The wheat xylanase inhibitor TAXI-III interacts with a xylanase secreted by Fusarium graminearum and limits wheat cell death.

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). These inhibitors are considered part of the defence mechanisms that plants use to counteract microbial pathogens: recently, we provided in planta evidences for the protective role of TAXI-III, a member of the TAXI type XIs. To elucidate the molecular mechanism underlying the capacity of the transgenic wheat plants expressing TAXI-III to limit Fusarium Head Blight (FHB) disease symptoms caused by Fusarium graminearum, we performed infiltration experiments on wild-type and transgenic wheat tissues with a xylanase strongly expressed by F. graminearum during wheat spike infection, which we have previously demonstrated to induce cell death and hydrogen peroxide accumulation. Experiments performed on glumes of flowering wild-type wheat spikes showed that the co-infiltration with TAXI-III significantly decreased cell death and hydrogen peroxide accumulation. Most interestingly, similar results were also obtained by infiltrating the same xylanase on glumes of transgenic wheat plants expressing TAXI-III. Molecular modelling studies predict an interaction between the TAXI-III and the active site of the xylanase, thus the formation of this complex might prevent the recognition of the xylanase by a plant receptor possibly involved in cell death elicitation. Therefore these results suggest that the reduced FHB symptoms on transgenic TAXI-III plants can be due to the direct inhibition of xylanase activity secreted by the pathogen but also to the capacity of TAXI-III to block the xylanase necrotizing activity.