The three classes of wheat xylanase-inhibiting proteins accumulate in an analogous way during wheat ear development and germination

Wheat contains high levels of the three classes of xylanase inhibitors (XIs), Triticum aestivum xylanase inhibitor (TAXI), xylanase-inhibiting protein (XIP) and thaumatin-like xylanase inhibitor (TLXI). These proteins have been linked to plant defense. In this study, expression of XIs during wheat ear development and germination was examined using immunoblotting. The three types of XIs accumulated at high levels between the milky and the soft dough stages of ear development, and reached the highest levels at the hard kernel stage. From the hard kernel stage to harvest ripeness, a slight drop in inhibitor levels was observed, which was more marked for TAXI and TLXI than for XIP. During germination, the levels of the three types of XIs initially decreased, but XIs accumulated again after 1–2 d, reaching maximum levels between 5 and 9 d after imbibition. The levels of TAXI, XIP and TLXI in the seedlings then gradually and continuously declined as a function of time. 1D- and 2D-immunoblotting indicated that the three types of XIs occur in a wide variety of forms. This polymorphism is maintained throughout ear development and germination, although the proportions of the different (iso)forms vary with time. A differential temporal profile was observed for the unprocessed and processed forms of TAXI-type proteins. Finally, the occurrence of TAXI and XIP, but not TLXI, in roots and shoots of young seedlings was demonstrated. No XIs were detected in roots, leaves or stems at later stages of ear development. Overall, the three classes of XIs show remarkable similarities in their temporal distribution, indicating a related function within the wheat plant.