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Tri-methylated H3 lysine 4 (H3K4me3) is associated with transcriptionally active genes, but its function in the transcription process is still unclear.Point mutations in the catalytic domain of ATX1 (ARABIDOPSIS TRITHORAX1), a H3K4 methyltransferase, and RNAi knockdowns of subunits of the AtCOMPASS-like (Arabidopsis Complex Proteins Associated with Set) were used to address this question.We demonstrate that both ATX1 and AtCOMPASS-like are required for high level accumulation of TBP (TATA-binding protein) and Pol II at promoters and that this requirement is independent of the catalytic histone modifying activity.However, the catalytic function is critically required for transcription as H3K4me3 levels determine the efficiency of transcription elongation.The roles of H3K4me3, ATX1, and AtCOMPASS-like may be of a general relevance for transcription of Trithorax-activated eukaryotic genes.Tri-methylated H3 lysine 4 (H3K4me3) is induced with dehydration.Meanwhile, Pre-exposure to stress may alter plants subsequent responses by producing faster and/or stronger reactions implying that plants exercise a form of stress memory.we show that during recurring dehydration stresses Arabidopsis plants display transcriptional stress memory demonstrated by an increase in the rate of transcription and elevated transcript levels of a subset of the stress-response genes (trainable genes).During recovery (watered) states, trainable genes produce transcripts at basal (pre-induced) levels, but remain associated with atypically high H3K4me3 and Ser5P polymerase II levels, indicating that RNA polymerase II is stalled.This is the first example of a stalled RNA polymerase II and its involvement in transcriptional memory in plants.These newly discovered phenomena might be a general feature of plant stress-response systems and could lead to novel approaches for increasing the flexibility of a plants ability to respond to the environment.