Transgene-specific and event-specific molecular markers for characterization of transgenic papaya lines resistant to Papaya ringspot virus

The commercially valuable transgenic papaya lines carrying the coat protein (CP) gene of Papaya ringspot virus (PRSV) and conferring virus resistance have been developed in Hawaii and Taiwan in the past decade. Prompt and sensitive protocols for transgene-specific and event-specific detections are essential for traceability of these lines to fulfill regulatory requirement in EU and some Asian countries. Here, based on polymerase chain reaction (PCR) approaches, we demonstrated different detection protocols for characterization of PRSV CP-transgenic papaya lines. Transgene-specific products were amplified using different specific primer pairs targeting the sequences of the promoter, the terminator, the selection marker, and the transgene, and the region across the promoter and transgene. Moreover, after cloning and sequencing the DNA fragments amplified by adaptor ligation-PCR, the junctions between plant genomic DNA and the T-DNA insert were elucidated. The event-specific method targeting the flanking sequences and the transgene was developed for identification of a specific transgenic line. The PCR patterns using primers designed from the left or the right flanking DNA sequence of the transgene insert in three selected transgenic papaya lines were specific and reproducible. Our results also verified that PRSV CP transgene is integrated into transgenic papaya genome in different loci. The copy number of inserted T-DNA was further confirmed by real-time PCR. The event-specific molecular markers developed in this investigation are crucial for regulatory requirement in some countries and intellectual protection. Also, these markers are helpful for prompt screening of a homozygote-transgenic progeny in the breeding program.