interaction(Fig. 8.4). Plant breeders have historically taken advantage of this system,
although it can sometimes take many years to identify a plant line with the desired resist-
ance and to breed that trait into useful cultivars. Another disadvantage to the breeding
approach is that unwanted or undesirable genes may sometimes be linked to theRgene,
and it can be difficult to separate them from the Rgene using traditional breeding
methods. Finally, usefulRgenes are sometimes not easy to transfer because of barriers
in crossing different species. Therefore, the ability to clone and transfer a singleRgene
from one plant variety or species to another represents an encouraging option to adapt
and speed up the process.
A promising approach at engineering resistance is seen in the application of a specific
resistance gene to ward off a bacterial disease in rice (Ronald 1997). Bacterial blight is a
destructive disease of domesticated rice (Oryza sativa) in Africa and Asia, caused by the
pathogenXanthomonas oryzaepathovaroryzae. Scientists looking for alternative sources
of resistance to bacterial blight identified a wild relative of rice, O. longistaminata,
native to Mali, which is resistant to the pathogen but has very low quality and yield in
terms of grain production. Through careful genetic and molecular studies, anRgene
calledXa21was isolated from the wild species. This gene has been introduced into
domesticated rice using particle bombardment and it confers strong resistance against
strains ofX. oryzaecarrying theAvrgene recognized byXa21. Through efforts of scientists
scattered across the globe, theXa21gene has been incorporated into several rice varieties of
agricultural importance. The use of transgenic rice as a food crop is still controversial, and
its adoption has been slow compared to crops like soybean, corn, and cotton. So although
transgenic lines of blight-resistant rice are poised for application, they are currently not
widely grown for food production. At least one-third of the world’s population, including
many developing countries, depends on rice as the major source of calories they consume.
Figure 8.4.Resistance to specific strains of plant pathogens can be conferred by the protein product
of a single resistance (R) gene. Most plantRgenes function by recognizing the activity or presence of
a specific virulence factor from the pathogen. In addition to the ability to induce basal defenses, these
pathogen “effectors” are also active in attacking various host proteins. The protein products ofRgenes
guard against pathogens via surveillance of specific targeted host proteins. When theseR-gene-
mediated defenses are triggered, the plant responds with a hypersensitive response and rapid acti-
vation of defense gene expression.
8.3. TRAITS FOR IMPROVED CROP PRODUCTION 203