Epigenetic control of a transcription factor at the cross section of two antagonistic pathways.

The expression of the Arabidopsis gene WRKY70 is known to be antagonistically regulated by the salicylic acid (SA) and jasmonic acid (JA) signaling pathways. The gene encodes a transcription factor functioning at the crossroad of the two pathways. Here we show that the Arabidopsis homolog of Trithorax, ATX1, activates the expression of the WRKY70 gene and is involved in establishing the trimethylation pattern of histone H3 tail lysine 4 (H3K4me3) residues of its nucleosomes. Chromatin immunoprecipitation (ChIP) analyses with antiATX1 specific antibodies demonstrated that WRKY70 is a primary target for the ATX1 histone methylase activity, while the SA-responsive gene, PR1, and the JA-responsive gene, THI2.1, are secondary targets. The unexpected finding that PR1 and THI2.1 nucleosomes carryH3K4me3-marks unrelated to their transcription states suggests that the defense-response genes PR1 and THI2.1 keep their nucleosomes in 'actively' modified state, perhaps, in preparation for quick-changes of transcription when needed by the cell. Based on the experimental data, we propose a model that could explain the ability of a single epigenetic factor to orchestrate expression of a large number of genes, particularly in cases involving response reactions.