RNA silencing is a mechanism of gene regulation by sequence specific RNA degradation and is involved in controlling endogenous gene expression and defense against invasive nucleic acids such as viruses. RNA silencing has been proven to be transmitted between scions and rootstocks through grafting, mostly using transgenic plants. It has been reported that RNA silencing of tobacco endogenous genes, NtTOM1 and NtTOM3, that are required for tobamovirus multiplication, resulted in high resistance against several tobamoviruses. In the present study, we examined the graft transmission of RNA silencing for conferring virus resistance to non-transgenic scions of the same and different Nicotiana species grafted onto rootstocks in which both NtTOM1 and NtTOM3 were silenced. Non-transgenic Nicotiana tabacum (cvs. Samsun and Xanthi nc) and N. benthamiana were used as scions for grafting onto the rootstocks silenced with both genes. Short interfering RNA (siRNA) of NtTOM1 and NtTOM3 was detected in both the scions and the rootstocks eight weeks after grafting. The leaves were detached from the scions and inoculated with several tobamoviruses. The virus accumulation was tested by ELISA and northern blot analysis. The viruses were detected in grafted scions at extremely low levels, showing that virus resistance was conferred. These results suggest that RNA silencing was induced in and virus resistance was conferred to the non-transgenic scions by grafting onto silenced rootstocks. The effect of low temperature on siRNA accumulation and virus resistance was not significantly observed in the scions.

Attenuated viruses have been isolated and studied not only as a practical means of controlling virus diseases but also to gain a molecular understanding of viral virulence and cross protection. They have been isolated from crop fields and generated through high/low temperature treatment or by mutagens such as nitrous acid and ultraviolet irradiation. Some viruses have been beneficially used in fields and evaluated for one or more decades. Molecular genetic studies on attenuated viruses have revealed that amino acid substitutions are located in replicase and the movement protein in tobamovirus, protein 2b for cucumovirus, and P1 and HC-Pro for potyvirus. In most cases, with a few exceptions, symptom attenuation is positively correlated with a reduced level of RNA silencing suppression. Molecular mechanisms underlying virus attenuation and cross protection and the rationale for practical use of attenuated viruses for effective virus disease control are discussed.

Small RNA-mediated gene silencing pathways play important roles in the regulation of development, genome stability and various stress responses in many eukaryotes. Recently, a new type of small interfering RNAs (qiRNAs) approximately 20–21 nucleotides long in Neurospora crassa have been shown to mediate gene silencing in the DNA damage response (DDR) pathway. However, the mechanism for RNA silencing in the DDR pathway is largely unknown in plants. Here, we report that a class of small RNAs (qiRNAs) derived from rDNA was markedly induced after treatment by DNA-damaging agents [ethyl methanesulphonate (EMS and UV-C)], and that aberrant RNAs (aRNAs) as precursors were also highly induced after the DNA damage treatment in rice. However, these RNAs were completely abolished in OsRecQ1 (RecQ DNA helicase homologue) and OsRDR1 (RNA-dependent RNA polymerase homologue) mutant lines where either gene was disrupted by the insertion of rice retrotransposon Tos17 after the same treatment. DNA damage resulted in a more significant increase in cell death and a more severe inhibition of root growth in both mutant lines than in the WT. Together, these results strongly suggest that both OsRecQ1 and OsRDR1 play a pivotal role in the aRNA and qiRNA biogenesis required for the DDR and repair pathway in rice, and it may be a novel mechanism of regulation to the DDR through the production of qiRNA in plants.