Fusarium graminearum SnodProt proteins protect fungal cell wall polysaccharides from enzymatic degradation

The genome of Fusarium graminearum, a necrotrophic fungal pathogen causing Fusarium head blight (FHB) disease of wheat, barley and other cereal grains, contains five genes putatively encoding for proteins similar to cerato platanins, which contribute to the virulence of the fungal pathogens Botrytis cinerea and Magnaporthe grisea. Two of them (FGSG_10212 and FGSG_11205) belong to the class of SnodProt proteins, with reported phytotoxic activity. To verify the contribution of the two F. graminearum SnodProt proteins to the infection process, single and double gene knock-out mutants were produced but no reduction in symptoms severity was observed compared to the wild type strain on both soybean and wheat spikes. Histological analysis performed by fluorescence microscopy on wheat spikelets infected with mutants constitutively expressing the dsRed confirmed that the F. graminearum SnodProt proteins do not contribute to fungal virulence. In particular, the formation of compound appressoria on wheat glumes was unchanged. Looking for other functions of these proteins, the double mutant was characterized by in vitro experiments. Wild type and mutants were similarly inhibited by salt and H2O2 stress. Surprisingly, the mutant grew better than wild type on carboxymethyl cellulose, while no difference was observed on glucose. Furthermore, conidia and mycelium of the mutant were more affected by treatments with chitinase and β-1,3-glucanase, thus indicating that the F. graminearum SnodProt proteins could protect fungal cell wall polysaccharides from enzymatic degradation.