Priming

The term priming is mostly used for biotic interactions (e.g., with herbivores or parasites), while the term acclimation is commonly used for abiotic stress events (e.g., heat or frost). They both denote the sensitivity and responsiveness to stress that results from a prior experience and often causes enhanced defense readiness. For simplicity, we will use the term 'priming' in this section for both biotic and abiotic stressors (Mauch-Mani et al. 2017). Priming is a robust defense because it has no or minimal fitness costs in terms of growth or reproduction, and often it is transient and only activated by a stimulus (Mauch-Mani et al. 2017).

‌Defense priming has been observed in a wide range of plant taxa, including wild species and model organisms. Notably, experimental evidence suggests that epigenetic changes were observed following priming. Several studies have hypothesized that epigenetic changes could influence the way how plants respond to biotic and abiotic stresses (Bruce et al. 2007; Mauch-Mani et al. 2017). The general idea is that an induced defense can associate molecular, biochemical, and physiological cues with stronger and/or activated phenotypic defense mechanisms in primed versus unprimed plants (Fig 2)(Bruce et al. 2007; Mauch-Mani et al. 2017). A possible response could be in the form of DNA hypo-hyper methylation (Boyko et al. 2007; Verhoeven et al. 2010), inducing methylation of small, non-coding RNAs as the stress response.