Enhanced viral intergenic region–specific short interfering RNA accumulation and DNA methylation correlates with resistance against a geminivirus

Abstract

RNA silencing mediated by short-interfering RNA (siRNA) is used by plants as a defense against viruses. In the case of geminiviruses, viral DNA is targeted at the transcriptional level, while virus-derived transcripts are targeted by posttranscriptional silencing. Mungbean yellow mosaic India virus (MYMIV), a bipartite geminivirus, causes yellow mosaic disease in soybean (Glycine max). A soybean variety resistant to this disease has been identified (line PK416). To understand the molecular mechanism underlying this resistance, distribution of MYMIV-derived siRNAs along the viral genome was compared in resistant and susceptible plants, using samples obtained in the first few days following inoculation. We observed that, in the resistant soybean variety, most of the virus-derived siRNAs were complementary to the intergenic region (IR), while in the susceptible variety (line JS335), a majority of the siRNAs corresponded to coding regions of the viral genome. Most of the IR-specific siRNA molecules produced in the resistant plants were 24 nt in size. Bisulfite sequencing showed that, in the resistant plants, a higher level of methylation occurred in the IR of viral DNA.