Papaya and the aphid transmitted virus are cosmopolitan in distribution . PRSV is controlled by different methods such as rouging of infected plants, use of barrier crops, cross protection, and transgenic resistance . PRSV disease management, via vector controlling, is very difficult to conduct whilst cross protection for controlling PRSV disease is not effective worldwide. Resistance against PRSV has not been found in Carica papaya . On the other hand, several wild Carica species such as C. cauliflora, C. pubescens, and C. quercifolia are resistant to PRSV, but these are sexually incompatible with C. papaya . The most effective method of controlling plant viruses is through enhancing population resistance . Genetic transformation of plants has made it possible to introduce selected genes into plants for controlling plant diseases and pests. The concept of pathogen derived resistance has stimulated research into obtaining virus resistance in papaya through gene technology. Pathogen-derived resistance is mediated either by proteins encoded by transgenes (protein-mediated) or by the transcripts produced from the transgene (RNA-mediated). Recently, research has indicated that pathogen-derived resistance is mediated by an RNA-based posttranscriptional gene-silencing mechanism. Protein-mediated resistance provides moderate protection against a broad range of related viruses whilst RNA-mediated resistance offers high levels of protection to closely related strains of the virus . The RNAi technology has enabled the induction of an immune reaction to PRSV. This technology has been at the forefront of the new era in the development of eco-friendly molecular tools, which can be suppressed by specific genes which are responsible for disease management.
PRSV is the most destructive viral disease of papaya. Control of PRSV includes rouging infected plants and spraying them with aphicides. However, rouging cannot stop the spread of the disease once it is established. Similarly, spraying with aphicides is often ineffective since the virus is transmitted to the plants before the aphids are killed . The PRSV disease management has been focused on developing tolerant or resistant varieties of papaya, but these varieties are rarely planted due to poor fruit quality and vigour . PRSV-resistant gene is available in some wild varieties related to the Carica species. But the development of PRSV-resistant varieties through conventional breeding methods has been complicated due to the sexual incompatibility of wild species and cultivated papaya [51, 52]. Disease tolerance in back crosses with commercial papaya also limits this approach for PRSV disease management. Cross protection was used to control PRSV which involved the use of a mild virus strain against economic damage caused by severe strains of the same virus [6, 53]. The cross protection strategy of inoculating papaya with a mild strain of PRSV provides resistance against severe PRSV strain infection in Taiwan . Cross protection depends on the availability of mild strains that can be used for effective protection against the target virus. Cross protection needs extra agricultural practice and care. However, strain specificity and the technical difficulties associated with propagating pure strains of mild forms of the virus and the unavailability of such mild strains limit the benefits of this approach . Field evaluations revealed that cross protection was marginally effective for PRSV management evaluation in the field . Researchers from Cornell University and the University of Hawaii initiated the development of PRSV-resistant papaya by gene technology. The concept of pathogen derived resistance was proposed by Sanford and Johnston  for developing resistance against pathogens. This research group has applied the concept of pathogen derived resistance which has stimulated research into obtaining virus resistance through gene technology. Pathogen derived resistance is governed either by protein-mediated or RNA-mediated methods. An alternative strategy using RNA-mediated gene silencing with transgenic plants expressing viral genes has been developed . Resistance levels of PRSV differ with environmental factors and plant development stages despite of the success with this approach. Broad spectrum resistance against different PRSV isolates depends on the homology of transgenes with viral target genes and the genetic divergence of different PRSV strains which are correlated with their geographical distribution . The transgenic papaya varieties resistant to PRSV against different viral strains must be developed individually for various papaya growing regions. The development of PRSV-resistant lines is generally considered the best strategy for efficient PRSV disease control in papaya for long-term protection . 2b1af7f3a8