Transfer of Plum Pox Virus Coat Protein Genes from a Plum Pox-resistant Transgenic Clone of Prunus domestica Plum to Its Progeny through Hybridization

Sharka or plum pox virus (PPV) is a major disease of stone fruit and causes severe economic losses in Europe. There is little resistance to PPV in most Prunus species, thus genetic engineering represents a potentially useful approach to obtain resistant germplasm. Transgenic plums containing the PPV coat protein (CP) or the related papaya ringspot virus (PRV)-CP gene were produced through Agrobacterium tumefaciens-mediated transformation. These transgenic plum clones were then evaluated for resistance to PPV infection in the greenhouse by graft or aphid inoculation with PPV. While symptoms of PPV appeared in most transgenic clones, all plants of PPV-CP transgenic clone C5 were symptomless and ELISA and immunocapture-reverse transcriptase PCR negative for over three years following inoculation with two strains of PPV (Ravelonandro et al., Plant Dis. 81:1231-1235, 1997). Clone C5, which contains multiple copies of the PPV-CP gene, was hybridized with PRV-CP transgenic plants or untransformed plum cultivars. Progeny were obtained containing no transgenes, only the PPV-CP, only the PRV-CP, or both the PRV-CP and PPV-CP transgenes. Seedlings were inoculated with PPV. At 5 and 11 months post-inoculation, seedlings containing the PPV-CP genes from C5 were symptomless and ELISA negative. Seedlings containing only PRV-CP transgenes or non-transformed controls showed symptoms of PPV infection and were ELISA positive. These results indicate that the PPV-CP transgenes can be transferred to progeny through hybridization and that these genes can impart resistance to PPV in transgenic seedlings. The inheritance of the multicopy inserts of the PPV-CP and PRV-CP transgenes is being analyzed. The combined effects of both transgenes on resistance to PPV and the stability of PPV resistance in the progeny of the resistant C5 transgenic line are currently under evaluation.