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.

The highly phosphorylated nucleotide, guanosine tetraphosphate (ppGpp), functions as a secondary messenger in bacteria and chloroplasts. The accumulation of ppGpp alters plastidial gene expression and metabolism, which are required for proper photosynthetic regulation and robust plant growth. However, because four plastid-localized ppGpp synthases/hydrolases function redundantly, the impact of the loss of ppGpp-dependent stringent response on plant physiology remains unclear.