The Fusarium graminearum Xyr1 transcription factor regulates xylanase expression but is not essential for fungal virulence

The fungal pathogen Fusarium graminearum is the causal agent of the Fusarium head blight disease of wheat and other small grain cereals. This fungus is known to produce high amount of cell wall degrading enzymes during infection of wheat spikes; besides, wheat tissue is particularly rich in xylan which can be hydrolyzed by fungal xylanases. In order to establish the role of F. graminearum xylanase activity in pathogenicity, we performed a targeted gene disruption of the F. graminearum xyr1 gene, encoding the major regulator of xylanase genes expression. The Δxyr1 mutant resulted strongly affected in growing on wheat cell walls and its xylanase activity was dramatically reduced compared to wild-type; besides, the disruption of the xyr1 gene heavily reduced the expression of xylanase encoding genes both in vitro and during wheat spike infection, thus confirming the involvement of the F. graminearum Xyr1 in the regulation of genes controlling xylan degradation. However, despite of the deep impact caused by xyr1 gene disruption on the expression of xylanase genes and on total xylanase activity, the virulence of the Δxyr1 mutant appeared not affected on Triticum aestivum and T. durum spikes and on soybean seedlings. In conclusion, although a possible role of the xylanase activity in the virulence of F. graminearum cannot be conclusively excluded, our results seem to question the importance of a large amount of xylanase activity during the infection process.