Epigenetics is now recognized as a major determinant of plant stress adaptation, and in the context of climate change, it is essential to understand how it may contribute to temperature acclimation. Taking advantage of a robust experimental system of steady-state seedlings grown in mineral water (see Réthoré et al., 2019, Plant J. 99, 302-315), we aim to establish how temperature increases sustained for several days affect the epigenome, and how it can translate into changes in the transcriptome, both at the nuclear and mitochondrial levels. We have analysed the tolerance of Arabidopsis seedlings from wild-type and a series of epigenetic mutants to sustained heat stress for four days at different temperatures. We observed that two mutant lines affected in DNA methylation (ddm) and demethylation (ros1) showed a higher thermal sensitivity in comparison to the wild-type plants since they progressively died after the 34°C treatment. To investigate the link between temperature sensitivity and the epigenome dynamics, we analyzed both transcriptome and methylome changes in the thermosensitive mutants with respect to wild-type lines. Nuclear and mitochondrial DNA and RNA were used to profile transcriptomes and methylomes from seedlings at the end of four days of heat stress at different temperatures (25, 28, 31, 34 °C) and in WT and thermosensitive lines. We will present the preliminary analysis of the data, with emphasis on the role of mitochondria, that uncover the missing link between epigenetic regulation and mitochondrial functions in temperature stress tolerance.