Rice Genetics IV - IRRI books - International Rice Research Institute

Mutant replicase. With replicase genes, deletions or substitutions have been madein conserved domains such as glycine-aspartate-aspartate (see Waterhouse andUpadhyaya 1998). In most of these cases, the resistance has been effective only againstthe homologous virus or its close relatives.Mutant movement protein. Mutant movement proteins seem to confer broaderresistance. Use of a mutated open reading frame (ORF) of one of the so-called “tripleblock”genes (ORF2 encoding a ~13 kDa protein) from the potexvirus white clovermosaic virus (WCMV), having the six conserved hydrophilic amino acids convertedto hydrophobic amino acids, conferred resistance to WCMV and other potexvirusesand a carlavirus (Beck et al 1991) in tobacco. Similarly, broad resistance has beenfound using mutant movement proteins from TMV (Cooper et al 1995) and potatoleafroll virus (Tacke et al 1996). In both cases, the plants produced resistance to virusesoutside their respective families.PDR using RNARecent studies suggest that most of the PDR genes operate at the nucleic acid leveland not at the protein level. The three different RNA-mediated mechanisms that havebeen shown to be successful in transgenic plants are (1) expression of an RNA thatcompetes against viral RNA for viral proteins, (2) expression of a ribozyme designedto cleave viral RNA at a specific sequence, and (3) expression of sense and/or antisenseviral RNA in plants to induce specific degradation of viral RNA (i.e., posttranscriptionalgene silencing or PTGS).Competitive RNA. Some viruses replicate and package small single-stranded satelliteRNAs that are devoid of replicase or coat protein genes. Transgenic plants expressingcloned copies of satellite RNA of tobacco rattle virus (Gerlach et al 1987) orCMV (Harrison et al 1987) showed reduced levels of helper virus replication andamelioration of symptom development when challenged with respective helper viruses.It is thought that this attenuation may be caused by the transgene satellite RNAcompeting with viral RNA for replicase and/or coat protein. However, certain virus/satellite/host-plant combinations give enhanced, rather than reduced, viral symptoms(Tien and Wu 1991).Defective interfering (DI) RNAs (which are small RNAs derived from the helpervirus) can also ameliorate symptoms caused by their helper virus and probably by thesame competitive mechanism as mentioned before. Transgenic plants expressing a DIRNA from cymbidium ringspot virus (CRSV) showed reduced virus symptoms whenchallenged with the virus (Koller et al 1993).Ribozymes. The discovery by Haseloff and Gerlach (1988) of a sequence-dependentself-cleavage of satellite RNA (to produce unit-size satellite RNA from multimers)has led to the development of the “ribozyme” concept. They showed that ribozymescan be made with desired specificity to cleave (in cis or trans) any target RNA moleculesat a GUC sequence (cleavage occurs immediately after the C). Transgenictobacco expressing a ribozyme targeted against potato spindle tuber viroid showed avery high level of resistance to infection by the viroid (Yang et al 1997).408 Upadhyaya et al