Priming is an adaptive strategy that enhances defensive capacity of plants to respond faster and/or stronger against the biotic and abiotic stress. This phenomenon is marked by induced resistance. Stimuli from pathogens/ beneficial microbes/ herbivores/ chemicals, can trigger the establishment of priming by acting as warning signal (Mauch-Mani et al., 2017; Bagheri and Fathipour 2021). Priming initiates a state of readiness that does not confer resistance but permit accelerated induced resistance as soon as the invasion takes place. The process of priming is by which a biotic or abiotic stress modifies plant response to a future challenge (Ramirez-Prado et al., 2018). The readiness executed by means of priming is termed as “primed state” (Conrath et al., 2015).
Priming is an intrinsic part of induced resistance (Mauch-Mani et al., 2017). Induced resistance general mechanisms are direct activation of defense response in systemic tissue after local stimuli and priming (Aranega-Bou et al., 2014, De Kesel et al., 2021). Plant innate immune system recognize and respond to pathogens. Immune recognition of pathogen-associated molecular patterns or effector leads to defense activation at pathogen challenged site (Jones and Dangl 2006; Espinas et al., 2016). Followed by systemic defense activation at distant healthy sites termed systemic acquired resistance (SAR). These inducible defenses are accompanied by transcriptional reprogramming of defense related genes (Espinas et al., 2016).
Salicylic acid (SA) accumulation and coordinated activation of PR gene is required for SAR establishment preventing spread of plant pathogen (Van Loon and Van Strien 1999; Dong 2004). In response to endogenous accumulation of SA, the defense regulatory protein NPR1 (Nonexpresser of Pathogenesis-related Genes1) moves to the nucleus where it interacts with TGA (TGACG-binding) transcription factors to induce defense gene expression thus activating SAR (Dong 2004). Application of methyl jasmonate induces resistance against necrotrophic pathogen, while SA conferred resistance against biotrophic pathogen (Oliveira et al., 2015; Hu et al., 2018). Rhizoctonia solani exhibited distinct preference for unprimed plants by growing directly towards the leaves. This indicates fungus can detect an unprimed host (Koley et al., 2021).
SAR is induced after pathogen attack or by exogenous application of SA or its synthetic analogs (Kohler et al., 2002). Plant defense priming can be activated by chemical compound such as b-aminobutyric acid, salicylic acid, jasmonic acid, hexanoic acid, pipecolic acid, chitosan, and volatile organic compounds (Conarth et al., 2002; Conrath et al., 2015; Llorens et al., 2016; Zhou and Wang 2018; Tiwari et al., 2022). The chemical changes in wounded leaf may act as wound signal to undamaged region within plant or to neighboring plants. The recognition of these signals may initiate priming, which comprises changes at the molecular levels and lead to a so-called primed state in undamaged leaves (Frost et al., 2008). The non-SA SAR pathway is driven by pipecolic acid or its presumed bioactive derivative N-hydroxy-pipecolic acid (Vlot et al., 2020). Soil drench application of hexanoic acid compound remains in roots, provoking molecular changes that may trigger the defensive response in the rest of the plant mediated by the changes in the mevalonic and linolenic pathways and enhancing the emission of volatile compounds suggesting the involvement of mevalonic pathway in response to hexanoic application (Llorens et al., 2016).
Plant immunity involves regulation by chromatin remodeling and DNA methylation (Luna et al., 2012). Plant Molecular studies on defense priming recorded altered chromatin structure through a variety of post translational modification of histone proteins and histone modification is associated with priming memory in plants (Buzas 2017; Lamke and Baurle 2017).
Epigenetic phenomenon: A stable and heritable (through cell division) change in gene expression which is independent of DNA sequence changes and is, in principle reversible (Lamke and Baurle 2017).
Epigenetic modification: Describes a change in nucleosome structure caused by histone modification, histone variants or modification (methylation) of the DNA (Lamke and Baurle 2017).
Past studies reveal defense priming can pass down generation, indicating an epigenetic component of transgenerational defense priming (Luna et al., 2012; Martinez-Medina et al., 2016).Host resistance increase towards pathogen through phytohormone priming (Bawa et al., 2019). Priming provides enhanced disease protection at low cost to the plant.
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