Figure 4.

Biotic stress memory. A) Exposure of conifers to biotic stress or other stimuli (grey arrow), such as methyl jasmonate (MeJA) application, induces complex defense responses that include primed defenses and prolonged upregulated defenses. Primed defenses are latent until a trigger stress (black arrow) results in a strong and rapid induction. B) Conifer inducible defense responses include the accumulation of defensive molecules, formation of specialized defense structures, and synthesis of defensive enzymes. These defenses are executed in different tissues and over different time scales. Epigenetic mechanisms underlaying biotic stress memory likely include changes in DNA methylation, as many components of the RNA-directed DNA methylation (RdDM) pathway are temporarily downregulated during the onset of priming and after a trigger stress. Additionally, miRNAs that are differentially expressed may help maintain the primed state and the primed defense response by post-transcriptional silencing of mRNAs or guiding DNA methylation via the RdDM pathway.