In this protocol we describe a cell wall stress assay for the fungal pathogen F. oxysporum, based on exposure to the two anionic dyes Calcofluor White (CFW) and Congo Red (CR). Both compounds have been used to exert stress upon the fungal cell wall in vitro (Perez-Nadales and Di Pietro, 2015; Perez-Nadales and Di Pietro, 2011; Leach et al., 2012; Heilmann et al., 2013; Garcia et al., 2015). CFW perturbs chitin assembly, whereas CR interferes with β-glucan synthesis, resulting in cell wall-weakening and activation of the cell wall stress response (Ram and Klis, 2006; Kopecka and Gabriel, 1992; Roncero and Duran, 1985). Presumably, the signaling pathways and cell wall changes associated with this response reflect cell wall homeostasis during normal growth as well as cell wall remodeling events in response to stresses encountered during the fungus-host interactions. The conditions for preparation of CFW and CR culture medium specified in this protocol are based on the paper by Ram and Klis entitled “Identification of fungal cell wall mutants using susceptibility assays based on Calcofluor white and Congo red”, published in Nature protocols (Ram and Klis, 2006). This paper established the optimum conditions for preparation of CFW and CR stock solutions and suggested maintaining the culture medium at a constant pH to avoid acidification, protonation and precipitation of these dyes. This cell wall stress assay has been widely used in our group for the characterization of F. oxysporum mutants in mitogen activated protein kinase (MAPK) signaling pathway genes involved in cell wall integrity (Perez-Nadales and Di Pietro, 2015; Perez-Nadales and Di Pietro, 2011; Turra et al., 2014).
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