Agriculture Reference
In-Depth Information
from selected individual garlic bulbs, a number of clonal families were
developed. Subsequently these families were planted in comparative trials and
the highest yielding families were selected as those with the highest mean
values of the product (weight of harvested bulb
weight multiplication
between planting and harvest). The weight multiplication is simply the weight
of the harvested bulb divided by the weight of the planted clove. High yielding
clones are selected on the basis of several years' results from such trials
(Messiaen
et al
., 1993). For shallots it is difficult to devise a simple selection
index unless mother bulbs originate from the same plot and are of similar size.
Eady (2002), in reviewing this subject, restricted the term 'genetic trans-
formation' to the transfer of a characterized set of genes using gene insertion
technologies as opposed to the introduction of non-characterized DNA from
crosses with wild relatives or by the artificial fusion of cells of different allium
species (Buiteveld
et al.
, 1998). To transform plants with discrete, charac-
terized genes, the DNA sequences have to be delivered to a cell and incor-
porated in the chromosomes. The gene has to be regulated so that it is
expressed in the desired way and the transformed cell must proliferate and
generate a plantlet that can be characterized and selected as containing the
inserted DNA. Finally, the selected transformed plant must be hardened-off and
grown and assessed under normal cultural conditions.
Successful genetic transformation has been reported in onion, shallot, garlic
and leek (Eady, 2002; Eady
et al.
, 2003a, b, 2005; Zheng
et al.
, 2004). The gene
insertion system using the bacterial vector
Agrobacterium
has been most widely
used. Wild-type
Agrobacterium
infects roots and transfers a specific section of DNA,
known as T-DNA, into the host plant's genome. Infected cells proliferate and form
a tumour that acts as a food source for the proliferating bacteria. It has been
possible to suppress tumour induction and add designed sequences of DNA to
Agrobacterium
T-DNA and utilize its DNA insertion ability to incorporate coding for
desired genes into plant genomes. Inserted along with the DNA required for a
desired new property must be 'promoter' DNA coding for genes to regulate the
novel gene, i.e. to 'switch it on' when required, and also DNA coding for 'reporter'
genes. The reporter genes encode properties that enable the selection of the
transformed plantlets from those with unmodified genomes, and which can be
used to verify that the transformation sequence has been incorporated.
Promoters derived from both cauliflower mosaic virus and also
chrysanthemum have been used in allium transformation. Herbicide resistance
and antibiotic resistance genes have been used as reporters to select out the
transformed cell cultures, since only those incorporating these properties can
survive in growth media to which are added the appropriate antibiotic or
herbicide. A gene that codes for a fluorescent protein has also been used to show
