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.
Keywords
Activating sequence-1
- Chromatin immunoprecipitation - Hyperacetylated histones -
Pathogenesis related gene-1a
- Salicylic acid - TGA factors