Therefore, development of disease-resistant rice could potentially make a major impact on
alleviating hunger.
It has been known for decades that a previous inoculation with a virus can often protect a
plant from subsequent infections by closely related viruses. This form of immunization of
the plant has been known ascross-protectionand has been employed with active viruses in
limited cases. Crop plants can be intentionally inoculated with mild strains of a virus in the
hope that this will protect the plant against future viral outbreaks. Much like vaccination
with live viruses in humans, this strategy does have certain risks. In the case of inoculating
with mild strains of a plant virus, there is a chance that the mild strain will present a drag on
yield or that a virulent strain will emerge from the population and cause severe disease.
With the advent of genetic engineering in plants, it became possible to express just a
portion of plant viruses within the host. It turns out that this approach can likewise lead
to resistance to closely related viruses.
Most plant viruses are relatively simple in terms of their genetic makeup, consisting of
just a few genes carried by either an RNA or DNA genome encased in a protein coat. By
expressing a portion of the viral genome constitutively in plants, a system of specific target-
ing of incoming, similar RNA sequences can be activated in a potential host plant. This
RNA silencingsystem is active in many organisms, including humans, and might have
evolved partially as a surveillance–protection system against invading viruses.
A great success story using RNA-mediated virus resistance has developed in the pro-
duction of papaya in Hawaii (Gonsalves 1998) (see Life Box 8.1). Virtually the entire pro-
duction of this crop in Hawaii was threatened in the mid-1990s by the spread of the papaya
ringspot virus (PRSV). Infection with the virus was so common, and the effects on yield
were so severe by the late-1990s that many fields had already been abandoned. By expres-
sing the coat protein gene of a mild strain of PRSV in papaya (Fig. 8.5), transgenic plants
Figure 8.5.Transgenic resistance to papaya ringspot virus (PRSV) is possible because of the process
of RNA-mediated gene silencing. To make virus-resistant plants, a portion of the coat protein (CP)
gene of PRSV was transferred to and expressed in transgenic papaya plants. Following transcription,
the RNA triggers targeted, sequence-specific degradation of similar RNA sequences, such as that
found on incoming PRSV viral RNA. The initial degradation of RNA is carried out by an enzyme
calledDICER, and the process is mediated by an enzymatic structure called theRNA-induced silen-
cing complex(RISC). Ultimately, this can lead to RNA cleavage, as well as blockage of transcription
or translation of the target gene.
204 GENES AND TRAITS OF INTEREST FOR TRANSGENIC PLANTS