Differential induction of cinnamyl alcohol dehydrogenase during defensive lignification in wheat (Triticum aestivum L.): characterisation of the major inducible form

The induction and substrate specificity of cinnamyl alcohol dehydrogenase (CAD, EC 1.1.1.195) was investigated in relation to the deposition of a defensive, syringyl-rich lignin at wound margins in wheat (Triticum aestivum L. cv. Brigadier). Column chromatography of untreated, wounded and elicitor-treated tissues revealed three major CAD forms (CAD-A, -B and -C) of which only CAD-C was responsive to elicitors. Examination of the substrate preference of these fractions indicated p-coumaryl alcohol to be the preferred substrate of CAD-A and CAD-B, whereas sinapyl alcohol was favoured by CAD-C. Activity-stained isoelectric focussing gels revealed in untreated and wounded leaves four CAD isoenzymes with isoelectric points of 4.59 (i), 4.67 (ii), 4.81 (iii), 4.93 (iv). Elicitor treatment generally enhanced the staining of all isoenzymes and resulted in the appearance of two new isoenzymes of 5.22 (v) and pI 5.31 (vi). In activity stained non-denaturing PAGE gels, CAD-C further resolved into two distinct zones of CAD activity. Cinnamyl alcohol dehydrogenase-C was purified to apparent homogeneity and characterisation revealed a 45-kDa subunit peptide which in its native form demonstrated a marked substrate preference for sinapyl alcohol. Overall, the differential induction and substrate preference of CAD-C are consistent with a defensive role during defensive lignification at wound margins in wheat. Received: 20 September 1998 / Accepted: 22 October 1998