The simultaneous perception of self- and non-self-danger signals potentiates plant innate immunity responses.

MAIN CONCLUSION: The simultaneous perception of endogenous and exogenous danger signals potentiates PAMP-triggered immunity in tomato and other downstream defence responses depending on the origin of the signal. Abstract Plant cells perceive a pathogen invasion by recognising endogenous or exogenous extracellular signals such as Damage-Associated Molecular Patterns (DAMPs) or Pathogen-Associated Molecular Patterns (PAMPs). In particular, DAMPs are intracellular molecules or cell wall fragments passive or actively released to the apoplast, whose extracellular recognition by intact cells triggers specific immune signalling, the so-called DAMP-triggered immunity. The extracellular recognition of DAMPs and PAMPs leads to a very similar intracellular signalling, and this similarity has generated a biological need to know why plants perceive molecules with such different origins and with overlapped innate immunity responses. Here, we report that the simultaneous perception of DAMPs and a PAMP strengthens early and late plant defence responses. To this aim, we studied classical PTI responses such as the generation of ROS and MAPK phosphorylation, but we also monitored the biosynthesis of phytocytokines and performed a non-targeted metabolomic analysis. We demonstrate that co-application of the bacterial peptide flagellin with the DAMPs cyclic AMP or cellobiose amplifies PAMP-triggered immunity responses. Both co-applications enhanced the synthesis of phytocytokines, but only simultaneous treatments with cAMP strengthened the flagellin-dependent metabolomic responses. In addition, cAMP and cellobiose treatments induced resistance against the hemibiotrophic bacteria Pseudomonas syringae pv. tomato DC3000. Overall, these results indicate that the complex mixture of DAMPs and PAMPs carries specific information that potentiates plant defence responses. However, downstream responses seem more specific depending on the composition of the mixture.

Extracellular DNA as an elicitor of broad-spectrum resistance in Arabidopsis thaliana.

Like in mammals, the plant immune system has evolved to perceive damage. Damaged-associated molecular patterns (DAMPs) are endogenous signals generated in wounded or infected tissue after pathogen or insect attack. Although extracellular DNA (eDNA) is a DAMP signal that induces immune responses, plant responses after eDNA perception remain largely unknown. Here, we report that signaling defenses but not direct defense responses are induced after eDNA applications enhancing broad-range plant protection. A screening of defense signaling and hormone biosynthesis marker genes revealed that OXI1, CML37 and MPK3 are relevant eDNA-Induced Resistance markers (eDNA-IR). Additionally, we observed that eDNA from several Arabidopsis ecotypes and other phylogenetically distant plants such as citrus, bean and, more surprisingly, a monocotyledonous plant such as maize upregulates eDNA-IR marker genes. Using 3,3'-Diaminobenzidine (DAB) and aniline blue staining methods, we observed that H2O2 but not callose was strongly accumulated following self-eDNA treatments. Finally, eDNA resulted in effective induced resistance in Arabidopsis against the pathogens Hyaloperonospora arabidopsidis, Pseudomonas syringae, and Botrytis cinerea and against aphid infestation, reducing the number of nymphs and moving forms. Hence, the unspecificity of DNA origin and the wide range of insects to which eDNA can protect opens many questions about the mechanisms behind eDNA-IR.