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Abstract
BackgroundRNA-dependent RNA polymerases (RDRs) function in anti-viral silencing in Arabidopsis thaliana and other plants. Salicylic acid (SA), an important defensive signal, increases RDR1 gene expression, suggesting that RDR1 contributes to SA-induced virus resistance. In Nicotiana attenuata RDR1 also regulates plant-insect interactions and is induced by another important signal, jasmonic acid (JA). Despite its importance in defense RDR1 regulation has not been investigated in detail.Methodology/Principal FindingsIn Arabidopsis, SA-induced RDR1 expression was dependent on ‘NON-EXPRESSER OF PATHOGENESIS-RELATED GENES 1’, indicating regulation involves the same mechanism controlling many other SA- defense-related genes, including pathogenesis-related 1 (PR1). Isochorismate synthase 1 (ICS1) is required for SA biosynthesis. In defensive signal transduction RDR1 lies downstream of ICS1. However, supplying exogenous SA to ics1-mutant plants did not induce RDR1 or PR1 expression to the same extent as seen in wild type plants. Analysing ICS1 gene expression using transgenic plants expressing ICS1 promoter:reporter gene (β-glucuronidase) constructs and by measuring steady-state ICS1 transcript levels showed that SA positively regulates ICS1. In contrast, ICS2, which is expressed at lower levels than ICS1, is unaffected by SA. The wound-response hormone JA affects expression of Arabidopsis RDR1 but jasmonate-induced expression is independent of CORONATINE-INSENSITIVE 1, which conditions expression of many other JA-responsive genes. Transiently increased RDR1 expression following tobacco mosaic virus inoculation was due to wounding and was not a direct effect of infection. RDR1 gene expression was induced by ethylene and by abscisic acid (an important regulator of drought resistance). However, rdr1-mutant plants showed normal responses to drought.Conclusions/Significance RDR1 is regulated by a much broader range of phytohormones than previously thought, indicating that it plays roles beyond those already suggested in virus resistance and plant-insect interactions. SA positively regulates ICS1.
Highlights
RNA silencing refers to a set of gene regulation mechanisms occurring in most eukaryotes, whereby transcript stability or translatability is suppressed in a sequence-specific manner, guided by small 19–24 nt RNA molecules [1,2]
1 mM Salicylic acid (SA), samples were taken over a time course spanning 72 hours (h) and were analysed by reverse transcription coupled with quantitative PCR (RTqPCR) for RDR1 and pathogenesis-related 1 (PR1) expression (Figure 1)
This is in contrast to the work of Yu and colleagues, who reported that AtRDR1 induction took longer to become detectable (4 to 8 h) with no diminution of AtRDR1 expression apparent at 24 h post-treatment, the point at which the analysis was terminated [16]
Summary
RNA silencing refers to a set of gene regulation mechanisms occurring in most eukaryotes, whereby transcript stability or translatability is suppressed in a sequence-specific manner, guided by small 19–24 nt RNA molecules [1,2]. RNA silencing is an important component of anti-viral defense in plants [3,4]. Doublestranded structures within viral RNA can be cleaved by dicer-like (DCL) nucleases to generate double-stranded small interfering (si)RNAs. In Arabidopsis thaliana, there are four DCL enzymes, of which DCL4 and DCL2 are the most important in the generation of virus-derived siRNAs [5,6,7,8]. RNA-dependent RNA polymerases (RDRs) function in anti-viral silencing in Arabidopsis thaliana and other plants. Salicylic acid (SA), an important defensive signal, increases RDR1 gene expression, suggesting that RDR1 contributes to SA-induced virus resistance. In Nicotiana attenuata RDR1 regulates plant-insect interactions and is induced by another important signal, jasmonic acid (JA). Despite its importance in defense RDR1 regulation has not been investigated in detail
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