Facilitating transcriptional transitions: an overview of chromatin bivalency in plants

Chromatin is an essential contributor to the regulation of transcription. The two histone post-translational modifications H3K4me3 and H3K27me3 act as an activator and repressor of gene expression, respectively, and are usually described as being mutually exclusive. However, recent work revealed that both marks might co-exist at several loci, forming a distinctive chromatin state called bivalency. While this state has been detected on a handful of genes involved in plant development and stress responses, its role in the regulation of transcription remains unclear. In an effort to shed more light on the putative function(s) of bivalency in plants, this review details the potential players involved in its setting and reading, and explores how this chromatin state might contribute to the control of gene expression. We propose that bivalency maintains transcriptional plasticity by facilitating transitions between a repressed and an active state and/or by preventing irreversible silencing of its targets. We also highlight recently developed techniques that could be used for further investigating bivalency.