Acclimation to high temperature increases plants’ tolerance of subsequent lethal high temperatures. Although epigenetic regulation of plant gene expression is well studied, how plants maintain a memory of environmental changes over time remains unclear. Here, we show that JUMONJI (JMJ) proteins, demethylases involved in histone H3 lysine 27 trimethylation (H3K27me3), are necessary for Arabidopsis thaliana heat acclimation. Acclimation induces sustained H3K27me3 demethylation at HEAT SHOCK PROTEIN22 (HSP22) and HSP17.6C loci by JMJs, poising the HSP genes for subsequent activation. Upon sensing heat after a 3-day interval, JMJs directly reactivate these HSP genes. Finally, jmj mutants fail to maintain heat memory under fluctuating field temperature conditions. Our findings of an epigenetic memory mechanism involving histone demethylases may have implications for environmental adaptation of field plants.

1 Acclimation to high temperature increases plants' tolerance of subsequent 2 lethal high temperatures 1-3 . Although epigenetic regulation of plant gene 3 expression is well studied, how plants maintain a memory of environmental 4 changes over time remains unclear. Here, we show that JUMONJI (JMJ) 5 proteins 4-8 , demethylases involved in histone H3 lysine 27 trimethylation 6 (H3K27me3), are necessary for Arabidopsis thaliana heat acclimation. 7 Acclimation induces sustained H3K27me3 demethylation at key HEAT SHOCK 8 PROTEIN (HSP) loci by JMJs, poising the HSP genes for subsequent 9 activation. Upon sensing heat after a 3-day interval, JMJs directly reactivate 10 HSP genes. Finally, jmj mutants fail to maintain heat memory under fluctuating 11field temperature conditions. Our findings of an epigenetic memory mechanism 12 involving histone demethylases may have implications for environmental 13 adaptation of field plants. 14 15 3 Main 1 The ability to adapt to environmental changes is essential for plants, as sessile 2 organisms, to survive 1 . Plants memorize heat experience over several days and 3 develop future responsiveness 2 . Exposure to moderate temperature enables 4 plants to acquire thermotolerance for subsequent lethal high temperature 3 . In 5 Arabidopsis thaliana, HEAT SHOCK TRANSCRIPTION FACTOR A2 (HSFA2) 6 is necessary for the maintenance of acquired thermotolerance 9, 10 . HEAT 7 SHOCK PROTEIN (HSP) genes encode molecular chaperones that protect 8 cellular proteins from denaturation. Upon sensing of high temperature, the 9 transient binding of HSFA2 at HSPs governs the sustained increase of histone 10 marks such as histone H3 lysine 4 trimethylation (H3K4me3) and the 11 expression of the HSP genes 11-13 . Then, the expression of heat-memory genes 12 declines gradually while H3K4me3 levels remain high. In that situation, proper 13 maintenance of repressive histone marks should also play important roles in the 14 downregulation of HSP genes 13 . Despite the importance of histone modification 15 enzymes, little is known about the underlying mechanism of those enzymes in 16 flexible and reversible HSP gene expression. 17 JUMONJI (JMJ) proteins 4-8 , demethylases involved in histone H3 18 lysine 27 trimethylation (H3K27me3), are evolutionarily conserved and regulate 19diverse biological processes. In this study, we investigate the role these 20 H3K27me3 demethylases on acquired thermotolerance in response to recurring 21 heat. We demonstrate that JMJ proteins keep repressive histone marks at low 22 levels on chaperone-encoding small HSPs 11-13 that function as memory genes. 23Using inducible JMJs and mutants of small HSPs, we demonstrate the 24 underlying cause of heat memory is the lower H3K27me3 on small HSPs. This 25 histone modification-based transcriptional memory is well-aligned with a 26 mathematical stochastic model that we developed, which predicts expression 27 levels of sHSPs. Moreover, we recapitulate fluctuating temperature conditions 28 and suggest that JMJ-mediated sustained H3K27me3 dem...