PEROXIDASE AND NOT LACCASE IS THE ENZYME RESPONSIBLE FOR CELL-WALL LIGNIFICATION IN THE SECONDARY THICKENING OF XYLEM VESSELS IN LUPINUS

The post-exponential growth phase of lupin (Lupinus albus cv. Multolup a) hypocotyls is characterized by a strong deposition of lignins in th e primary and secondary walls of the xylem vessels. Coinciding with th is phenomenon, there is a clearly peroxidatic activity in both the pri mary cell walls and the outer-most layers of the secondary thickening of the xylem vessels, as demonstrated by 3,3'-diaminobenzidine cytoche mistry. This activity was completely inhibited by KCN and the removal of H2O2 and was not due to laccase since this enzyme shows an almost t otal inability to oxidize 3,3'-diaminobenzidine both in the presence a nd in the absence of H2O2 The absence of laccase-like activities in ce ll walls of vascular cells was supported by the fact that cell wall pr oteins from vascular cells were only capable of oxidizing 3,3'-diamino benzidine and coniferyl alcohol in the presence of H2O2 These results support the idea of an exclusive role of peroxidase (and exclude any r ole for laccase) in lignin formation in the secondary thickening of xy lem vessels in Lupinus.