Recent advances in plant thermomemory.

KEY MESSAGE: This review summarizes the process of thermal acquired tolerance in plants and the knowledge gap compared to systemic acquired resistance that a plant shows after pathogen inoculation. Plants are continuously challenged by several biotic stresses such as pests and pathogens, or abiotic stresses like high light, UV radiation, drought, salt, and very high or low temperature. Interestingly, for most stresses, prior exposure makes plants more tolerant during the subsequent exposures, which is often referred to as acclimatization. Research of the last two decades reveals that the memory of most of the stresses is associated with epigenetic changes. Heat stress causes damage to membrane proteins, denaturation and inactivation of various enzymes, and accumulation of reactive oxygen species leading to cell injury and death. Plants are equipped with thermosensors that can recognize certain specific changes and activate protection machinery. Phytochrome and calcium signaling play critical roles in sensing sudden changes in temperature and activate cascades of signaling, leading to the production of heat shock proteins (HSPs) that keep protein-unfolding under control. Heat shock factors (HSFs) are the transcription factors that read the activation of thermosensors and induce the expression of HSPs. Epigenetic modifications of HSFs are likely to be the key component of thermal acquired tolerance (TAT). Despite the advances in understanding the process of thermomemory generation, it is not known whether plants are equipped with systemic activation thermal protection, as happens in the form of systemic acquired resistance (SAR) upon pathogen infection. This review describes the recent advances in the understanding of thermomemory development in plants and the knowledge gap in comparison with SAR.