Abstract: 【Objective】The objective of this study is to analyze the RNA silencing suppressor activity of SP6 protein encoded by SRBSDV segment 6 (S6) and to clear whether SRBSDV encodes RNA silencing suppressor to interfere with RNA silencing pathway. 【Method】To investigate the suppress activity of SP6 on local and systemic silencing induced by sense RNA, SP6 was coinfiltrated with the reporter green fluorescent protein (GFP) in transgenic Nicotiana benthamiana line 16c carrying GFP. SP6 was also coinfiltrated with GFP and dsGFP to determine whether it interfered with local and systemic silencing induced by dsRNA. GFP and SP6 were infiltrated in different parts of leaves in a same plant, respectively, to characterize whether SP6 can block the spread of RNA silencing signal. SP6 was over-expressed in the PVX heterogenous system to find out whether it was a pathogenicity determinant. 【Result】The local and systemic silencing can be delayed, but not inhibited, in N. benthamiana leaves agroinfiltrated with SP6 and GFP. SP6 can block the spread of the silencing signal and reverse the GFP silencing, but can not inhibit the silence induced by dsRNA. Expression of SP6 enhanced Potato virus X pathogenicity in N. benthamiana. 【Conclusion】SP6 is an RNA silencing suppressor encoded by Southern rice black-streaked dwarf virus. It may target the initiate step and prevent the spread of RNA silencing signal in the RNA silencing pathway.

Key words: Southern rice black-streaked dwarf virus, SP6, gene silencing, suppressor