Gene coding for SigA-binding protein from Arabidopsis appears to be transcriptionally up-regulated by salicylic acid and NPR1-dependent mechanisms

SigA-binding protein (SIB A) is a nuclear-encoded chloroplast-targeted protein that interacts with the plastid-encoded plastid RNA polymerase sigma-factor SigA (Sig1). In this study, the SIB A gene responded rapidly to salicylic acid (SA) treatment, but responded slowly to ethylene (ET) and jasmonic acid (JA) treatments as determined with microarray and quantitative real-time RT-PCR analyses. Expression of the SIB A gene increased rapidly, with a peak at 2 h after SA treatment and then decreased gradually. In contrast, expression of the PR-1 gene, a marker gene on the SA defense signaling pathway, increased gradually between 0 and 24 h after SA treatment. In addition, transcription levels of SIB A and PR-1 decreased between 0 and 24 h in the benzothiadiazole (BTH)-treated npr1-1 plants. We suggest that the SIB A gene is downstream of NPR1 on the SA signaling pathway. From the results of our analysis of the 1.5-kbp promoter region of the SIB A gene in Arabidopsis, the isolated promoter region of the SIB A gene seems able to drive the transcription of the GUS gene to elevated levels more rapidly in transgenic Arabidopsis plants undergoing SA treatment compared with levels of PR-1. To analyze biological function of SIB A and investigate SIB A-regulated genes, we generated transgenic Arabidopsis plants in which SIB A was overexpressed. As a result, ROS-related genes, such as glutathione transferase, were up-regulated in plants overexpressing SIB A. However, SIB A-overexpressing plants were not resistant to Pseudomonas syringae pv. tomato DC3000 strain. The SIB A gene has promise as a new tool to analyze the SA-signaling pathway and the role of the chloroplast in plant defense responses.