SILENCING IN PRUNUS: A NATURAL DEFENSE DEVELOPED BY WOODY FRUIT TREES IN RESPONSE TO VIRUS INFECTION

Transgenic research permits elucidating with relevancy the molecular interactions between the engineered virus gene and plant genome expression. Silencing is among the natural phenomenon involving the natural regulation of endogeneous gene. Recently, Hily et al. (2005) have demonstrated that it represents the basic machinery implied in the blockage of virus spread in transgenic plum C-5. To anticipate the future exploitation of a transgenic plum resistant to Plum pox potyvirus (PPV), greenhouse studies were carried out between 2003–2005 by inoculating more than one hundred plums with the 3 combinations of PPV co-existing with either Ilarviruses (Prune dwarf, PDV and Prunus necrotic ringspot viruses, PNRSV) or Trichovirus (Apple chlorotic leaf spot virus, ACLSV). Observations on symptom expression and diverse analyses of virus genome replication elucidate the co-existence of these viruses, the case of uneven distribution of these viruses and the subsequent plant phenotypes. As expected, the resistant plum C-5 can be infected with Ilarviruses and Trichoviruses, but interestingly, their association with PPV did not lead to the resistance breaking of sharka. These are consistent with the lack of PPV RNA detection in C-5 scions, and notably, the molecular detection of siRNA (small interfering RNAs) in leaf samples collected from experiments confirm the specific knock-down of PPV genome replication that is correlated with the phenotypic behaviour of the plum C-5 to sharka disease.