Generation of a Papaya Hybrid Variety with Broad-Spectrum Transgenic Resistance to Papaya ringspot virus and Papaya leaf-distortion mosaic virus

The aphid-borne potyvirus, Papaya ringspot virus (PRSV), is the major limiting factor for papaya production worldwide. Transgenic resistance conferred by the PRSV-coat protein (CP) gene and based on the mechanism of post-transcriptional gene silencing (PTGS), has become the most effective method to protecting papaya from infection by the noxious virus. The PRSV-CP transgenic lines of cultivars ‘Rainbow’ and ‘SunUp’ have been commercialized in Hawaii since 1998, and to date, they have retained their resistance against the virus. However, another aphid-borne potyvirus, Papaya leaf-distortion mosaic virus (PLDMV), which occurs in Okinawa and Taiwan, has emerged as a serious threat for PRSV-CP transgenic papaya. To deal with the new emerging problem, double resistance in transgenic papaya carrying a chimeric construct containing partial CP genes of PRSV and PLDMV was generated. In addition, a super strain of PRSV was recently identified, that contains a stronger gene silencing suppressor capable of effectively shutting off PTGS and single or double CP-transgenic resistance in a homology-independent way. To solve this problem, transgenic resistance generated by an untranslatable construct targeting the PRSV HC-Pro gene was developed and the transgenic papaya lines provided broad-spectrum resistance against the PRSV super strain and PRSV isolates from different geographical locations. The event-specific molecular markers, derived from the flanking sequences of the transgene integration, in combination with the sex-linked markers, significantly fastened the molecular breeding process for pyramiding of single, double and super transgenic resistance into a commercial hybrid papaya cultivar. The super commercial hybrid cultivar of papaya with broad-spectrum resistance to different strains of PRSV and PLDMV has a great potential for application in different geographic regions of the world.