Influence of coat protein transgene copy number on resistance in transgenic line 63-1 against Papaya ringspot virus isolates

Line 63-1 is a 'Sunset'-derived transgenic papaya expressing the coat protein (CP) gene from a mild mutant of a Hawaiian isolate of Papaya ringspot virus (PRSV). Previous work showed that line 63-1 R, plants exhibited a range of resistance to severe PRSV isolates from Hawaii (HA), Jamaica (JA), Thailand (TH), and Brazil (BR). Genetic and molecular data obtained in this study confirm that line 63-1 has two CP transgene insertion sites; segregation analysis shows that the CP and the npt H genes are present at both loci. To study the potential effect of gene dosage on resistance, various populations of R-1, R-2, and R-3 seedlings were challenged by PRSV HA, BR, and TH. A R, population obtained by self-pollination of line 63-1 hermaphrodite R-0 plant exhibited resistance to all three isolates. The percentage of plants resistant to all three PRSV isolates increased in 63-1-derived populations as a result of recurrent selection. Additional genetic studies demonstrate that the number of resistant plants in a 63-1-derived population is directly correlated with the number of plants with multiple transgene copies. We conclude that transgene dosage plays a major role in affecting the resistance of 63-1 to PRSV isolates from various geographical locations.