Abstract
BackgroundHydrophobins are small, cysteine rich, surface active proteins secreted by filamentous fungi, forming hydrophobic layers on the walls of aerial mycelia and spores. Hydrophobin mutants in a variety of fungi have been described to show 'easily wettable' phenotypes, indicating that hydrophobins play a general role in conferring surface hydrophobicity to aerial hyphae and spores.ResultsIn the genome of the grey mould fungus Botrytis cinerea, genes encoding three hydrophobins and six hydrophobin-like proteins were identified. Expression analyses revealed low or no expression of these genes in conidia, while some of them showed increased or specific expression in other stages, such as sclerotia or fruiting bodies. Bhp1 belongs to the class I hydrophobins, whereas Bhp2 and Bhp3 are members of hydrophobin class II. Single, double and triple hydrophobin knock-out mutants were constructed by consecutively deleting bhp1, bhp2 and bhp3. In addition, a mutant in the hydrophobin-like gene bhl1 was generated. The mutants were tested for germination and growth under different conditions, formation of sclerotia, ability to penetrate and infect host tissue, and for spore and mycelium surface properties. Surprisingly, none of the B. cinerea hydrophobin mutants showed obvious phenotypic defects in any of these characters. Scanning electron microscopy of the hydrophobic conidial surfaces did not reveal evidence for the presence of typical hydrophobin 'rodlet' layers.ConclusionsThese data provide evidence that in B. cinerea, hydrophobins are not involved in conferring surface hydrophobicity to conidia and aerial hyphae, and challenge their universal role in filamentous fungi. The function of some of these proteins in sclerotia and fruiting bodies remains to be investigated.
Highlights
Hydrophobins are small, cysteine rich, surface active proteins secreted by filamentous fungi, forming hydrophobic layers on the walls of aerial mycelia and spores
With blastp and tblastn searches using known hydrophobin proteins, no further hydrophobin genes were identified in the B. cinerea genome
In order to identify further candidates for B. cinerea hydrophobins, a systematic search was performed in the published genome sequences of B. cinerea strains B05.10 and T4
Summary
Hydrophobins are small, cysteine rich, surface active proteins secreted by filamentous fungi, forming hydrophobic layers on the walls of aerial mycelia and spores. Hydrophobins are characterised by their small size and the presence of eight cysteine residues which are arranged in a conserved array and form four pairs of disulphide bridges By their ability to aggregate to amphipathic membranes, they attach to the surface of the hydrophilic fungal cell wall, thereby exposing the hydrophobic layer to the outside [2]. In the rice pathogen Magnaporthe oryzae mutants in the class I hydrophobin Mpg produced wettable conidia and hyphae lacking rodlets, and were defective in appressorium formation and host infection. This was attributed to the inability of the germ tubes to firmly attach to the hydrophobic plant cuticle and to appropriately sense surface features leading to appressorium differentiation [4,5]. Hydrophobin mutations in the tomato pathogen Cladosporium fulvum did not impair the mutant strains to cause disease [9]
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