Comparative reactions of recombinant papaya ringspot viruses with chimericcoat protein (CP) genes and wild-type viruses on CP-transgenic papaya

Transgenic papaya cultivars SunUp and Rainbow express the coat protein (CP) gene of the mild mutant of papaya ringspot virus (PRSV) HA. Both cultivars are resistant to PRSV HA and other Hawaii isolates through homology-depend ent resistance via post-transcriptional gene silencing. However, Rainbow, w hich is hemizygous for the CP gene, is susceptible to PRSV isolates from ou tside Hawaii, while the CP-homozygous SunUp is resistant to most isolates b ut susceptible to the YK isolate from Taiwan. To investigate the role of CP sequence similarity in overcoming the resistance of Rainbow, PRSV HA recom binants with various CP segments of the YK isolate were constructed and eva luated on Rainbow, SunUp and non-transgenic papaya. Non-transgenic papaya w ere severely infected by all recombinants, but Rainbow plants developed a v ariety of symptoms. On Rainbow, a recombinant with the entire CP gene of YK caused severe symptoms, while recombinants with only partial YK CP sequenc es produced a range of milder symptoms. Interestingly, a recombinant with a YK segment from the 5' region of the CP gene caused very mild, transient s ymptoms, whereas recombinants with YK segments from the middle and 3' parts of the CP gene caused prominent and lasting symptoms. SunUp was resistant to all but two recombinants, which contained the entire CP gene or the cent ral and 3'-end regions of the CP gene and the 3' non-coding region of YK, a nd the resulting symptoms were mild. It is concluded that the position of t he heterologous sequences in the recombinants influences their pathogenicit y on Rainbow.