Miryam Pérez-Cañamás

To establish productive infections, viruses must be able both to subdue the host metabolism for their own benefit and to counteract host defences. This frequently results in the establishment of viral–host protein–protein interactions that may have either proviral or antiviral functions. The study of such interactions is essential for understanding the virus–host interplay. Plant viruses with RNA genomes are typically translated, replicated, and encapsidated in the cytoplasm of infected cells. Despite this, a significant array of their encoded proteins has been reported to enter the nucleus, often showing high accumulation at subnuclear structures such as the nucleolus and/or Cajal bodies. However, the biological significance of such a distribution pattern is frequently unknown. Here, we explored whether the nucleolar/Cajal body localization of protein p37 of Pelargonium line pattern virus (PLPV, genus Pelarspovirus, family Tombusviridae), might be related to potential interactions with the nucleolar/Cajal body marker proteins, fibrillarin and coilin. The results revealed that p37, which has a dual role as coat protein and as suppressor of RNA silencing, a major antiviral system in plants, is able to associate with these cellular factors. Analysis of (wildtype and/or mutant) PLPV accumulation in plants with up- or downregulated levels of fibrillarin or coilin have suggested that the former might be involved in an as yet unknown antiviral pathway, which may be targeted by p37. The results suggest that the growing number of functions uncovered for fibrillarin can be wider and may prompt future investigations to unveil the plant antiviral responses in which this key nucleolar component may take part.

DNA cytosine methylation is one of the main epigenetic mechanisms in higher eukaryotes and is considered to play a key role in transcriptional gene silencing. In plants, cytosine methylation can occur in all sequence contexts (CG, CHG, and CHH), and its levels are controlled by multiple pathways, including de novo methylation, maintenance methylation, and demethylation. Modulation of DNA methylation represents a potentially robust mechanism to adjust gene expression following exposure to different stresses. However, the potential involvement of epigenetics in plant-virus interactions has been scarcely explored, especially with regard to RNA viruses. Here, we studied the impact of a symptomless viral infection on the epigenetic status of the host genome. We focused our attention on the interaction between Nicotiana benthamiana and Pelargonium line pattern virus (PLPV, family Tombusviridae), and analyzed cytosine methylation in the repetitive genomic element corresponding to ribosomal DNA (rDNA). Through a combination of bisulfite sequencing and RT-qPCR, we obtained data showing that PLPV infection gives rise to a reduction in methylation at CG sites of the rDNA promoter. Such a reduction correlated with an increase and decrease, respectively, in the expression levels of some key demethylases and of MET1, the DNA methyltransferase responsible for the maintenance of CG methylation. Hypomethylation of rDNA promoter was associated with a five-fold augmentation of rRNA precursor levels. The PLPV protein p37, reported as a suppressor of post-transcriptional gene silencing, did not lead to the same effects when expressed alone and, thus, it is unlikely to act as suppressor of transcriptional gene silencing. Collectively, the results suggest that PLPV infection as a whole is able to modulate host transcriptional activity through changes in the cytosine methylation pattern arising from misregulation of methyltransferases/demethylases balance.