Gene silencing and plant antiviral immunity

Abstract

Posttranscriptional gene silencing (PTGS) refers to the trans-inactivation of homologous genes caused by increased mRNA degradation. Examples of PTGS include gene quelling in Neurospora and RNA interference (RNAi) in Caenorhabditis elegans and Drosophila. Antisense-mediated gene silencing (ASGS) is another mechanism used for posttranscriptional silencing of gene expression. These epigenetic means for controlling gene expression also exist in plants, although their function is not clear, but it has been observed that PTGS is associated with antiviral resistance. Our understanding of PTGS and ASGS is derived from numerous studies in plants and their associated viruses. Virus-mediated suppression of PTGS leads to reduced resistance in the host; plants with mutations in genes related to PTGS have abnormal responses when infected with viruses and show subsequent overexpression of viral genes encoding suppressor proteins.One such component implicated in PTGS in plants is RNA-dependent RNA polymerase (RdRP). Studies of a mutant RdRP strain of Arabidopsis that exhibited increased susceptibility to viral infection suggested that RdRP might play a role in eliciting an antiviral response. To this end, Xie et al. 1xAn important role of an inducible RNA-dependent RNA polymerase in plant antiviral defense. Xie, Z. et al. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 6516–6521Crossref | PubMed | Scopus (122)See all References1 isolated the tobacco gene encoding RdRP and observed increased RdRP enzyme activity and gene expression after infection with tobacco mosaic virus (TMV). Compared with wild-type plants, transgenic tobacco plants expressing antisense RdRP were more susceptible to TMV and potato virus X (PVX), had increased levels of viral RNA, and had more pronounced disease symptoms. The generation of these antisense RdRP plants was feasible, suggesting that RdRP was probably not required for development.In another recent study on PTGS, DiSerio et al. 2xSense- and antisense-mediated gene silencing in tobacco is inhibited by the same viral suppressors and is associated with accumulation of small RNAs. DiSerio, F. et al. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 6506–6510Crossref | PubMed | Scopus (43)See all References2 used viral suppression to examine more fundamental aspects of the mechanism underlying posttranscriptional sense and antisense silencing of genes in tobacco. Small RNAs diagnostic for PTGS were associated with both forms of gene silencing. This observation suggests that PTGS and ASGS share parts of a common mechanism leading to silencing of genes and strongly supports an earlier hypothesis that ASGS is an antisense form of PTGS. Understanding the fundamental aspects of PTGS mechanisms in plants should shed light on many basic biological properties of other organisms.