Viral Genome Methylation as an Epigenetic Defense against Geminiviruses

Abstract

Geminiviruses encapsidate single-stranded DNA genomes that replicate in plant cell nuclei through doublestranded DNA intermediates that associate with cellular histone proteins to form minichromosomes. Like most plant viruses, geminiviruses are targeted by RNA silencing and encode suppressor proteins such as AL2 and L2 to counter this defense. These related proteins can suppress silencing by multiple mechanisms, one of which involves interacting with and inhibiting adenosine kinase (ADK), a cellular enzyme associated with the methyl cycle that generates S-adenosyl-methionine, an essential methyltransferase cofactor. Thus, we hypothesized that the viral genome is targeted by small-RNA-directed methylation. Here, we show that Arabidopsis plants with mutations in genes encoding cytosine or histone H3 lysine 9 (H3K9) methyltransferases, RNA-directed methylation pathway components, or ADK are hypersensitive to geminivirus infection. We also demonstrate that viral DNA and associated histone H3 are methylated in infected plants and that cytosine methylation levels are significantly reduced in viral DNA isolated from methylation-deficient mutants. Finally, we demonstrate that Beet curly top virus L2؊ mutant DNA present in tissues that have recovered from infection is hypermethylated and that host recovery requires AGO4, a component of the RNA-directed methylation pathway. We propose that plants use chromatin methylation as a defense against DNA viruses, which geminiviruses counter by inhibiting global methylation. In addition, our results establish that geminiviruses can be useful models for genome methylation in plants and suggest that there are redundant pathways leading to cytosine methylation.

RNAi in Plants: An Argonaute-Centered View

“…AGO4 is also involved in plant defense against DNA viruses. In tissues that have recovered from infection, DNA of Beet curly top virus L2 2 mutant is hypermethylated and host recovery requires AGO4 (Raja et al, 2008), suggesting that DNA methylation mediated by AGO4 is a mechanism used by plants to defend against DNA viruses. Rice encodes four AGO4 homologs (AGO4a, AGO4b, AGO15, and AGO16) (Kapoor et al, 2008).…”

The role of virus-derived small interfering RNAs in RNA silencing in plants

“…Recent research has indicated that transcriptional gene silencing (TGS) caused by small RNA-guided methylation is also a defense mode against DNA viruses [65]. Tomato yellow leaf curl China virus (TYLCCNV), a member of the geminiviruses, has a satellite RNA called DNA  (betasatellite).…”

La bataille du silence

“…Étant donné qu'AGO2, AGO4 et AGO7 recrutent majoritairement les sARN avec une adénosine en 5'-terminal et AGO5 les sARN commençant par une cytosine, leur association avec le RISC antiviral n'est pas à exclure [24,32,33]. De même, AGO4 et AGO6 étant responsables du silencing transcriptionnel chez Arabidopsis, leur rôle potentiel dans la défense contre les virus à ADN est probable [34,35].…”