Rice Genetics IV - IRRI books - International Rice Research Institute

an ambisense coding strategy (Fig. 1D). Genome segment S1 encodes the replicase.Segments S2, S3, and S4 use ambisense expression to produce six proteins includinga nucleoprotein (N protein), which is associated with the virus particle and sometimesreferred to as the coat protein.Rice stripe virus. Hayakawa et al (1992) first mentioned virus resistance in ricevia PDR for RSV. Two japonica rice varieties (Kinuhikari and Nipponbare) were transformedwith the N protein (referred to by the authors as the coat protein) gene of RSVunder the control of the CaMV 35S promoter containing the castor bean catalaseintron in the 5’ untranslated region of the transcript. Three out of five primary transgeniclines screened for RSV resistance were symptomless, which correlated with thetransgene protein presence, and two lines showed segregation of transgene proteinsand symptomless plants. Similar results were obtained when using the coat proteingene of a Chinese isolate of RSV (Yan et al 1997). This demonstration of N protein–mediated resistance for RSV suggests that it will be a successful approach for resistanceto other tenuiviruses such as RGSV and RHBV. Because this mechanism seemsto be acting before viral replication commences, the unusual nature of the tenuivirusreplication strategy is perhaps irrelevant for this mechanism of resistance.Rice hoja blanca virus. Encouraged by the N protein–mediated resistance in riceagainst RSV (Hayakawa et al 1992), a research group at the Centro Internacional deAgricultura Tropical (L. Calvert et al, personal communication) has investigated virusprotection by expression of the RHBV N gene in rice. They have transformed theLatin American indica rice variety Circa-8, using the biolistic method of transformation,with a construct encoding the N gene under the control of the CaMV 35S promoter.Plant lines obtained show an extreme resistance to RHBV, Mendelian inheritanceof this trait, and unaltered agronomic performance characteristics (in glasshousetests) in the absence of the virus. The best lines hold great promise for breeding withrice containing conventional RHBV resistance to produce a superior rice variety foruse in Latin America. The CIAT team has found that some of its resistant lines containN genes with partial deletions. The most resistant lines had no detectable levels of Nprotein and very low levels of mRNA. This suggests that, in contrast to the RSVresults, the mechanism is operating at the RNA level, is probably posttranscriptionaldegradation, and can operate against viruses using ambisense gene expression.SobemovirusRice yellow mottle virus. The disease caused by rice yellow mottle virus (RYMV) is aserious problem for African rice growers using large-scale irrigated systems. RYMVcontains a 4,550-nt-long nonpolyadenylated single-stranded positive-sense RNA withfour ORFs (Yassi et al 1994, Fig. 1E). The ORF1 product (P1) is required for viralreplication (Bonneau et al 1998). The ORF2 product is a polyprotein comprising viralprotease, the helicase, and the RNA-dependent RNA polymerase. ORF4 encodes thecoat protein and the function of ORF3 is yet to be determined.Kouassi and others (1997) have produced transgenic Taipei 309 rice expressingthe RYMV-cp gene under the control of the Ubi1 promoter and demonstrated transgeneproduct accumulation and resistance to RYMV. In the majority of cases, the type ofEngineering for virus resistance in rice 415