Overexpression of the xylanase inhibitor TAXI-III reduces Fusarium Head Blight symptom in durum wheat.

Xylanase inhibitor proteins (XIs) inhibit microbial xylanases from glycoside hydrolase families 10 and 11. The wheat genome contains three types of XIs: Triticum aestivum XI (TAXI), xylanase inhibitor protein (XIP) and thaumatin-like XI (TLXI). The genes encoding these inhibitors may exist in multiple copies in the wheat genome and some of these are localized on chromosome group 5. XIs are induced by pathogen infection and are responsive to wounding, 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. On the basis of these features XIs are considered part of the defence mechanisms that plants use to counteract the activity of xylanases secreted by microbial pathogens during infection. An additional aspect that reinforces this possibility is the observation that xylaneses are important components during pathogenesis for the fungal pathogens Botrytis cinerea and Septoria tritici. In this report we focus our attention on TAXI-III, one member of the TAXI type XIs that have been reported to be induced by Fusarium graminearum infection. The expression of the endogenous Taxi-III is regulated in a tissue-specific manner, being expressed only in developing caryopsis and roots. In order to facilitate the understanding of its contribution in wheat resistance, we expressed Taxi-III under control of the constitutive maize Ubiquitin promoter. Results show that TAXI-III accumulates in all tissues, including those that normally do not contain this inhibitor and its presence endows the transgenic wheat with new inhibition capacities. We show also that the transgene-encoded protein is correctly secreted into the apoplast and possesses the expected inhibition properties against microbial xylanases. In particular, total protein wheat extract or purified TAXI-III from transgenic plants fully inhibit a purified xylanase from Fusarium graminearum and to some extent the total xylanase activity produced in vitro by this fungal pathogen. These new inhibition properties of the transgenic tissues correlate with a significant reduction of Fusarium Head Blight (FHB) disease symptom caused by F. graminearum. In conclusion, our results provide for the first time specific evidences at the plant level 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.