Priming and memory of stress responses in organisms lacking a nervous system.

Experience and memory of environmental stimuli that indicate future stress can prepare (prime) organismic stress responses even in species lacking a nervous system. The process through which such organisms prepare their phenotype for an improved response to future stress has been termed 'priming'. However, other terms are also used for this phenomenon, especially when considering priming in different types of organisms and when referring to different stressors. Here we propose a conceptual framework for priming of stress responses in bacteria, fungi and plants which allows comparison of priming with other terms, e.g. adaptation, acclimation, induction, acquired resistance and cross protection. We address spatial and temporal aspects of priming and highlight current knowledge about the mechanisms necessary for information storage which range from epigenetic marks to the accumulation of (dormant) signalling molecules. Furthermore, we outline possible patterns of primed stress responses. Finally, we link the ability of organisms to become primed for stress responses (their 'primability') with evolutionary ecology aspects and discuss which properties of an organism and its environment may favour the evolution of priming of stress responses.