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 defe nsive, syringyl-rich lignin at wound margins in wheat (Triticum aestivum L, cv. Brigadier). Column chromatography of untreated, wounded and elicitor-t reated tissues revealed three major CAD forms (CAD-A, -B and -C) of which o nly CAD-C was responsive to elicitors. Examination of the substrate prefere nce of these fractions indicated p-coumaryl alcohol to be the preferred sub strate of CAD-A and CAD-B, whereas sinapyl alcohol was favoured by CAD-C. A ctivity-stained isoelectric focussing gels revealed in untreated and wounde d 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, CA D-C further resolved into two distinct zones of CAD activity. Cinnamyl alco hol dehydrogenase-C was purified to apparent homogeneity and characterisati on revealed a 45-kDa subunit peptide which in its native form demonstrated a marked substrate preference for sinapyl alcohol. Overall, the differentia l induction and substrate preference of CAD-C are consistent with a defensi ve role during defensive lignification at wound margins in wheat.