SUMMARY
Transgenic plums of Prunus domestica L. transformed with the Plum pox virus coat protein gene (PPV-CP) were the subjects of three experiments undertaken in Romania. In the first experiment PPV-CP transgenic clones C2, C3, C4, C5, C6 and PT3 were evaluated for Sharka resistance under high natural infection pressure. Transgenic clone C5, subsequently named „HoneySweet?, showed high resistance to PPV. None of the C5 trees became naturally infected by aphids for more than ten years. The resistance of C5 was based on post-transcriptional gene silencing (PTGS). In the second experiment we assessed the effect of two heterologous viruses (Prunus necrotic ringspot virus and Prune dwarf virus) on the efficacy and stability of PTGS - mediated resistance to PPV displayed by the C5 plum. The engineered resistance to PPV in C5 transgenic plums was stable and was not suppressed by the presence of the assayed heterologous viruses. Some PPV-CP transgenic plum clones that are susceptible to PPV including C2, C3, C4 and PT3 display a constitutive transcription of PPV-CP sequence. In the third experiment we used these plants to assess the environmental safety issues related to potential hazards concerning the emergence of PPV variants. The serological and molecular variability of PPV detected in transgenic and in conventional plums revealed that the transgenic plums do not affect the diversity of indigenous PPV populations. We suggest that the safety and efficacy data developed in Romania and other European countries and in the United States of America over the last 15 years justifies an expansion of the field tests in Romania and in other countries that are experiencing the damages to plum production caused by PPV.