Agriculture Reference
In-Depth Information
Genetic transformation
Genetic transformation is a means of transferring a specific gene by biotech-
nology from an organism, which may or may not be of the same species, into
a cultivar to achieve a specific improvement.
The basic processes involved are:
Identification of the desired gene.
Its isolation.
Creation of a modified genetic sequence by fusing the desired gene, a
promotersequencewhichcontrolsthefunctioningofthegeneandamarker
gene which allows the gene's presence to be detected.
Multiplication of the recombinant sequence.
Insertion of copies of the desired gene into the cultivar.
Selection of the organisms which have taken up the gene as indicated by
presence of the marker gene.
Multiplication of the modified plants.
Apples and pears show enormous genetic variation, with genes relating to
tree vigour control, precocity of flowering, aspects of fruit quality and storage
life, and resistance to many diseases and some pests already identified. Under-
standing of the mechanisms of these processes has also facilitated the selection
of potentially useful genes from bacteria and insects. Some potentially useful
gene transfers have also been developed semi-empirically.
James
et al.
(
) achieved transformation of
apple and pear, respectively, using disarmed binary vectors with genes for
kanomycinresistancetransferredby
Agrobacterium tumefaciens
.Leavesfromshoot
cultures were inoculated either by cutting out discs or strips and placing these
in the suspension of
A. tumefaciens
carrying the binary vector, or by wounding
with a scalpel dipped in the bacterial suspension. Selection of transformed ma-
terial was by growing on media containing kanomycin. Subsequent cloning
was by the standard tissue culture techniques. Optimization of the differ-
ent stages has been described by De Bondt
et al.
(
) and Mourgues
et al.
(
). Most major
apple cultivars, e.g. 'Braeburn', 'Delicious', 'Elstar', 'Fuji', 'Gala', 'Golden
Delicious' and 'Jonagold' have been transformed with different bacterial con-
structs as have the pears 'Beurre Bosc', 'Comice', 'Conference' and 'Passe
Crassane' (Norelli and Aldwinkle,
,
). Transformation using
Agrobacterium
spp. is the most widely used method of gene transfer to apples and pears. In the
field situation
A. tumefaciens
causes crown gall disease and
A. rhizogenes
causes
hairy root disease. They naturally infect the host plants and insert some of
their own DNA, the transfer or T-DNA, into the chromosomal DNA of the
