Suppressor activity against cosuppression conferred by the petunia vein clearing virus genome

The petunia genome contains an endogenous pararetrovirus, petunia vein clearing virus (PVCV). Previous analyses indicate that PVCV has suppressor activity against RNA silencing, but the suppressor protein has not been identified. Here we tested whether an open reading frame (ORF) of PVCV confers the activity that can suppress cosuppression of the CHS-A genes encoding chalcone synthase, which has a high rate of RNA turnover in the petal tissues of petunia. Petunia transformants that express PVCV ORF under the control of cauliflower mosaic virus 35S promoter were produced. The transgenic plants were crossed with those that have CHS-A cosuppression to produce plants that contain both the PVCV ORF transgene and CHS-A transgene. The coexistence of these transgenes resulted in phenotypic changes: pigmentation of various extents occurred on the originally white petals of CHS-A cosuppression phenotype. The generation of pigmented portions in flower petals coincided with higher transcript levels of CHS-A and PVCV ORF and less CHS-A short interfering RNA. These results indicate that the PVCV ORF can suppress CHS-A cosuppression and change the flower color phenotype when it is expressed as a transgene.