Inducible epigenome editing probes for the role of histone H3K4 methylation in Arabidopsis heat stress memory.

Histone modifications play a crucial role in the integration of environmental signals to mediate gene expression outcomes. However, genetic and pharmacological interference often causes pleiotropic effects, creating the urgent need for methods that allow locus-specific manipulation of histone modifications, preferably in an inducible manner. Here, we report an inducible system for epigenome editing in Arabidopsis (Arabidopsis thaliana) using a heat-inducible dCas9 to target a JUMONJI (JMJ) histone H3 lysine 4 (H3K4) demethylase domain to a locus of interest. As a model locus, we target the ASCORBATE PEROXIDASE2 (APX2) gene that shows transcriptional memory after heat stress (HS), correlating with H3K4 hyper-methylation. We show that dCas9-JMJ is targeted in a HS-dependent manner to APX2 and that the HS-induced overaccumulation of H3K4 trimethylation (H3K4me3) decreases when dCas9-JMJ binds to the locus. This results in reduced HS-mediated transcriptional memory at the APX2 locus. Targeting an enzymatically inactive JMJ protein in an analogous manner affected transcriptional memory less than the active JMJ protein; however, we still observed a decrease in H3K4 methylation levels. Thus, the inducible targeting of dCas9-JMJ to APX2 was effective in reducing H3K4 methylation levels. As the effect was not fully dependent on enzyme activity of the eraser domain, the dCas9-JMJ fusion protein may act in part independently of its demethylase activity. This underlines the need for caution in the design and interpretation of epigenome editing studies. We expect our versatile inducible epigenome editing system to be especially useful for studying temporal dynamics of chromatin modifications.