Fusarium graminearum hydrophobins: role in fungal growth and plant infection.

Hydrophobins are small fungal-specific proteins which are present at the surface of aerial hyphae and spores. They may play a role in several processes such as formation of fungal aerial structures, attachment to hydrophobic surfaces, interaction with the environment and protection against the host defense system by masking the fungal cell wall. The plant pathogen Fusarium graminearum, a necrotrophic fungus which causes Fusarium head blight (FHB) of wheat, barley and other cereal grains, contains five hydrophobin genes. To determine their role in F. graminearum, single and triple mutants of the five genes were produced and characterized. Interestingly, Hyd1 is dramatically upregulated during initial epiphytical growth on wheat paleas and glumes. A reduction in hydrophobicity was observed for the single mutants ∆hyd1 and ∆hyd3 as well as the triple mutants including ∆hyd1 or ∆hyd3. Single disruptants ∆hyd2, ∆hyd4 or ∆hyd5 behaved wild type-like. The single mutant ∆hydr3 and the triple mutants including ∆hydr3 showed a decrease in growth compared to the wild type in a complete or minimal growth medium. Surprisingly, a reduced growth was registered when these mutants were grown under osmotic stress conditions or in the presence of H2O2. In order to verify possible defects in their cell wall, the growth of the mutants is currently under scrutiny in the presence of β-1,3-glucanase, chitinase and two fungicides. Point inoculation of wheat spikes with all mutants showed no role of the hydrophobins during infection. Preliminary data suggest a reduction in virulence of the Hyd1 mutant after spray inoculation.