Role of Plant Laccases in Lignin Polymerization - Botanical Research - page 149

oxidize coniferyl alcohol ( Ranocha et al.,1999 ). A 90 kDa H 2 O 2 -independent
phenoloxidase displayed in vitro laccases activity and was able to oxidize the
three monolignols with the highest affinity for coniferyl alcohol ( Ranocha
et al., 1999 ). Moreover, laccases down-regulated transgenic poplars were
produced which displayed higher levels of soluble phenolics and irregular
fibres, albeit the global lignin level was not reduced ( Ranocha et al., 2002 ).
Only two studies have reported changes of lignin levels in transgenic plants
showing modified laccases activity ( Liang et al., 2006; Wang et al., 2008 ).
A decreased lignin content was reported in one arabidopsis tt10/lac15 mutant
that shows reduced laccases activity particularly in the seeds ( Liang et al.,
2006 ). In contrast, an increased lignin level was observed in transgenic poplar
plants expressing the cotton laccases GaLAC1 gene ( Wang et al., 2008 ).
These poplar lines showed a 2-13-fold increase in laccases activity when
compared to the wild-type (WT) poplar and a 2-19% increase in lignin
content measured by the thioglycolic assay.
B. PLANT LACCASES: HOW TO IDENTIFY FUNCTION(S)
IN LARGE MULTIGENIC FAMILIES?
Phylogenetic and expression studies of two laccases multigenic families (ara-
bidopsis and poplar) can be used to emphasize the relevancy of these
approaches for the functional determination of laccases genes.
In arabidopsis, 17 laccases have been reported ( McCaig et al., 2005 ). The
comparison between the phylogenetic and expression trees for these 17
laccases can provide useful indications as to their putative roles ( Fig. 1 ).
These laccases are discriminated mainly in five different clusters on the
expression tree ( Fig. 1 A). Among these clusters, three organ-specific groups
can be observed: A, C, and D that are stem-specific, root-specific, and seed
specific, respectively. The protein alignment allowed us to discriminate seven
groups in the arabidopsis laccases gene family ( Fig. 1 B), unlike previous
classifications that have identified six groups in arabidopsis ( Caparr ´ s-Ruiz
et al., 2006; McCaig et al., 2005; Turlapati et al.,2011 ). Each of the AtLAC1,
AtLAC6, AtLAC14,andAtLAC15 genes is the unique representative of
groups 1, 3, 6, and 7, respectively. According to their geneCAT expression
profiles, AtLAC1, AtLAC6,andAtLAC14 are weakly expressed without any
organ specificity. In contrast, AtLAC15 is strongly expressed in seeds suggest-
ing a seed-specific function ( Pourcel et al.,2005 ). In spite of its distinct locali-
zation in group 5 of the phylogenetic tree ( Fig. 1 B), AtLAC5 is coexpressed
with AtLAC15 (group D, Fig. 1 A). Both group 4 (with AtLAC7, AtLAC8,
AtLAC9) and group 5 (AtLAC3, AtLAC5, AtLAC12,andAtLAC13)contain
root-specific laccases. The largest group (group 2, Fig. 1 B) comprises six