Agriculture Reference
In-Depth Information
culture. Regeneration frequencies are improving con-
tinuously, however, which will not only improve the
applicability of the technique in a range of species but
will also increase the potential for their application
in other ways. For example, the possibility of delib-
erately applying positive selection pressure during the
culture phase for certain characteristics, that is
in vitro
selection, will become even more attractive. Also this
might be combined with induced mutagenesis during
microsporogenesis, for example allowing production of
novel resistance to fungal or bacterial pathogens or to
herbicides.
these are now being revised and would appear to offer
practical possibilities.
PLANT TRANSFORMATION
The stable introduction of foreign genes into plants
represents one of the most significant developments
affecting the production of crop species in a continuum
of advances in agricultural technology relating to plant
breeding. The progress in this area has depended largely
on the tissue culture systems having been developed
which, at least, initially, provide an amenable vehicle
for the transformation induction.
The term transformation comes from that used for a
much longer period, bacterial transformation, in which
DNA has been successfully transferred from one isolate
to another or between species of bacteria, and integrated
into the genome. It was shown that the stably trans-
formed bacteria then expressed the new genes and had
appropriately altered phenotypes. In eukaryotes, trans-
formation has a further complicating dimension, at least
in many plant breeding contexts. The transforming
DNA must not only be integrated into a chromosome, it
must be a chromosome of a cell, or cells that will develop
into germ-line cells. Otherwise the 'transformation' will
not be passed on to any sexual progeny.
Using plant transformation techniques it is possible
to transfer single genes (i.e. simply inherited traits) into
plants, have such transgenes expressed and to func-
tion successfully. Theoretically at least, specific genes
can be transformed from any source into developed
cultivars or advanced breeding lines in a single step.
Plant transformation, therefore, allows plant breed-
ers to bypass barriers, which limit sexual gene transfer
and exchange genes (and traits) from unrelated species
where incompatibility does not allow sexual hybridiza-
tion. These recombinant DNA techniques therefore
apparently allow breeders to transfer genes between
completely unrelated organisms. For example, bacte-
rial genes can be transferred and expressed in plants.
This therefore appears to break the barrier that sex-
ual reproduction generally imposes. However, as we
learn more and more about the DNA and hence the
genes involved, the perspective of the picture some-
what changes. Increasing direct evidence of the presence
in different species of the same basic gene, or clear
variants of it, demonstrate the greater conservation of
In vitro
multiplication
In vitro
multiplication of breeding lines can have two
main benefits (particularly in clonal species) in relation
to plant breeding programmes:
•
Plants propagated
in vitro
can generally be initi-
ated to be disease-free, and can: be used to help
maintain stocks of breeding lines; facilitate long-
term germplasm storage and facilitate international
exchange of material
Short 'generation' times and fast growth means that
rapid increases in plant number can readily be
achieved
•
Both the above have particular importance to clonal
crops which tend to have a relatively low multi-
plication rate as a result of their vegetative mode
of propagation and which are particularly suscepti-
ble to viral and bacterial diseases, which tend to
be multiplied and transmitted through each clonal
generation.
Good examples of maintaining high disease status
and offering rapid plant regeneration potential include
potato and strawberry. Other, perhaps less well devel-
oped examples include
in vitro
propagation of date and
oil palms. In these crops it was found that rapid plant
regeneration would indeed offer an alternative to the
slow and lengthy process of propagating side shoots in
date palm and a more uniform planting material in
the case of oil palm. However, in date palm the pro-
cess is still very genotype dependent and with oil palm
there proved to be an unacceptably high frequency of
sterile palms produced with initial protocols, however,
