Sources of target specificity associated with the recovery against Pea seed-borne mosaic virus infection mediated by RNA silencing in pea.

SUMMARY Transgenic peas containing a copy of the Pea seed-borne mosaic virus (PSbMV) isolate DPD1 NIb sequence develop a 'recovery' phenotype when challenged with either the homologous (DPD1) or a heterologous (NY) PSbMV isolate. However, the specificity of the subsequent resistance differs with respect to the initiation by and targeting of different virus isolates. Analysis of tissue in which recovery had been induced by either of the isolates, revealed the presence of low molecular RNA molecules (siRNA) derived from degradation of the NIb transgene mRNA. When non-transgenic scions were grafted on to transgenic stocks in which recovery had been induced, all the scions became infected, indicating that the virus can be exported from recovered tissue. Experiments in which recovered scions were grafted on to non-transgenic stocks revealed that the recovery phenotype could be maintained in the apparent absence of a source of virus. However in a number of cases, side-shoots which developed on the non-transgenic stock became infected. These results indicate that recovered tissue contains extremely low levels of infectious virus with the potential, directly or indirectly, to confer the observed resistance specificity. Indirectly, the viral genome could act as a source of specific siRNA molecules, which are present in infected tissues but are below the level of detection in recovered tissues. These could act in conjunction with siRNAs derived from the transgene mRNA to maintain a level of PTGS (post-transcriptional gene silencing) activity which is effective in preventing further accumulation of homologous or related viruses. We suggest a model to explain the differential specificity.