the F. oxysporum genome
Host cells impede invading fungal pathogens through the rapid production of Reactive oxygen species (ROS) by membrane-bound NADPH oxidases (Marino et al, 2012); and successful fungal pathogens must evolve strategies to overcome the cellular damage from ROS during infection (Heller and Tudzynski, 2011)
Summary
The ascomycete fungus Fusarium oxysporum is an important pathogen that can infect and cause disease on a wide range of hosts including plants, animals, and humans (Gordon, 2017). Characterization of the Extracellular FoSod in F. oxysporum (Fov) is responsible for Fusarium wilt of cotton, and is a significant disease found worldwide in all growing areas (Davis et al, 2006) This soil-borne fungus invades the vascular tissue via the roots and rapidly spreads to the aboveground portion of the host. The oxidative burst may result in the hypersensitive response (HR) which inhibits the spread of the pathogen to surrounding tissue (Zurbriggen et al, 2010), and can serve as an important signal that initiates a series of other plant defense responses or the production of plant hormones (Sauer et al, 2001) These processes stimulate host plants to alter the expression of genes involved in defense response, leading to the production of phytoalexins, callose deposition, and systemic acquired resistance; thereby impeding further pathogen spread and disease development (Sauer et al, 2001; Forman et al, 2010)
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