Identification of the TuNI gene causing systemic necrosis in Arabidopsis ecotype Ler infected with Turnip mosaic virus and characterization of its expression

We previously observed that Turnip mosaic virus (TuMV) induces systemic necrosis in Arabidopsis ecotype Ler but not in Col-0. Sharing most of the common features of the hypersensitive reaction, this systemic necrosis has been found to be a result of a gene-for-gene interaction between the host TuNI gene and viral P3 gene. We here analyzed the TuNI locus in detail and identified three very similar candidates for TuNI gene(s). Our functional and expression analyses using transgenic plants expressing each of the candidates and a protoplast transient expression assay suggested that the expression of the best candidate, named RGX, is responsible for the systemic necrosis although concomitant expression with the other two candidates may be necessary. To understand the expression profile of RGX, we analyzed promoter activity by producing transgenic Col-0 plants that express the reporter GFP gene under the control of the 500-1400-bp upstream regions of RGX. The transgenic plants reproduced the expression patterns of RGX; GFP expression increased after TuMV infection but decreased in the shade treatment. The expression of two other candidates was also upregulated by TuMV infection, suggesting that transcriptional activation of the TuNI candidate genes has a role in controlling TuMV-mediated systemic necrosis.