Complete Loss of RelA and SpoT Homologs in Arabidopsis Reveals the Importance of the Plastidial Stringent Response in the Interplay between Chloroplast Metabolism and Plant Defense Response.

This article focuses on the role of guanosine tetraphosphate (ppGpp), a highly phosphorylated nucleotide acting as a secondary messenger in plastids, in regulating plant metabolism, photosynthesis, and defense responses. Using CRISPR/Cas9 technology, researchers generated an Arabidopsis thaliana mutant lacking all four plastid-localized ppGpp synthases/hydrolases (RSH1, RSH2, RSH3, and CRSH), resulting in over a 20-fold reduction in ppGpp levels. The mutant exhibited leaf chlorosis, impaired non-photochemical quenching (NPQ) under nitrogen-starvation conditions, altered metabolite profiles, increased reactive oxygen species, and elevated levels of defense-related phytohormones such as salicylic acid and jasmonic acid. These findings demonstrate that plastidial ppGpp homeostasis is essential for coordinating plastid metabolism, photosynthetic regulation, phytohormone signaling, and plant defense, particularly during nitrogen deficiency.