validates and provides evidence that both the biolistics
and the Agrobacterium-mediated gene transfer technologies
are equally effective and reliable methods for incorporating blackheart
resistance based on RNA silencing transgenic constructs into
‘Smooth Cayenne’, a cultivar that has been difficult to improve
by conventional breeding. Using these technologies and based on
the results from our field studies, 15 of 39 (38%) transgenic lines
under evaluation were regarded as blackheart resistant, having no
blackheart symptoms in two or more trials. Further selection was
required to remove unwanted off-types or poor performing lines,
however 5 blackheart resistant lines were selected that performed
as well or better than the control in terms of fruit characteristics and
quality. These included V59(1) and V61(1), both developed through
Agrobacterium-mediated transfer of the ppo gene construct, as well
as 140-10.1 and 158-2, both developed through biolistics with
the former containing an opp.ppo gene construct and the latter
an opp.i.ppo construct. A fifth blackheart resistant line with suitable
fruit characteristics was 161-19, developed through biolistic
gene transfer of the opp.cca.i.acc.ppo construct, but requires further
evaluation to determine if natural flowering is also controlled.
Blackheart resistant lines were produced at a higher frequency
for Agrobacterium-mediated transformation, even though there
have been reports that Agrobacterium traditionally has a limited
host range, whereby monocotyledonous crops and cereals in particular
appear to be more recalcitrant to Agrobacterium infection
than dicotyledons (Yang and Christou, 1994). Nevertheless other
workers (Firoozabady et al., 2006) have successfully transformed
‘Smooth Cayenne’ pineapple using Agrobacterium-mediated transformation
with an efficiency of up to 60%. The reported efficiency
for Agrobacterium-mediated transformation may be attributed to
a more targeted incorporation of the introduced gene construct
in the plant’s genome compared to the more random and incomplete
integration through the use of biolistics. This is supported by
the work of Cheng et al. (2001), who compared the two transformation
methods in rice, using several plasmids. It was found that
irrespective of the plasmid used, the re-arrangement frequency of
the transgene (consisting of the promoter, specific gene of interest
and terminator) was six times lower in transgenic plants obtained
through Agrobacterium-mediated transformation as compared to
biolistics resulting in higher transformation efficiency.
Blackheart resistant plants were obtained regardless of the
transgenes used. Constructs based on sense (ppo), antisense-sense
(opp.ppo) and hairpin RNA with a spliceable intron (opp.i.ppo;
opp.cca.i.acc.ppo) were all capable of causing gene silencing and