Bacterial Pathogen Infection Triggers Magic Spot Nucleotide Signaling in Arabidopsis thaliana Chloroplasts Through Specific RelA/SpoT Homologs

Magic spot nucleotides (p)ppGpp are important signaling molecules in bacteria and plants. In the latter, RSH enzymes are responsible for (p)ppGpp turnover. Profiling of (p)ppGpp is more difficult in plants than in bacteria due to lower concentrations and more severe matrix effects. Here, we report that capillary electrophoresis mass spectrometry (CE-MS) can be deployed to study (p)ppGpp abundance and identity in Arabidopsis thaliana. This goal is achieved by combining a titanium dioxide extraction protocol and pre-spiking with chemically synthesized stable isotope labeled internal reference compounds. The high sensitivity and separation efficiency of CE-MS enables monitoring of changes in (p)ppGpp levels in A. thaliana upon infection with the pathogen Pseudomonas syringae pv. tomato (PstDC3000). We observed a significant increase of ppGpp post infection that is also stimulated by the flagellin peptide flg22 only. This increase depends on functional flg22 receptor FLS2 and its interacting kinase BAK1 indicating that pathogen-associated molecular pattern (PAMP) receptor-mediated signaling controls ppGpp levels. Transcript analyses showed an upregulation of RSH2 upon flg22 treatment and both RSH2 and RSH3 after PstDC3000 infection. A. thaliana mutants deficient in RSH2 and RSH3 activity display no ppGpp accumulation upon infection and flg22 treatment, supporting involvement of these synthases in PAMP-triggered innate immunity responses to pathogens within the chloroplast.