Importance of secreted lipases for virulence of the phytopathogenic fungus Fusarium graminearum

Abstract

Two secreted lipases FGL1 and FgLIP1 of Fusarium graminearum have been characterized as important factors for plant infection and for nutrition acquisition, respectively. In this report, we identified 15 more putatively secreted lipases of the fungus which share the same lipase motif of five amino acids GXSXG. Disruption of the entire coding sequence of the secreted lipase genes by gene replacement techniques revealed their function during plant infection. While most of the lipases were not required for fungal infection, disruption of lipase genes FGL2 and FGL5 led to the reduced infection on wheat heads and maize cobs, indicating that these lipases are general virulence factors required for plant infection. Measurements of the gene transcripts by real-time PCR during plant infection revealed that transcription of the characterized lipase FGL1 is highly induced after 3 days post inoculation (dpi). Interestingly, we observed the decreased induction of FGL1 in the reduced virulence mutants delta FGL2 and delta FGL5 but not in the delta FGL4, a virulent strain, after 3 dpi. This implicates that lipase activity of FGL2 and FGL5 is required for induction of FGL1. By complementing the delta FGL2 and delta FGL5 inoculums with different free fatty acids, it iswas revealed that gamma-linolenic acid induced the expression of the gene FGL1 in the delta FGL2 inoculums. Therefore, we hypothesized that free fatty acids liberated from FGL2 and FGL5 activity are required for activation of the important virulence factor FGL1 during plant infection.