Role of Plant Laccases in Lignin Polymerization - Botanical Research - page 153

conserved or specific roles of these laccases ( Fig. 2 ). The results of the
protein-based similarity analysis showed that both poplar and arabidopsis
laccases were grouped in seven main clusters. One major specificity of poplar
is the overall higher level of duplicated genes found in the genome when
compared to arabidopsis. Important differences were also observed
concerning the number of poplar homologs for one particular arabidopsis
gene. For example, AtLAC1, belonging to group 1, has five putative poplar
co-orthologs. In contrast, AtLAC6 and AtLAC14 from groups 3 and 6,
respectively, have only one predicted ortholog each. An increase in poplar
genes was observed to cluster with arabidopsis laccases from groups 1, 2, and
7. Further information was provided by the analysis of poplar in silico
expression data available through the efp Browser http://bar.utoronto.ca/
( Wilkins et al., 2009 ). Expression data from 29 gene models out of the 51
predicted in the genome annotation were collected. Figure 3 presents a
hierarchical clustering of the expression data available for the gene models
represented on the Affymetrix poplar array. Three main expression clusters
were observed: one grouping genes highly, but not exclusively expressed in
stem xylem (group X), a second cluster containing genes highly, but not
exclusively expressed in roots (group R), and the third cluster represented
by weakly expressed genes with no specific tissue expression (group O). Of
the five poplar genes belonging to group R, three of them, PtrLAC33, 39, and
40 clustered together with arabidopsis laccases AtLAC3, 7, 8, 9, and 13 that
are mainly expressed in roots ( Fig. 1 A, group C). One of them, PtrLAC47,
clustered with the AtLAC15/TT10 seed-specific laccase. The last one,
PtrLAC43, is phylogenetically linked to AtLAC4 and AtLAC10, which are
more specifically expressed in the stem.
Examination of the expression clustering revealed that PtrLAC2,
PtrLAC12, and PtrLAC21 are the only genes presenting a specific expression
in both roots and stem xylem tissue ( Fig. 3 ). Interestingly, these genes are
phylogenetically linked to AtLAC2 ( Fig. 2 ) and more distant from PtrLAC1
and PtrLAC31, which are more specifically expressed in poplar stem xylem as
compared to roots. Of the 14 genes belonging to group X, three poplar genes
clustered with the arabidopsis seed-specific laccases AtLAC5 while the other
11 genes are linked to arabidopsis laccases highly expressed in the stem
(group A, Fig. 1 A). Finally, 10 poplar genes are clustered in group O.
Three of them are tightly linked to AtLAC6, AtLAC14, and AtLAC16 that
are weakly expressed with no particular organ specificity and belong to
arabidopsis group E ( Fig. 1 A). Two are phylogenetically linked to TT10/
AtLAC15, two are similar to AtLAC1, two others clustered together with the
stem-specific AtLAC11 from group A ( Fig. 1 A), and the last one is linked to
the root-specific laccases AtLAC7.