Rice Genetics IV - IRRI books - International Rice Research Institute

yield losses per annum of about US$700 million, $40 million, and $10 million, respectively(Herdt 1991, Ramasamy and Jatileksono 1996).Although viral spread can be controlled by regular insecticide application, themost cost-effective, lowest risk, and environmentally sound method of protection forrice from viruses is by the use of virus resistance genes. Protection of rice from virusescan be mediated by natural resistance genes that inhibit virus replication (orspread) in the plant and by genes that repel virus-vectoring insects. Resistance canrange from complete immunity to moderate tolerance and resistance sources havebeen identified in indica, japonica, and wild species of rice (see Waterhouse andUpadhyaya 1998). Sebastian et al (1996) have mapped the chromosomal location ofthe green leafhopper (GLH) and RTSV resistance gene(s). Progress has also beenmade in identifying the RYMV resistance locus (Pressoir et al 1998). However, manyof the resistance sources found for rice viruses are polygenic and in most cases theirmechanism of action is unknown and they are also difficult to transfer to elite varieties.A few of the single-gene resistance sources are under threat of being overcome byevolving virus strains. Since the first demonstration of virus resistance by a viral coatprotein transgene from the tobacco mosaic virus (TMV) in 1986 (Powell-Abel et al1986), there have been many examples of success stories of engineered resistance(virus-derived transgenes) in dicotyledonous plants. Recent advances in rice transformationand molecular characterization of several rice-infecting viruses have greatlyfacilitated the application of pathogen-derived resistance (PDR) in rice (see Waterhouseand Upadhyaya 1998). Here we describe various strategies of PDR that have beensuccessful in engineering virus resistance in crops other than rice. We also outline theprogress made in understanding the underlying mechanisms of PDR as well as theprogress, problems, and prospects of genetically engineered virus-resistant rice.Engineered virus resistance in plantsRecent advances in tissue culture, molecular biology, and transformation technologyhave facilitated engineering pathogen (originally termed as parasite)-derived resistancewith a wide range of potential PDR genes for crop plants. The concept of PDRis that genes from a pathogen’s genome can be expressed inappropriately in its host,thereby interrupting the infection cycle (Sanford and Johnston 1985). To use PDReffectively, it is important to understand the life cycle of the target virus at the molecularlevel. Three genes common to both DNA and RNA viruses are a replicase, acoat protein, and a movement protein gene. The replicase is responsible for initiatingreplication of the invading virus to produce many new viral RNA molecules. The coatprotein packages the newly produced RNA molecules to form virus particles, whereasthe movement protein interacts with the viral RNA and the plant cell’s plasmodesmatato facilitate the spread of the viral RNA from cell to cell. Interfering with anyone of these genes will interrupt the virus life cycle. These three genes have been themost common targets for PDR against plant viruses. According to the success stories,mostly with dicot plants and to a limited extent with monocot plants, PDR for viruscan be divided into three categories: (1) the expression in plants of wild-type virus406 Upadhyaya et al