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not specific to plant laccases as both fungal and bacterial laccases are also
capable of catalysing flavonoid polymerization (
Fowler et al., 2011;
Ghidouche et al., 2008
).
Zhao et al. (2010)
hypothesize that TT10 may be
the initial polymerizing enzyme for PA formation. However, seeds of tt10 fed
with epicatechin catalyse the formation of epicatechin oligomers (e.g. dehy-
drodiepicatechin A) that differ from PAs by the interflavan linkage (
Pourcel
et al., 2005
). It remains to be determined, whether epicatechin is the true
in vivo substrate. On the other hand, the TT10 protein is predicted to be
secreted in the apoplast. The experimental confirmation of this subcellular
localization may rule out the hypothesis that TT10 is a potential vacuolar-
condensing enzyme.
As laccases are known to have a broad substrate range (
Mayer and
Staples, 2002
), TT10 may not be specific for flavonoid substrates. As a matter
of fact,
Liang et al. (2006)
showed evidence of TT10 involvement in mono-
lignol polymerization. They analysed extractable lignin content in the mature
seeds using the TGA assay and detected a 30% reduction of lignin content in
tt10 mutant seeds. Moreover, they also suggest that TT10 is involved in
lignin polymerization, based on the results of in vitro assays where they
observed a decrease in monolignol polymerization in the mutant compared
to the WT. As genes encoding catechol oxidases are not present in the
arabidopsis genome (
Pourcel et al., 2005
), it is possible that TT10 laccases
activity has acquired the capacity to oxidize flavonoids in addition to oxidiz-
ing monolignols.
In conclusion, the physiological roles of the TT10 laccases are still a matter
of debate. As TT10 is accumulated mainly in the seed coat during seed
development, it could play a role in protecting the embryo and remaining
endosperm by providing a physicochemical barrier against various stresses
through quinone production (
Aniszewski et al., 2008; Pourcel et al., 2007
).
However, the absence of TT10 laccases in tt10-1 (Landsberg background)
does not affect seed dormancy, germination, longevity, and testa permeabili-
ty to tetrazolium salts, contrarily to what is observed for tt mutants lacking
tannins (
Debeaujon et al., 2000
). These results need to be confirmed using
several tt10 alleles in other ecotype backgrounds.
IV. TRANSCRIPTIONAL REGULATION
OF PLANT LACCASES
Lignification and secondary cell wall formation is regulated via a complex
network of TFs acting upon genes encoding monolignols biosynthetic
enzymes (
Zhao and Dixon, 2011; Zhong et al., 2010
). It is also becoming
