Agriculture Reference
In-Depth Information
as a genetic 'bridge species' that can make the introgression of useful genes from
A.
fistulosum
into common onion a practical possibility.
Interest in
A. roylei
arose when it was found to be completely resistant to
onion downy mildew and partially resistant to onion leaf blight,
B. squamosa
. The
hybrid between
A. roylei
and
A. cepa
produced pollen with few chromosomal
abnormalities and was therefore fertile, and could be backcrossed to
A. cepa
.
Downy mildew resistance was shown to be due to a single dominant gene, which
has been named '
Pd
' and which is located at one end of chromosome 2 in
A.
roylei
. This was shown from a genetic linkage map based on molecular markers
and by direct visualization of a DNA marker for the
Pd
gene on the chromosome
using a fluorescent stain linked to probe DNA.
A DNA sequence of 20 base pairs closely linked to
Pd
and which can be
amplified for ease of identification has been developed (termed a 'sequence-
characterized amplified region' (SCAR) marker). Genetic mapping of the crosses
between
A
.
roylei
and onion showed that the mildew resistance gene was located
at the end of chromosome 3 (Scholten
et al
., 2007). The discriminating power of
the SCAR marker to show only downy mildew-resistant genotypes declined
during the programme of repeated back-crossing of resistance-carrying
genotypes with onion to produce resistant, good-quality onions. Hence, new
AFLP markers for the resistance gene had to be developed (Scholten
et al.
,
2007). Furthermore, it was found that a region of the
A. roylei
fragment
introduced into onion was lethal when homozygous within an onion genetic
background.
However, one individual in which the
A. roylei
genetic fragment was
shorter carried the mildew resistance gene but had lost the lethal recessive
gene (Scholten
et al.
, 2007). From this, breeding lines homozygous for mildew
resistance that could transmit resistance to all their progeny were developed.
Several companies that have developed downy mildew-resistant onion cvs are
using genetic markers to follow the incorporation of the
Pd
resistance gene into
their breeding lines. This avoids having to perform time-consuming assess-
ments for resistance to the disease at every step, and represents a good example
of marker-assisted breeding. Commercial downy mildew resistant cvs are
expected to be available in 2008-2009, some 20 years after the original
identification of resistance in
A .roylei
(Scholten
et al.
, 2007).
The potentiality exists to introduce resistance to many diseases using genetic
transformation (Eady, 2002). Incorporation of engineered viral protein genes
into plant genomes has conferred resistance to viral disease in a number of crops.
The DNA sequences that code for the coat-proteins of allium carlaviruses and
potyviruses, groups that include a number of serious diseases (see Table 5.3),
have been determined. Using this information it should be possible to engineer
and express these sequences in alliums to induce resistance.
Many onion fungal pathogens cause damage by hyphal invasion, and this
can be combated by various resistance genes. A number of these have been
identified and shown to act by preventing the growth of fungal cell walls,
