Eight cDNA clones for laccase (Lac 1-Lac 8) were isolated and characterized
for analysis of the role of laccases in xylem formation by molecular and g
enetic approaches. Five of them (Lac 1-5) were isolated from cDNA libraries
derived from differentiating xylem through a cDNA sequencing project in lo
blolly pine xylem (Allona of al. 1998) and other three (Lac 6-8) were isola
ted using a probe from a tobacco blue copper oxidase. Analysis of the cDNAs
revealed that the proteins predicted had N-terminal signal sequences, 8-21
glycosylation sites, and four copper binding sites. The putative mature la
ccases range between 59.2 and 61.7 kD. The predicted isoelectric points var
y between 7.3 and 9.9. Phylogenetic analysis shows these laccase cDNAs form
three clusters in the cationic plant laccase group. Northern analysis indi
cated that all eight of the laccase transcripts are most abundant in differ
entiating xylem of the six tissues and organs tested. The Lad transcript is
also detectable in immature pollen cones, and the Lac 7 transcript is dete
ctable in several organs tested. The predominance of transcript abundance i
n differentiating xylem suggests that laccases play an important role in pi
ne xylem development, consistent with previous hypotheses that laccases are
involved in lignin synthesis in differentiating xylem.