HC-Pro viral suppressor from tobacco vein banding mosaic virus interferes with DNA methylation and activates the salicylic acid pathway

Salicylic acid (SA) is an important signalling molecule that is synthesized by plants and induces the expression of resistance genes. The SA pathway is typically activated by DNA viruses as well as RNA viruses. Here, we demonstrated that heper-component protease (HC-Pro) encoded by tobacco vein banding mosaic virus (TVBMV) decreases in DNA methylation at the promoters of the regulators ACD6 and NPR1 in the SA pathway. We found that the overexpression of HC-Pro increases the expression of components in the SA pathway in plants. The results revealed that HC-Pro interferes in DNA methylation and activates the SA pathway in the HC-Pro transgenic plants and TVBMV-infected plants. We further found that the accumulation of siRNAs derived from the promoter repeats of ACD6 and NPR1 is greatly reduced in the HC-Pro plants. Our results suggested that HC-Pro-mediated interference with DNA methylation is likely caused by a reduction in accumulation of siRNAs.