The xylanase inhibitor TAXI-III is involved in wheat resistance against Fusarium graminearum.

Cereals contain xylanase inhibitor proteins (XIs) which inhibit microbial xylanases from glycoside hydrolase families 10 and 11. In wheat, three types of XIs have been identified: Triticum aestivum XI (TAXI), xylanase inhibitor protein (XIP) and thaumatin-like XI (TLXI). Each type of XIs is represented by a multiple number of genes. These inhibitors are considered part of the defence mechanisms that plants use to counteract microbial pathogens. Indeed, XIs are induced by pathogen infection and are responsive to mechanical wounding and jasmonic acid and salicylic acid treatments. They are localized in the apoplastic region and are effective against xylanases of microbial origin and not against endogenous plant xylanases. An additional aspect that reinforces the possibility that the XIs are involved in plant defence derives from the observation that xylaneses are important components during pathogenesis for the fungal pathogens Botrytis cinerea and Septoria tritici. Nevertheless, more direct evidences have not been reported yet. Therefore, we produced a number of transgenic plants over-expressing TAXI-III, a member of the TAXI type XIs that is expressed only in developing caryopsis and roots and is induced by pathogen infection, wounding and elicitor treatments. In order to facilitate the understanding of its possible contribution in wheat defence, we designated the transgenic plants to express Taxi-III constitutively. Results showed that TAXI-III accumulated accumulates in all tissues including those that normally do not accumulate it and its presence endows the transgenic wheat with new inhibition capacity. We show also that TAXI-III is correctly secreted into the apoplast and possesses the expected inhibition capacity against microbial xylanases. In particular, total protein wheat extract or purified TAXI-III from transgenic plants inhibits a purified xylanase from Fusarium graminearum and to some extent the total xylanase activity produced in vitro by the fungal pathogens F. graminearum and Bipolaris sorokiniana. The new inhibition properties of the transgenic tissues correlate with a significant reduction of fusarium Fusarium head Head blight Blight (FHB) disease symptom caused by F. graminearum but do not influence significantly leaf spot symptoms caused by B. sorokiniana. Possible differences on the efficacy of TAXI-III to inhibit specific xylanases produced by the two fungal pathogens during host colonization or the presence of additional factors conditioning host colonization at floral or leaf tissues can be responsible for this different outcome. In conclusion, our results provide for the first time a straight evidence in planta that XIs are involved in plant defence against fungal pathogens and show the possibility to manipulate TAXI-III accumulation to improve wheat resistance against F. graminearum.