© 2000 Heron Publishing—Victoria, Canada
Oxidase activity in lignifying xylem of a taxonomically diverse range of trees: identification of a conifer laccase
Andrew Richardson (1), Julie Duncan (1) and Gordon J. McDougall (1, 2)
1. Unit for Plant Biochemistry, Division of Biochemistry and Cell Biology, Scottish Crop Research Institute, Invergowrie, Dundee
DD2 5DA, U.K. / 2. Author to whom correspondence should be addressed (gmcdou@scri.sari.ac.uk) / Received November 8, 1999
Summary
In a diverse taxonomic range of tree species, including representative species of ancient families of angiosperms (Magnolia × soulangiana Soul.-Bod.) and gymnosperms (Ginkgo biloba L.), oxidase activity was associated with cell walls of developing xylem and was enriched in extracts of cell wall-associated
glycoproteins. In all species where oxidase activity was detected histochemically, it was expressed in cell walls of lignifying,
differentiating xylem cells and was absent from old wood, cambium and phloem, suggesting that oxidases have a conservative
role in lignification of tree xylem.
An oxidase from the developing xylem of Picea sitchensis (Bong) Carr. (Sitka spruce) was partially purified by a combination of lectin affinity and immobilized metal ion affinity
chromatography. A portion of the total oxidase activity had high affinity for immobilized zinc ions and this feature allowed
it to be separated from the bulk of oxidase activity. Two polypeptides that could have been responsible for the bound oxidase
activity were enriched by this procedure. The smaller polypeptide of Mr ~73 kDa yielded an N-terminal amino-acid sequence that was homologous to laccase-like polyphenol oxidases (E.C. 1.10.3.2)
from loblolly pine (Pinus taeda L.), poplar (Populus euramericana (Dode) Guinier) and Arabidopsis. The larger polypeptide (Mr ~77 kDa) yielded an N-terminal amino-acid sequence that was homologous with a range of plant subtilisin-like serine proteinases.
The roles of oxidase and proteinase activities in developing xylem are discussed.
Keywords:
angiosperms, gymnosperms, lignin, purification, Sitka spruce, xylogenesis.
