Virus induced gene silencing(VIGS)and suppression are RNA-specific defense and counter-defense circuits in plant-virus interactions.These phenomena have been investigated extensively with an Agrobacterium-mediated transient expression system.In this study,a virus-based transient expression system was developed to study these phenomena.A Tomato bushy stunt virus(TBSV)viral vector with an inactivated P19 suppressor gene, referred to as pHST2-14,was chosen to express the P1 of Tobacco etch virus (TEV).TEV P1 is a component of a well-characterized VIGS suppressor, TEV P1/HC-Pro protein.A TBSV defective interfering RNA(DI)that contains the 3'proximal portion of a green fluorescence protein (GFP) gene,DI-P, was used as a silencing inducer of the homologous GFP gene on GFP transgenic Nicotiana benthamiana (NbGFP) plants.The TEV P1 gene was inserted into pHST2-14 to generate TBSV-P1.Transcripts of TBSV-P1 were then mixed with DI-P transcripts and inoculated onto NbGFP plants.DI-P consistently accumulated in NbGFP plants that were inoculated with TBSV-P1 and DI-P,and efficiently induced silencing of GFP transgene.These results demonstrate that a TBSV-based co-delivery system can provide a new alternative tool to investigate gene silencing and its influence by a TBSV-expressed foreign protein.It also can be used to elucidate functions of endogenous genes in plants.