Multiple exposures to drought 'train' transcriptional responses in Arabidopsis.

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'. The mechanisms of plants' stress memory responses are poorly understood leaving this fundamental biological question unanswered. Here 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 (preinduced) 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 plant's ability to respond to the environment.