Plants develop immune memory on interaction with pathogen, leading to systemic acquired resistance. Whereas when plant interact with beneficial microorganisms, they boost plant immune system (Nishad et al., 2020).Genome of every organism can store the information of the past experiences, which a particular organism collects under specialized situation, and recalls the same in future circumstances. All their stored genomic information and experiences are collectively known as memory. The memory can be short- or long-term depending on the type and intensity of the stressor. In plants second messenger such as reactive oxygen species, calcium, nitric oxide, salicylic acid (SA), jasmonic acid and pipecolic acid (Pip) play a crucial role in memory response (Bhar et al., 2022). SA and Pip are vital for SAR, with each one activating different set of defense-related genes (Nishad et al., 2020; Bernsdorff et al., 2016). Second messenger which are produced in response to extracellular stimuli are also produced in response to stimuli from within cell. Second messengers are small molecules and ions that relay signals received by cell surface receptors to effector proteins (Newton et al., 2016).
Plants adopt several survival mechanisms including epigenetic modifications. Epigenetic modification involves heritable gene expression changes that do not require changes in deoxyribonucleic acid (DNA) sequence (Sampson et al., 2023). Among epigenetic modification DNA methylation, histone modification and small ribonucleic acid (sRNA) mediated silencing play a role in stress tolerance (Joseph and Shah 2022). Plants response to pathogen attack relies on its ability to adjust the transcriptome. The external signals are transmitted to the nucleus, leading to activation of transcription factor that enhance the expression of defense-related genes (Mierziak and Wojtasik 2024). DNA methylation and histone modification are closely linked to chromatin, which regulate gene expression associated with defense against the pathogen attack (Mierziak and Wojtasik 2024). During interaction between fungi and plant, some fungi develop novel virulence mechanism sending sRNA as an effector molecule into plant cells to silence plant resistance gene, while plant transport sRNA through extracellular vesicles, into pathogens to suppress virulence-related genes (Huang et al., 2019; Mierziak and Wojtasik 2024). Gene expression is regulated by changes in chromatin structure. Less condensed region of chromatin is more transcriptionally active. Histones forming the core of chromatin are subject to many modifications e.g. acetylation, methylation or ubiquitination (Mierziak and Wojtasik 2024). DNA methylation can pass generation and develop transgenerational memory (Bhar et al., 2022).
Stress memory in plants is the ability of plant to store stress information when exposed to stressors so that it may respond when challenged by stress later in a different manner (Kambona et al., 2023). In plants stress signaling is transferred as genomic imprints in successive generation and such immunogenic priming of plants against biotic stresses is crucial (Bhar et al., 2022). Plants have high quantity of non-coding DNA in the form of repetitive and high GC rich region (Walkup 2000). The flexible DNA packaging with a histone core to form functional chromosome allows the nucleosome core to move along the string of DNA and permit transcriptional machinery to express prescribed portions of DNA. This transcriptional control can be altered by epigenetic modification in histone proteins (Kornberg 1999; Bhar et al., 2022). The pathogen-associated molecular pattern triggered immunity and effector triggered immunity initiate signaling cascade that subsequently lead to epigenetic modification (Mierziak and Wojtasik 2024). The epigenetic modification (such as DNA methylation, histone modification, chromatin remolding and involvement of non-coding RNAs) activate transcriptional regulator and defense gene leading to defense responses.
Short-term memory instigates the transition expression of transcription factor that would control defense-responsive genes if required (Bhar et al., 2022). Short-term memory (linked to the increase of metabolites or transcription factor) may persist for days/week (Galviz et al., 2022) and epigenetic mechanisms can persist for months and eventually be transferred to the offsprings (Crisp et al., 2016; Galviz et al., 2022). The heritable transgenerational changes in genome methylation are a part of plant strategies for stress adaptation. Change in DNA methylation patterns generate diversity in gene expression at both tissue specific and population levels. These changes at population level generate transient diversity between individual organisms within population of plants exposed to a specific environment (Alex and Igor 2011).
Plant constantly engages in beneficial or competitive interaction with pathogens or with other plants. The epigenetic mechanism emerges as a regulatory entity and as a target of pathogen that tries to over-come plant defense (Ramos-Cruz et al., 2021). DNA methylation is an epigenetic mark that stably inherit to multiple generations (Joseph and Shah 2022). Epigenetics modification is a part of adaptive and survival responses.
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BIOLOGICAL PROCESSES- A NATURE'S GIFT 