Peroxidase and not laccase is the enzyme responsible for cell wall lignification in the secondary thickening of xylem vessels inLupinus

Abstract

The post-exponential growth phase of lupin (Lupinus albus cv. Multolupa) hypocotyls is characterized by a strong deposition of lignins in the primary and secondary walls of the xylem vessels. Coinciding with this phenomenon, there is a clearly peroxidatic activity in both the primary cell walls and the outer-most layers of the secondary thickening of the xylem vessels, as demonstrated by 3,3′-diaminobenzidine cytochemistry. This activity was completely inhibited by KCN and the removal of H2O2 and was not due to laccase since this enzyme shows an almost total inability to oxidize 3,3′-diaminobenzidine both in the presence and in the absence of H2O2. The absence of laccase-like activities in cell walls of vascular cells was supported by the fact that cell wall proteins from vascular cells were only capable of oxidizing 3,3′-diaminobenzidine and coniferyl alcohol in the presence of H2O2. These results support the idea of an exclusive role of peroxidase (and exclude any role for laccase) in lignin formation in the secondary thickening of xylem vessels inLupinus.