Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/6/2005
Publication Date: 9/9/2005
Citation: Hily, J., Scorza, R., Webb, K.K., Ravelonandro, M. 2005. Accumulation of the long class of sirna is associated with resistance to plum pox virus throughout the life cycle of a transgenic woody perennial (Prunus domestica l.) plum. Molecular Plant-Microbe Interactions. MPMI 16:794-799 (2005). Interpretive Summary: The development of virus resistant transgenic annual herbaceous model plants, such as tobacco and Arabidopsis, have shown that a key component responsible for resistance is the production of small, interfering ribonucleic acids (siRNAs) by the inserted genes. The production of siRNA and resistance, produced by genetic engineering in tree fruits, had not, up to this time, been demonstrated. We show in this report that a transgenic plum tree produced in our laboratory through genetic engineering is highly resistant to plum pox virus, the most serious virus disease of stone fruit trees. We have further shown that this plum tree produces siRNAs and that these are responsible for the virus resistance. This work shows for the first time the association between siRNA production and resistance to an economically important virus in a stone fruit crop. Further, we show in this report that non-genetically engineered plum trees also respond to virus infection by naturally producing siRNA, but that this siRNA differs in its size from that produced in transgenic plums and is not as effective in resisting the virus.
Technical Abstract: We have previously demonstrated that plum (Prunus domestica L.), a woody perennial tree fruit species, transformed with the coat protein of Plum pox potyvirus (PPV-CP) exhibited characteristics of post transcriptional gene silencing (PTGS). This transgene-silenced clone (C5) was shown to be highly resistant to PPV. The transgene insert and the PPV resistance were mutually inherited in the progeny of C5. In order to better characterize RNA silencing in woody perennial crops, we investigated hallmarks of PTGS throughout the life cycle of this resistant clone. We show in the current report that C5 constitutively produces a short (22 nt) and a long (25-26 nt) species of short interfering RNA (siRNA) throughout the life cycle from embryo to mature plant in the absence of PPV inoculation. Unlike siRNA, methylation at the sites analyzed, and transcription of the PPV-CP transgene appears to be 're-set' following seed germination. Uninoculated transgenic and untransformed PPV susceptible clones do not display DNA methylation nor do they produce detectable levels of siRNA. Upon infection, these susceptible clones did produce siRNA, but only the short (22 nt) species. These findings show that plum trees naturally respond to virus infection by initiating PTGS or PTGS-like mechanisms that involve the production of siRNA. We further suggest that high-level virus resistance in Prunus species requires the production of the long size class of siRNA.