Cell wall composition plays a key role on sensitivity of filamentous fungi to chitosan.

Chitosan antifungal activity has been reported for both filamentous fungi and yeast. Previous studies have shown fungal plasma membrane as main chitosan target. However, the role of the fungal cell wall (CW) in their response to chitosan is unknown. We show that cell wall regeneration in Neurospora crassa (chitosan sensitive) protoplasts protects them from chitosan damage. Caspofungin, a β-1,3-glucan synthase inhibitor, showed a synergistic antifungal effect with chitosan for N. crassa but not for Pochonia chlamydosporia, a biocontrol fungus resistant to chitosan. Chitosan significantly repressed N. crassa genes involved in β-1,3-glucan synthesis (fks) and elongation (gel-1) but the chitin synthase gene (chs-1) did not present changes in its expression. N. crassa cell wall deletion strains related to β-1,3-glucan elongation (Δgel-1 and Δgel-2) were more sensitive to chitosan than wild type (wt). On the contrary, chitin synthase deletion strain (Δchs-1) showed the same sensitivity to chitosan than wt. The mycelium of P. chlamydosporia showed a higher (ca. twofold) β-1,3-glucan/chitin ratio than that of N. crassa. Taken together, our results indicate that cell wall composition plays an important role on -sensitivity of filamentous fungi to chitosan.