Biology Reference
In-Depth Information
the field. For more clarity, the results are classified according to the gene
target of the genetic modification.
II. EVALUATION OF LIGNIN-MODIFIED
TRANSGENIC TREES GROWN IN THE
GREENHOUSE VERSUS IN THE FIELD
A. PHENYLALANINE AMMONIA-LYASE
PAL is the first enzyme of the phenylpropanoid pathway and catalyses the
deamination of phenylalanine to produce trans-cinnamic acid. Surprisingly,
there is no report on transgenic trees modified for PAL expression: probably,
this is related to the pleiotropic effects that may be expected from altering
the expression of this enzyme, central to plant secondary metabolism. Only
Gray-Mitsumune et al.(1999)
described the pattern of expression of a reporter
gene directed by two poplar PAL promoters in 2-month-old growth chamber-
grown transgenic poplar plants. This expression appears to be developmental-
ly regulated with the highest levels of expression in roots, young leaves and
stems. In contrast, this level was low in woody stems and weak or absent in
developing secondary xylem suggesting that the corresponding PAL genes are
not involved in lignin biosynthesis in wood.
B. CINNAMATE 4-HYDROXYLASE
Cinnamate 4-hydroxylase (C4H) belongs to the CYP73A group of the cyto-
chrome P450-dependent monooxygenase protein family and catalyses the
hydroxylation of cinnamic acid to p-coumaric acid. The C4H gene family
has been characterized in poplar by localization of the expression of its
different members and by the enzymatic specificity of the corresponding
proteins (
Lu et al., 2006
). As described subsequently, a C4H promoter was
further used in transformation experiments to target the expression of other
transgenes to the xylem cells during secondary cell wall differentiation.
Bjurhager et al. (2010)
produced and analysed transgenic poplars down-
regulated for C4H expression. The analysis was performed on 1.5-m high
greenhouse-grown trees. Transgenic trees presented a 30% reduction in
Klason lignin content with no significant change in syringyl to guaiacyl (S/
G) ratio. The transgenic plants exhibited normal wood anatomy and were
not different from wild-type plants, except for a slight reduction in growth
height. The lignin reduction resulted in a 10% decrease in wood density that
may be related to a looser arrangement in microfibril cellulose aggregates
