Role of Plant Laccases in Lignin Polymerization - Botanical Research - page 158

expressed in both xylem and interfascicular fibres ( Turlapati et al.,2011 ).
However, the lac11 single mutant displayed neither reduced lignin content
( Table III ) nor altered lignin structure (data not shown). The situation was
similar for AtLAC2 and AtLAC10: while these genes were found to be
expressed in stem lignified tissues ( Turlapati et al.,2011 ), their silencing did
not induce any change in lignin content or structure ( Table III ). These results do
not rule out the possibility that AtLAC2, AtLAC10,orAtLAC11 might make
a minor contribution to the lignin polymerization process. Double and even
multiple knockout mutants would be necessary to confirm or reject this
hypothesis. In contrast, the hypothesis that AtLAC12 is involved in stem
lignification was supported not only by its expression pattern in stem lignified
tissues ( Turlapati et al., 2011 ) but also by a weakly, but consistently decreased
lignin content of the lac12 stems ( Table III ). Further support for the hypothesis
was provided by the observation that the double mutant lac12 lac17 showed
greater alterations in lignin content and structure than the single lac12 and lac17
mutants ( Table III ). Taken together, these results show that both AtLAC4 and
AtLAC17 indisputably contribute to lignin polymerization in arabidopsis
stems, and it is very likely that AtLAC12 might assist these enzymes.
D. FLAVONOID POLYMERIZATION IN SEEDS: OTHER ROLES FOR
ARABIDOPSIS LACCASES, NAMELY, TRANSPARENT TESTA 10/ATLAC15
In addition to their role in lignification, it is possible that laccases might
be involved in other physiological processes. Arabidopsis seeds accumulate
two types of flavonoids, that are, proanthocyanidins (PAs) and flavonols
( Lepiniec et al., 2006 ). Colourless PAs (condensed tannins), the major con-
tributors to the brown WT seed colour when oxidized, are localized specifi-
cally in the inner integument (mostly in the endothelium) and in the chalaza
zone of the seed ( Debeaujon et al., 2003 ). Arabidopsis tannins are polymers
of flavan-3-ols of the epicatechin type. Colourless to yellow flavonols accu-
mulate in the seed envelopes (seed coat and endosperm) and in the embryo as
glycosylated derivatives ( Routaboul et al.,2006 ).
The TRANSPARENT TESTA 10 gene (TT10, At5g48100) encodes the
TT10/AtLAC15 laccases ( Pourcel et al.,2005 ). It was cloned through a candi-
date gene approach using the tt10-1 mutant originally isolated by Koornneef
(1990) , mapped and first described by Shirley et al. (1995) .Thett10 mutant
alleles are characterized by delayed seed browning caused by the inability to
perform enzymatic oxidation of tannins ( Pourcel et al., 2005; Shirley et al.,
1995 ). The consequence is an increased amount of soluble (unoxidized) versus
insoluble (more oxidized) tannins. Detailed metabolomic analysis revealed
that tt10 is also affected in flavonol composition, characterized by a higher