Selective targeting of miRNA-regulated plant development by a viral counter-silencing protein.

The cucumber mosaic virus (CMV) 2b protein suppresses RNA silencing and determines viral symptoms. Among Arabidopsis thaliana lines expressing 2b proteins from mild (LS and Q CMV) or severe (Fny CMV) strains, only Fny 2b-transgenic plants displayed strong symptom-like phenotypes in leaves, stems and flowers, together with stunting of main root growth and increased emergence of lateral roots. However, LS and Fny 2b proteins both enhanced lateral root length. Micro (mi)RNA-mediated cellular mRNA turnover was inhibited in Fny 2b-transgenic plants, but there was no evidence for this in LS 2b-transgenic plants. Both 2b proteins efficiently suppressed small interfering (si)RNA-mediated RNA silencing, suggesting that 2b proteins can target the siRNA pathway without disrupting miRNA-regulated RNA turnover. Thus, symptom induction is not an inevitable consequence of RNA silencing suppression. For CMV, strain-specific differences between the 2b silencing proteins determine whether only one or both small RNA-guided RNA destruction pathways are disrupted.