Fig. 2.

Model for chromatin-level regulation of the response and acclimation of plants to abiotic stress. Under optimal growth conditions, a stress-responsive gene is kept in a repressed (silenced) state (1). Upon moderate or short stress exposure, the deposition of chromatin activating marks and/or the removal of repressive marks establish a sensitized state (2). Genes in this state may be weakly transcribed or continue to be inactive. Longer stress duration or higher intensity induces an activated state (3). This can be achieved directly or via a transient sensitized state. Upon stress release, the gene rapidly reverts to the sensitized state (2) and then slowly to the repressed state (1). As long as the repressed state has not been reestablished, a second moderate or brief stress is sufficient to reactivate the gene. Note that the acclimated/sensitized state does not necessarily depend on prior full activation. For some genes, a second intense or long stress will not only establish the activated but also a hyperactivated state (3′) reflected by a further increase in the rate of transcription