RNA silencing is a nucleotide sequence-specific process that regulates gene expression in diverse eukaryotic organisms. In addition to controlling development, RNA silencing also functions as an antiviral mechanism in higher plants and invertebrates. Plants infected with some viruses also contain satellite RNA (satRNA), which required the helper virus to supply proteins for replication. However, it is not known that how the trilateral interactions among host plants, helper viruses and satRNAs relevant to RNA silencing.

To address this issue, supported by grants from the Ministry of Science and Technology of China, GUO Huishan and her colleagues in the State Key Laboratory of Plant Genomics, Institute of Microbiology, Chinese Academy of Sciences, found that satRNA of diverse structures stimulate plant primary antiviral RNA silencing (Du et al., 2007. Journal of Virology) by characterization of satRNA-derived siRNAs (satsiRNAs) in plants infected with Cucumber mosaic virus (CMV).

They have further found that satRNA reduced the accumulation of the 2b protein, a CMV-encoding suppressor of RNA silencing, and attenuated the disease symptoms caused by CMV infection (Hou et al., 2011. Molecular Plant Pathology).

By detailed biochemical and functional analysis, they also found that a satsiRNA targeted to helper viral RNAs triggering the host RNA-dependent RNA polymerase 6 (RDR6)-mediated degradation of CMV RNAs. Competition experiment with wild type and mutant CMV RNAs further revealed that the satsiRNA-related RDR6-dependent degradation of CMV RNAs was inhibited by the CMV-2b. Their data show a new mechanism of satRNA in reducing helper viral RNAs by satRNA-derived siRNA that triggered host antiviral silencing. The related paper was published in Journal of Virology (Zhu et al., 2011) (doi:10.1128/JVI.05806-11).

This is the first functional study that provides evidence showing the important biological functions of satsiRNAs in homeostatic interactions among the host, virus, and satellite RNA to determine the final outcome of the virus infection. Their data also provide evidence that the plant silencing mechanism is involved in the pathogenicity of satRNA.