RESEARCH PAPER Overexpression of the Arabidopsis thaliana EDS5 gene enhances resistance to viruses T. Ishihara 1 , K.-T. Sekine 1 , S. Hase 1 , Y. Kanayama 2 , S. Seo 3 , Y. Ohashi 3 , T. Kusano 4 , D. Shibata 5 , J. Shah 6,7 & H. Takahashi 1 1 Department of Life Science, Graduate School of Agricultural Science, Tohoku University, Sendai, Japan 2 Department of Biological Resource Sciences, Graduate School of Agricultural Science, Tohoku University, Sendai, Japan 3 National Institute of Agrobiological Sciences (NIAS), Tsukuba, Japan 4 Graduate School of Life Sciences, Tohoku University, Sendai, Japan 5 Kazusa DNA Research Institute, Kisarazu, Japan 6 Division of Biology and Molecular, Cellular and Developmental Biology Program, Kansas State University, Manhattan, KS, USA 7 Present address: Department of Biological Sciences, University of North Texas, Denton, TX, USA INTRODUCTION Salicylic acid (SA), a phenolic metabolite involved in plant development and stress response, is synthesised via two pathways. The first route has been studied in tobacco, cucumber, potato and rice and involves the phenylalanine ammonia lyase catalysed conversion of phenylalanine to cinnamic acid, which is subsequently converted via a series of steps to SA (Sticher et al. 1997; Me ´traux 2002). The second route, which is the major mechanism by which SA is synthesised in pathogen-inoc- ulated Arabidopsis plants, involves isochorismate as an intermediate. Isochorismate is synthesised from choris- mate by isochorismate synthase (ICS) in Arabidopsis Keywords Basal resistance; CMV resistance gene RCY1; EDS5; MATE transporter; PR-1a; SA-dependent signal transduction. Correspondence H. Takahashi, Department of Life Science, Graduate School of Agricultural Science, Tohoku University, Sendai 981-8555, Japan. E-mail: takahash@bios.tohoku.ac.jp Editor Rob Goldbach Received: 2 October 2006; Accepted: 7 September 2007 doi:10.1111/j.1438-8677.2008.00050.x ABSTRACT The Arabidopsis thaliana ENHANCED DISEASE SUSCEPTIBILITY 5 gene (EDS5) is required for salicylic acid (SA) synthesis in pathogen-challenged plants. SA and EDS5 have an important role in the Arabidopsis RCY1 gene- conferred resistance against the yellow strain of Cucumber mosaic virus [CMV(Y)], a Bromoviridae, and HRT-conferred resistance against the Tom- busviridae, Turnip crinkle virus (TCV). EDS5 expression and SA accumula- tion are induced in response to CMV(Y) inoculation in the RCY1-bearing ecotype C24. To further discern the involvement of EDS5 in Arabidopsis defence against viruses, we overexpressed the EDS5 transcript from the con- stitutively expressed Cauliflower mosaic virus 35S gene promoter in ecotype C24. In comparison to the non-transgenic control, the basal level of salicylic acid (SA) was twofold higher in the 35S:EDS5 plant. Furthermore, viral spread and the size of the hypersensitive response associated necrotic local lesions (NLL) were more highly restricted in CMV(Y)-inoculated 35S:EDS5 than in the non-transgenic plant. The heightened restriction of CMV(Y) spread was paralleled by more rapid induction of the pathogenesis-related gene, PR-1, in the CMV(Y)-inoculated 35S:EDS5 plant. The 35S:EDS5 plant also had heightened resistance to the virulent CMV strain, CMV(B2), and TCV. These results suggest that, in addition to R gene-mediated gene-for- gene resistance, EDS5 is also important for basal resistance to viruses. How- ever, while expression of the Pseudomonas putida nahG gene, which encodes the SA-degrading salicylate hydroxylase, completely suppressed 35S:EDS5- conferred resistance against CMV(Y) and TCV, it only partially compromised resistance against CMV(B2), indicating that SA-dependent and -independent mechanisms are associated with 35S:EDS5-conferred resistance against viruses. Plant Biology ISSN 1435-8603 Plant Biology 10 (2008) 451–461 ª 2008 German Botanical Society and The Royal Botanical Society of the Netherlands 451