Plant DNA viruses and gene silencing

Gene silencing is a multifaceted phenomenon leading to propagative down-regulation of gene expression. Gene silencing, first observed in plants containing transgenes, can operate both at the transcriptional and post-transcriptional levels. Silencing effects can be triggered by nuclear transgenes and by cytoplasmic RNA viruses, and it can be propagated between these elements and endogenous plant genes that share sequence homology. Although some aspects of gene silencing are becoming better understood, little is yet known about the relationship between nuclear and cytoplasmic events. Plant DNA viruses-- both the ssDNA geminiviruses and the reverse-transcribing pararetroviruses-- have properties with the potential to initiate gene silencing in the nucleus and in the cytoplasm. Characteristics include production of multiple copies of viral DNA genomes in the nucleus, illegitimate integration of viral DNA into host chromosomes mimicking transgene transformation, and generation of abundant viral RNAs in the cytoplasm. Evidence is emerging that geminiviruses and plant pararetroviruses can interact with the gene silencing system either from introduced DNA constructs or during viral pathogenesis. Some observations suggest there are complex relationships between DNA viral activity, transcriptional and post-transcriptional gene silencing mechanisms. DNA viruses also have properties consistent with an ability to counteract the plant silencing response. In this article, features of plant DNA viruses are discussed in relation to gene silencing phenomena, and the prospects for understanding the interaction between nuclear and cytoplasmic silencing processes.