Chromatin immunoprecipitation analysis of the tobacco PR-1a- and the truncated CaMV 35S promoter reveals differences in salicylic acid-dependent TGA factor binding and histone acetylation.

Salicylic acid (SA) is a plant signalling molecule needed for the induction of defence responses upon attack by a variety of pathogens. Truncation of the Cauliflower Mosaic Virus (CaMV) 35S promoter down to 90 bp has identified activation sequence-1 (as-1) as an autonomous SA-responsive cis element. The as-1-like elements are found in a number of SA-inducible promoters like e.g. the tobacco PR-1a promoter. They are recognized by basic/leucine zipper (bZIP) transcription factors of the TGA family. In tobacco leaves, TGA2.2 is the most abundant TGA factor. TGA2.2 is required for the expression of as-1-containing promoters. Here we unravel clear differences between the "truncated" CaMV 35S and the PR-1a promoter with respect to in vivo TGA binding and histone acetylation. Chromatin immunoprecipitation (ChIP) analysis revealed SA-inducible recruitment of tobacco TGA2.2 as well as SA-inducible histone acetylation at the PR-1a promoter. In contrast, no influence of SA on TGA2.2 binding and histone acetylation was detectable at the "truncated" CaMV 35S promoter. The finding of SA-independent TGA factor binding in the absence of additional flanking regulatory sequences suggests that transcriptional activation is not necessarily mediated by inducible DNA binding of TGA factors. Plants with severely reduced TGA2.2 protein levels also showed SA-induced histone acetylation at the PR-1a promoter indicating that regulatory events independent from TGA2.2 function are initiated at the PR-1a promoter.