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The xylanase inhibitor protein I (XIP‐I), recently identified in wheat, inhibits xylanases belonging to glycoside hydrolase families 10 (GH10) and 11 (GH11). Sequence and structural similarities indicate that XIP‐I is related to chitinases of family GH18, despite its lack of enzymatic activity. Here we report the identification and biochemical characterization of a XIP‐type inhibitor from rice. Despite its initial classification as a chitinase, the rice inhibitor does not exhibit chitinolytic activity but shows specificities towards fungal GH11 xylanases similar to that of its wheat counterpart. This, together, with an analysis of approximately 150 plant members of glycosidase family GH18 provides compelling evidence that xylanase inhibitors are largely represented in this family, and that this novel function has recently emerged based on a common scaffold. The plurifunctionality of GH18 members has major implications for genomic annotations and predicted gene function. This study provides new information which will lead to a better understanding of the biological significance of a number of GH18 ‘inactivated’ chitinases.

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