Event-specific qualitative and quantitative detection of three genetically modified papaya events using a single standard reference molecule

A novel standard reference plasmid (pGEM-PAPAYA3) was constructed as a positive control and reference standard for event-specific qualitative and quantitative detection of genetically modified (GM) papaya (55-1, 16-0-1, and Huanong No.1). The plasmid pGEM-PAPAYA3 contained the specific fragments of papaya endogenous reference gene chymopapain and the 3 GM papaya events. The qualitative PCR assay using pGEM-PAPAYA3 was established with a limit of detection correlating to approximately 5–50 copies of papaya haploid genomes. In the quantitative PCR assay, the square regression coefficients (R2) ranged from 0.993 to 0.997. The standard deviation and relative standard deviation values for repeatability ranged from 0.04 to 0.25 and 0.05%–0.86%, respectively. The method was used to test 11 papaya products purchased from Thailand, China, the Philippines and the USA, and the results revealed 2 varieties of GM papaya in 5 of the 11 samples tested. These results indicate the developed detection methods with the standard reference plasmid are applicable for identifying 3 GM papaya events.