Accumulated Evidence Substantiates a Role for Three Classes of Wheat Xylanase Inhibitors in Plant Defense

Since their discovery in 1997, work on proteinaceous wheat xylanase inhibitors (XIs) led to the identification and thorough biochemical and structural characterization of three classes of XIs, namely TAXI-type (Triticum aestivum xylanase inhibitor), XIP-type (xylanase inhibiting protein), and TLXI-type (thaumatin-like xylanase inhibitor) XIs. Already early on, a role in plant defense has been put forward for these proteins, mainly based on the observation that these XIs are only active against xylanases of microbial origin and can not inhibit the plant endogenous xylanases known so far. Considerable effort has been devoted to substantiate this plant defense hypothesis. Data at the genomic, transcriptomic and proteomic levels increasingly provide evidence that XIs, occurring as large polymorphic families, do indeed participate in plant defense. This review summarizes the current knowledge on XIs and the accumulated evidence on their role in plant defense. TLXI and XIP can be classified as pathogenesis-related (PR) proteins of classes PR-5 and PR-8, respectively, based on their homology with thaumatin-like proteins and chitinases. For TAXI, more evidence on the induction of TAXI proteins by a pathogen in tissues that normally do not express TAXI is required before it can be categorized as a new class of PR proteins. At the moment, TAXI proteins should be referred to as inducible defense-related proteins.