Plant viruses encode suppressors of post-transcriptional gene silencing (PTGS), an adaptivedefense response that limits virus replication and its spread in plants.The helper component proteinase (HC-Pro) of the potato virus A (PVA,genus Potyvirus) suppresses PTGS of silenced transgenes.Here,the effectof HC-Pro on siRNA-directed interference in the tobacco mosaic virus (TMV) was examined by using atransient Agrobacterium tumefaciens-based delivery system in intact tissues.It was shown that the interferenceeffect was completely blocked by co-infiltration with HC-Pro plus siRNA constructs in both systemic andhypersensitive hosts.In the system host,all plants agro-infiltrated with HC-Pro plus siRNA constructsdisplayed the same symptoms as the negative control.Meanwhile,TMV RNA accumulation was found to beabundant in the upper leaves using reverse transcriptase-PCR (RT-PCR) and Northern blot assays.On thecontrary,plants agro-infiltrated with the siRNA construct alone were free of symptoms.Therefore,ourstudy suggests that the transient expression of HC-Pro inhibited the siRNA-directed host defenses againstTMV infection. Plant viruses encode suppressors of post-transcriptional gene silencing (PTGS), an adaptivedefense response that limits virus replication and its spread in plants. The helper component proteinase (HC-Pro) of the potato virus A (PVA, genus Potyvirus) suppresses PTGS of silenced transgenes. Here, the effect of HC-Pro on siRNA-directed interference in the tobacco mosaic virus (TMV) was examined by using atransient Agrobacterium tumefaciens-based delivery system in intact tissues. It has been shown that the interference effect was completely blocked by co- infiltration with HC-Pro plus siRNA constructs in both systemic and hypersensitive hosts.In the system host, all plants agro-infiltrated with HC-Pro plus siRNA constructsdisplayed the same symptoms as the negative control.Meanwhile, TMV RNA accumulation was found to be abdant in the upper leaves using reverse transcriptase-PCR (RT-PCR) and Northern blot assays. On the contrary, plants agro-infiltrated with the siRNA construct alone were free from symptoms. herefore, ourstudy suggests that the transient expression of HC-Pro inhibited the siRNA-directed host defenses against TMV infection.