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.

The important plant hormone salicylic acid (SA; 2-hydroxybenzoic acid) regulates several key plant responses including, most notably, defence against pathogens. A key enzyme for SA biosynthesis is isochorismate synthase (ICS), which converts chorismate into isochorismate, and for which there are two genes in Arabidopsis thaliana. One (AtICS1) has been shown to be required for increased SA biosynthesis in response to pathogens and its expression can be stimulated throughout the leaf by virus infection and exogenous SA. The other (AtICS2) appears to be expressed constitutively, predominantly in the plant vasculature. Here, we characterise the enzymatic activity of both isozymes expressed as hexahistidine fusion proteins in Escherichia coli. We show for the first time that recombinant AtICS2 is enzymatically active. Both isozymes are Mg2+-dependent with similar temperature optima (ca. 33°C) and similar Km values for chorismate of 34.3 ± 3.7 and 28.8 ± 6.9 µM for ICS1 and ICS2, respectively, but reaction rates were greater for ICS1 than for ICS2, with respective values for Vmax of 63.5 ± 2.4 and 28.3 ± 2.0 nM s−1 and for kcat of 38.1 ± 1.5 and 17.0 ± 1.2 min−1. However, neither enzyme displayed isochorismate pyruvate lyase (IPL) activity, which would enable these proteins to act as bifunctional SA synthases, i.e. to convert chorismate into SA. These results show that although Arabidopsis has two functional ICS enzymes, it must possess one or more IPL enzymes to complete biosynthesis of SA starting from chorismate.