Assessment of Emerging Virus Threats for Application of Transgenic Papaya Resistant to Papaya Ringspot Virus

Abstract

The transgenic resistance conferred by the coat protein ( CP ) gene of a virus has become the most effective method to prevent papaya from infection by the noxious Papaya ringspot most virus ( PRSV ) . In May 1998, PRSV - CP gene transgenic papaya Rainbow and Sun Up were deregulated and granted approval for commercialization, representing the first successful application of a transgenic fruit tree in the world . Although the transgenic varieties are not resistant to most other PRSV strains from different geographic areas, breakdown of the resistance in Hawaii has not been recorded. Apart from Hawaii, a CP gene of a native Taiwan strain PRSV YK was used to transform Taiwan papaya cultivars by Agrobacterium-mediated transformation. The transgenic lines obtained showed various levels of resistance, ranging from delay of symptom development to complete immunity. However, during the4-year- long field trials, the highly resistant PRSV CP-transgenic papaya lines were found to be susceptible to an unrelated potyvirus Papaya leaf distortionmosaic virus ( PLDMV ) which has been identified in many places in Taiwan . To overcome the potential threat of PLDMM papaya lines carrying a chimeric transgene , including parts of the CP genes of PRSV and PLDMM and conferring resistance against both PRSV and PLDMM were developed . These transgenic papaya lines with double resistance are considered to have great potential for the control of PRSV and PLDMV in Taiwan. During the field test period, a super strain , PRSV 519 , able to break down the resistance of YK-CP transgenic lines , was also noticed . The breakdown of the transgenic resistance by 519 15 controlled by a strong gene- silencing suppressor, HC-Pro gene , of the virus in a transgene - homology - independent manner . we suggest that a chimeric construct targeting multiple viral genes , such as the genes of the CP , the replicase and the gene silencing suppressor HC - Pro of a potyvirus , may minimize the chances of the emergence of a super virus strain that can overcome the transgenic resistance , and provides more durable resistance against different viruses.