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Crustaceans, insects and plants do not have an adaptive immune system like higher vertebrates. Instead they use only their innate immune system for protection against infection and these pathways are activated in response to pattern recognition receptor (PRR) activation. The PPRs perception of host-derived endogenous molecules called DAMPs (Damage Associated Molecular Patterns) activates the ‘damaged-self recognition’ pathway, whereas exogenous molecules called HAMPs (Herbivore Associated Molecular Patterns), MAMPs (Microbe Associated Molecular Patterns) and PAMPs (Pathogen Associated Molecular Patterns) activate the ‘non-self-recognition’ pathway.
Crustaceans, insects and plants do not have an adaptive immune system like higher vertebrates. Instead they use only their innate immune system for protection against infection and these pathways are activated in response to pattern recognition receptor (PRR) activation. The PPRs perception of host-derived endogenous molecules called DAMPs (Damage Associated Molecular Patterns) activates the 'damaged-self recognition' pathway, whereas exogenous molecules called HAMPs (Herbivore Associated Molecular Patterns), MAMPs (Microbe Associated Molecular Patterns) and PAMPs (Pathogen Associated Molecular Patterns) activate the 'non-self-recognition' pathway [1,2]. Recently DAMPs have been categorized as elicitors that have effect in the activation of the plant immune system. They have been proposed as “vaccines” that enhance plant resistance in an environmentally friendly way , as well as their use in other agricultural approaches . This is the case of extracellular fragmented DNA (eDNA), recently considered as a type of DAMP in plants. The aim of this work is to review some molecules considered as DAMPs in plants focusing on eDNA and its effect in activation of different pathways related with the plant immune system.
DAMPs in Plants
In plants a few number of molecules has been identified acting as a DAMP in comparison with metazoans. Gust et al.  proposed that molecular patterns passively released from injured plant tissue (‘debris’) should be termed primary endogenous danger signals or classical DAMPs. Molecularly defined signaling peptides, which are processed and released in response to damage, are analogous to immunomodulatory metazoan cytokines and are thus more appropriately called phytocytokines. Some of these molecules identified on different plant species are cellobiose, cellodextrins, cutin, extracellular ATP, lectin-RK, eDNA, extracellular NAD (P), GLV (green leaf volatile)/VOC (volatile organic compound), High-mobility group protein B3 (HMGB3), methanol, oligogalacturonides.
The results of the experiments showed the formation of ROS and the activation of MAPKs, the reduction of a bacterial infection and an effect in the extra floral nectar production caused by the self-eDNA in Phaseolus vulgaris. More recently, a study to elucidate the role of eDNA as a DAMP analyzing the changes in CpG DNA methylation and defense-related responses was performed by Vega-Munoz et al. . The use of lettuce (Lactuca sativa) as a target plant and Capsicum chinense and Acaciella angustissima as source of non-self-DNA. In this study, the effect of self-eDNA caused also an inhibition of growth and germination in lettuce. The plants L. sativa and C. chinense belong to the Asterids clade but to different orders (Asterales and Solanales, respectively), whereas A. angustissima belongs to the clade Rosids I and the order Fabales. Thus C. chinense is closer phylogenetically to L. sativa than A. angustissima, and this situation might explain the similar effects of self-eDNA and nsDNA from C. chinense on the lettuce plants evaluated and the significant CpG DNA hypomethylation at similar eDNA concentrations in their study. This latter result is the first experimental demonstration that phylogenetic closeness is an important feature for plant responses to DAMPs along an epigenetic pathway. With these results they suggests that if the epigenetic pathway is a possible mechanism in DAMP signaling, high doses of self-eDNA, at least in lettuce, induces a switch (on-off) in several epigenetic mechanisms, unlike to CpG DNA methylation (i.e., methylation in CHH and CHG, or changes in the methylation and acetylation of histones); although it might be also possible that the induction of genetic mechanisms to control gene expression under these stress conditions.
Vega-Munoz et al.  also mentioned that probably, the DNA excreted by plants and further metabolized to sequences 50-2000 bp long have a very specific signature for each species to be recognized as self-DNA by the plant. Through DNA restriction-modification, bacteria distinguish the same from the strange through DNA methylation. The same effect is observed for TLR9, which specifically recognizes unmethylated CpGs ; in plants, specific responses depend on DNA fragmentation . Thus, DNA methylation patterns could be one possible mechanism for self-eDNA recognition in plants, although more research should be undertaken on this topic. This discovery opens new opportunities for exploiting the best characteristics of self-eDNA in the agricultural, horticultural and pharmacological industries, as highly species-specific inhibitory products, limiting effects on other species, as suggested elsewhere .
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