Transgenic expression of tomato bushy stunt virus silencing suppressor P19 via the pOp/LhG4 transactivation system induces viral-like symptoms in tomato.

During natural infection, the Tomato bushy stunt virus (TBSV) silencing suppressor protein P19 is expressed at high levels, which are required for optimum viral pathogenicity and silencing suppression. To date, expression of P19 in transgenic host plants has failed to achieve comparable expression levels and thus has provided only limited information on its in planta effects. To obtain high P19 expression and study its effects on host plant development in the absence of virus infection, we generated HA-tagged P19 (P19HA)-transgenic tomato reporter plants using the pOp/LhG4 transactivation system, which separates transformation from transgene expression. Upon reporter plant activation with a strong constitutive promoter, the transactivated F1 plants expressed high levels of a functional P19HA protein and displayed multiple abnormal phenotypes, some of which were highly reminiscent of the symptoms described previously for TBSV-infected tomato. Moreover, phenotype severity correlated with P19HA expression level, amount of bound miRNA/miRNA* duplexes, and accumulation of miRNA target transcripts. Together our results demonstrate that the tomato miRNA pathway is markedly compromised by P19, in particular when this protein is relatively abundant, as occurs during natural infection. We suggest that such interference with endogenous silencing may be responsible for at least some of the symptoms characteristic of TBSV-infected tomato.