Biology Reference
In-Depth Information
Improvement in delivery of ds RNAs is highly desirable (
Yu et al. 2012
) and
experiments with placing the ds RNA into liposomes or other protectants may be
needed to enhance stability of the ds RNA. In some cases, RNAi is systemic (trans-
ferred into cells throughout the body), whereas in other cases it is not. Sometimes
the knockdown of gene expression is brief and sometimes it is lengthy. At present
it is difficult to predict the outcome of RNAi experiments (
Terenius et al. 2011
).
Additional research is needed to select appropriate target genes and targeted
regions within the gene. The optimal length of the ds RNA is another issue that
needs to be resolved to enhance efficacy of the RNAi response (
Saleh et al. 2006,
Whyard et al. 2009
).
Table 14.6
lists some of the relevant concerns regarding mak-
ing RNAi a useful pest-management tool in the future.
14.5.7.1 RNAi in Crop Plants
RNAi is both systemic and heritable in plants. The small interfering RNAs (siRNAs)
can move between cells through channels in cell walls and thus can be distrib-
uted throughout the plant (
Gordon and Waterhouse 2007, Perrimon et al. 2010
).
Although much RNAi work in plants involves improving the production or nutri-
tional value of crops, some efforts are being made to make crop plants resistant
to insects (
Baum et al. 2007, Mao et al. 2007, Price and Gatehouse 2008, Huvenne
and Smagghe 2010, Zha et al. 2011
).
These efforts are just beginning and require additional research before this tech-
nology can become a commercial success. Issues that need to be resolved before
transgenic crops can be used effectively in agricultural pest management include
resolving the concentration of ds RNA needed to induce optimal silencing (note
that 100% suppression of gene expression is unlikely). The sequence used will
determine whether off-target effects occur in the plant, or in other insects. The
ideal situation would have the target pest, and only the target pest, affected
at an appropriate level. It also will be critical to resolve the effective length of
the ds RNA; this determines how well the ds RNA is taken up and how efficiently
the silencing is obtained. ds RNA that is too short or too long can cause nontar-
get effects or result in poor uptake, respectively. The persistence of the silencing
effect also is important; transient expression may result in reduced control. Another
issue is which life stage of the pest to target; younger stages often exhibit greater
silencing effects. To date, RNAi directed against Lepidoptera appears very difficult
to achieve, which is unfortunate because this order contains serious pest species
(although lepidopterans may be well controlled by the insertion of
Bacillus thuring-
iensis
[
Bt
] toxin genes into crop plants). An important target would be insects that
feed on phloem such as aphids, leafhoppers, psyllids, and whiteflies;
Bt
toxin genes
cannot control these pests. At present, it is not known whether ds RNAs can be
transmitted in phloem. Finally, many RNAi experiments result in a knockdown of
gene function in the pest, but not a complete silencing of the gene. Thus, the effect
