A Fusarium graminearum xylanase expressed during wheat infection is a necrotizing factor but is not essential for virulence

Fusarium graminearum is the fungal pathogen mainly responsible for Fusarium head blight (FHB) of cereal crops, which attacks wheat spikes, reducing crop production and quality of grain by producing trichothecene mycotoxins. Several cytohistological studies showed that spike infection is associated with the production of cell wall degrading enzymes. Wheat tissue, as in other commelinoid monocot plants, is particularly rich in xylan which can be hydrolyzed by fungal endo-1,4-β-xylanase. The FG_03624 is one of the most expressed xylanase genes in wheat spikes 3 days after inoculation and was heterologously expressed in the yeast Pichia pastoris. The recombinant protein (22.7 kDa) possessed xylanase activity and induced cell death and hydrogen peroxide accumulation in wheat leaves infiltrated with 10 ng/μl or in wheat lemma surface treated with 20 ng/μl. This effect reflects that observed with other described fungal xylanases (from Trichoderma reesei, Trichoderma viride and Botrytis cinerea) with which the FG_03624 protein shares a stretch of amino acids reported as essential for elicitation of necrotic responses. Several F. graminearum mutants with the FG_03624 gene disrupted were obtained, and showed about 40% reduction of xylanase activity in comparison to the wild type when grown in culture with xylan as carbon source. However, they were fully virulent when assayed by single floret inoculation on wheat cvs. Bobwhite and Nandu. This is the first report of a xylanase able to induce hypersensitive-like symptoms on a monocot plant. © 2013 Elsevier Masson SAS.