Constitutive expression of the xylanase inhibitor TAXI-III delays Fusarium Head Blight symptoms in durum wheat transgenic plants

Cereals contain xylanase inhibitor proteins (XIs) which inhibit microbial xylanases and are considered part of the defence mechanisms to counteract microbial pathogens. Nevertheless, in planta evidences for this role have not been reported yet. Therefore, we produced a number of transgenic plants constitutively over-expressing TAXI-III, a member of the TAXI type XIs that is induced by pathogen infection. Results showed that TAXI-III endows the transgenic wheat with new inhibition capacities. We showed also that TAXI-III is correctly secreted into the apoplast and possesses the expected inhibition parameters against microbial xylanases. The new inhibition properties of the transgenic plants correlate with a significant delay of Fusarium Head Blight (FHB) disease symptoms caused by Fusarium graminearum but do not influence significantly leaf spot symptoms caused by Bipolaris sorokiniana. We showed that this contrasting result can be due to the different capacity of TAXI-III to inhibit the xylanase activity of these two fungal pathogens. These results provide for the first time straight evidence in planta that XIs are involved in plant defence against fungal pathogens and show the potential to manipulate TAXI-III accumulation to improve wheat resistance against F. graminearum.