Induction of cell wall thickening by the antifungal compound dihydromaltophilin disrupts fungal growth and is mediated by sphingolipid biosynthesis

Dihydromaltophilin (heat-stable antifungal factor [HSAF]) is an antifungal metabolite produced in Lysobacter enzymogenes biocontrol strain C3. This compound induces cell wall thickening in Aspergillus nidulans. Here we show that the cell wall thickening is a general response to HSAF in diverse fungal species. In the A. nidulans model, the thickened cell wall negatively affects hyphal growth. Growth of HSAF-pre-treated hyphae failed to resume at hyphal tips with thick cell wall and the actin cable could not re-polarize at the thickened region of the cell wall, even after the treated hyphae were transferred to drug-free medium. Moreover, HSAF-induced cell wall thickening is mediated by sphingolipid synthesis: HSAF failed to induce cell wall thickening in the absence of ceramide synthase BarA and the sphingolipid synthesis inhibitor myriocin was able to suppress HSAF-induced cell wall thickening. The thickened cell wall could be digested by chitinase suggesting that chitin contributes to the HSAF-induced thickening. Furthermore, HSAF treatment activated the transcription of two chitin synthase encoding genes chsB and chsC.