Rice Genetics IV - IRRI books - International Rice Research Institute

esistance obtained was delayed symptom development but not complete immunity.By integrating a transgene encoding the RNA-dependent RNA polymerase of RYMV,Pinto and others (1999) have produced rice conferring resistance to RYMV strainsfrom different African locations. Study of the most resistant line, using transcriptionanalysis, indicated that the resistance is driven from an RNA-based mechanism associatedwith posttranscriptional gene silencing.Recently, a self-cleaving circular satellite RNA has been found to be associatedwith RYMV (Collins et al 1998). It may be possible to make transgenes derived fromthis satellite RNA that will confer RYMV resistance.Problems and prospectsAt least two of the PDR approaches have proven to be successful in rice: the cpmediatedresistance to RTSV, RRSV, RSV, RHBV, and RYMV and replicase-mediatedresistance to RYMV. To date, efforts by different research groups to engineerresistance to RTBV have not been successful. Although controlling RTSV may preventthe disease complex from spreading within the rice crop, resistance to RTSV willnot protect rice from RTBV coming from other sources. Nevertheless, we believe thata cosuppression or PTGS approach may be successful, as the RTBV RNA transcriptsmay be vulnerable early in the infection cycle.It seems very likely that rice with resistance to RGSV could be readily achievedusing the N gene as this approach has worked well for the other tenuiviruses. The factthat introducing one or more of the RRSV PDR genes has yielded RRSV resistance isencouraging because it opens the door for delivering resistance for the other six riceinfectingreoviruses. Reoviruses spend most of their life within a particle core and it istherefore possible that they are less vulnerable to straightforward PDR strategies.Several strategies, special to reoviruses, could be pursued, such as overexpressing theprotein that covers the channels of the core particle through which viral RNA is exported.Another approach might be to produce RNAs that contain prematurely terminatedreading frames but have the “panhandle” sequences that label them as specificsegments. Such molecules should compete with viral RNAs for packaging into newcore particles but not produce wild-type protein in the next round of translation/replication.An advantage of using a PDR transgene is that it can be followed in breedingprograms using defined markers, whereas natural resistance genes are often polygenicand usually have, at best, only linked markers. However, polygenic resistance islikely to be more durable in the field. We will probably see, in the future, the use ofcombinations of natural virus and insect resistance genes with virus and insect resistancetransgenes. In cases where there is no effective or easily transferred natural ricevirus resistance gene, a synthetic resistance gene might be made based on the PDRapproach and then introduced into a rice background containing natural virus resistanceto other viruses.The use of virus resistance transgenes poses concerns over issues other than theireffectiveness. In the late 1980s, when transgenic plants containing viral genes were416 Upadhyaya et al