Biology Reference
In-Depth Information
B. SOME HINTS FOR THE FUTURE
1. Needs for widening transformable species/genotypes
To date, very few genera have been subjected to genetic engineering aiming at
lignin modification: Populus, Eucalyptus, Picea, Larix and Pinus,and,within
these genera, only a few species have been successfully transformed: P. radiata,
P. abies, Larix leptoeuropae, E. camaldulensis, etc. For poplar, the array of
transformed species is somewhat wider but remains mostly concentrated on
aspen species: P. tremula, P. alba, P. tremuloides and related hybrids, whereas
very few cottonwood species and hybrids have been successfully transformed:
P. trichocarpa, P. deltoides
P. nigra, etc. Even within these species, transfor-
mation procedures have generally been optimized on only one or a very few
number of genotypes. For example, a large number of the studies, described in
the first part of this chapter, were carried out on the same genotype (INRA
717-1B4, P. tremula
P. alba) as this genotype is rather easy to transform and
is freely available to the scientific community (for more information contact
pilate@orleans.inra.fr). Generic transformation protocols need to be opti-
mized that are efficient on a wide number of genotypes within the same
species. Transformation of conifers is a different case, as it involves the
production of transgenic material upon regeneration of plants through
somatic embryogenesis, a technique thatseemsmucheasiertoextendtoa
large number of genotypes.
2. Use of suitable promoters
Analyses of the corpus of experiments described above indicated that it seems
preferable to use a promoter conferring a specific expression during the
lignification step in differentiating xylem such as p4CL1 or pC4H instead
of the single or double pCaMV 35S promoter, potentially conferring a
constitutive expression, that is still used in most studies: the use of such
specific promoters may reduce the occurrence of pleiotropic effects of the
genetic modification on plant growth and development.
3. The right combination
Only a few studies were devoted to the production and evaluation of trans-
genic trees modified for more than one gene. Although, double transforma-
tion will clearly increase the chance of obtaining trees affected in their growth
and development, with the knowledge acquired from the analyses of single
transformants, this can be anticipated through the judicious choice of target
genes and enhance beneficial effects. This will probably be the case with, for
example, the production of transgenic trees where both a reasonable reduc-
tion in lignin content and an increased S/G ratio have been generated. This
was probably the goal of producing trees both downregulated for 4CL and
