Biology Reference
In-Depth Information
J. PEROXIDASES AND LACCASES
Peroxidases and/or laccases catalyse the oxidation of monolignols, the last
step in lignin biosynthesis. Using hydrogen peroxide as an oxidant, perox-
idases can generate phenoxy radicals from monolignols which are then
coupled to generate lignin polymers. Laccases can promote polymerization
of monolignols in the absence of H
2
O
2
, resulting in either lignans or lignins
(
Sterjiades et al., 1992
).
The expression pattern of a promoter from an anionic peroxidase gene
(PRXA3) potentially involved in lignin synthesis was determined in transgenic
poplars transformed with a construct carrying a fusion of this promoter with
the reporter gene uidA: the reporter gene appeared strongly expressed in the
xylem and weakly in the phloem (
Morohoshi and Kajita, 2001
). Transgenic
poplars (Populus sieboldii
P. grandidentata) downregulated for this peroxi-
dase gene were produced using an antisense copy of this gene under the control
of its own promoter (
Li et al., 2003b
). These authors reported that 3-month-
old greenhouse-grown poplar trees, with decreased total peroxidase activity in
the stem (55% residual activity), exhibited decreased lignin content (up to 20%
reduction) and increased S units in the lignin polymer compared to wild-type
trees. Growth and development were similar in wild-type and transformed
trees. More surprisingly, poplar (P. tremula) transformation with an antisense
sequence of a cationic peroxidase gene (ShPX6a) from a forage legume species,
Stylosanthes humilis, under the control of the 35S promoter, gave a similar 10-
20% reduction in lignin content (
Ipek¸i et al., 1999
). None of these transgenic
trees was, however, evaluated further in a field test.
Poplar plants (INRA 717-1B4, P. tremula
P. alba) transformed with an
antisense copy of three different laccase genes were analysed for their poten-
tial effects on lignification. Despite severe reduction in the expression of each
gene in transformants, neither plant growth and development nor their lignin
content and composition were altered. Interestingly, repression of one of
these laccase genes (LAC3) induced a two to threefold increase in total
soluble phenolic content that was preferentially accumulated in xylem ray
parenchyma cells, whereas xylem fibre cell wall structure appeared dramati-
cally altered (
Ranocha et al., 2002
).
K. MULTIPLE TRANSFORMATIONS
Based on the results of the single gene transformation approach, several
studies were undertaken to try to combine the modification of several lignin
genes in a single tree and also to evaluate any possibility of interactions
between enzymes from the lignin biosynthetic pathway. This was the case
for a combinatorial approach developed by
Li et al.(2003a)
, who, using an
