<p style="text-align: justify;">Papaya ringspot virus (PRSV) is the most important papaya pathogen and a major limiting factor in commercial papaya production in the world. All major production areas I the Western Hemisphere [Brazil, the Caribbean region, Mexico, and USA (Florida and Hawaii)] and Eastern Hemisphere (the Philippines, Taiwan, Thailand, and China) are affected. Once introduced to a locality, PRSV becomes established in weeds or perennial plants and has never been successfully eradicated from any region. The aphid vectors involved in dissemination of PRSV are found worldwide. Thus, even if new PRSV-free papaya trees are planted in an area where PRSV occurs, they soon become infected.</p>

<p style="text-align: justify;">Transgenic papaya expressing the coat protein (CP) gene of Hawaiian PRSV isolate HA5-1 (see description of events 55-1/63-1) is highly resistant to PRSV isolates from Hawaii but more susceptible to a number of PRSV isolates from other geographic locations. In general, resistance conferred by the CP gene of a potyvirus (such as PRSV) is RNA-mediated and sequence-specific. Resistance is only effective when the transgene has a high similarity to the CP gene of the infecting virus and appears to be due to post-transcriptional gene silencing (PTGS). PTGS can take place when the antisense RNA, either directly from transcription of an antisense CP transgene or indirectly from sense transgene mRNA via RNA-dependent RNA polymerases, binds to the complementary regions of viral RNA transcripts in the cytoplasm to form duplexes, which are subsequently cleaved by dsRNA-specific nucleases into 21-23 nt RNAs that interfere with transcription of homologous RNAs. This requirement for sequence homology points to a need to utilize local PRSV isolates as sources of the CP gene in order to obtain effective PRSV resistance in transgenic papaya.<br />
In addition to translatable CP genes, transgene constructs with non-translatable CP genes (e.g., antisense constructs, or frame-shift mutants, or three-in-frame stop-codon mutations) have been shown to be effective in conferring resistance to potyviruses. Papaya line X17-2 was developed to contain a frame-shift mutant of the CP gene from PRSV isolate H1K (a thymidine residue inserted after the initiation codon to yield a frame-shift). Subsequent analysis of the fifth generation progeny of the original transformant revealed that the mutation was repaired and that the CP transgene was both translatable and expressed in later generations.</p>
