Biology Reference
In-Depth Information
Despite these effects, the authors demonstrated that water transport was not
the factor responsible for the observed decreased growth in these transgenic
plants. The main reason postulated by the authors was due to the limitation of
photosynthesis as a consequence of the reduction in carbon sink strength: this
resulted in an accumulation of photosynthates as starch and soluble sugars in
the leaves, leading to a decreased rate of photosynthesis and a reduction in
stomatal conductance (
Coleman et al., 2008b
). These observations were car-
ried out on 4- to 18-month-old greenhouse-grown trees whereas dramatic
growth reductions were observed only 6 weeks after acclimatizing. The defects
in water transport observed in these young plants would undoubtedly become
even more serious in bigger trees making the evaluation of such material
useless in field conditions.
E. FERULATE 5-HYDROXYLASE
Ferulate 5-hydroxylase (F5H), also designated coniferaldehyde 5-hydroxylase
(CAld5H), is a cytochrome P450-dependent monooxygenase of the CYP84A
group. In in vitro experiments, it preferentially converts coniferaldehyde and/
or coniferyl alcohol to sinapaldehyde and/or sinapyl alcohol, respectively
(
Humphreys et al., 1999; Osakabe et al., 1999
).
Young transgenic poplar trees (INRA 717-1B4 P. tremula
P. alba)
transformed with the F5H sense sequence under the control of the C4H
promoter (expressed in lignifying tissues) exhibited a strong increase in
S unit content, up to 85% compared to an average 55% observed in the
wild-type poplar plants, while lignin content was unaffected (
Franke et al.,
2000
). In tobacco, similar results were obtained using the same construct,
whereas when the C4H promoter was replaced by the CaMV 35S ''constitu-
tive'' promoter, no changes in S/G ratio were observed suggesting that the
35S promoter does not confer high levels of expression in lignifying tissues.
Analyses performed on 1-year-old transgenic poplar plants from the same
transgenic lines found even stronger increases in S units, up to 93.4% com-
pared to 65.6% for control trees (corresponding to an increase S/G ratio from
1.9 in control to 14.2 in the transgenic plants;
Huntley et al., 2003
). This
clearly demonstrated that F5H is a key-enzyme controlling lignin monomer
composition in angiosperms. Two-year-old greenhouse-grown transgenic
poplar plants exhibited no change in total lignin content, no apparent effect
on conduction and no alteration in their growth and development (
Huntley
et al., 2003
). However, mechanical evaluation of transgenic wood revealed
significant increases in elastic modulus, whereas bending stiffness remained
unchanged due to a reduction in stem diameter (
Koehler and Telewski,
