Significance of accumulation of the alarmone (p)ppGpp in chloroplasts for controlling photosynthesis and metabolite balance during nitrogen starvation in Arabidopsis

The regulatory nucleotides, guanosine 5'-triphosphate 3'-diphosphate (pppGpp) and guanosine 5'-diphosphate 3'-diphosphate (ppGpp), were originally identified in Escherichia coli, and control a large set of gene expression and enzyme activities. The (p)ppGpp-dependent control of cell activities is referred to as the stringent response. A growing number of (p)ppGpp synthase/hydrolase homologs have been identified in plants, which are localized in plastids in Arabidopsis thaliana. We recently reported that the Arabidopsis mutant overproducing ppGpp in plastids showed dwarf chloroplasts, and transcript levels in the mutant plastids were significantly suppressed. Furthermore, the mutant showed more robust growth than the wild type (WT), especially under nutrient-deficient conditions, although the mechanisms are unclear. To better understand the impact of the ppGpp accumulation on plant responses to nutrient deficiency, photosynthetic activities and metabolic changes in the ppGpp-overproducing mutant were characterized here. Upon transition to the nitrogen-deficient conditions, the mutant showed reduction of ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) contents, and effective and maximum quantum yield of photosystem II compared with WT. The mutant also showed more obvious changes in key metabolite levels including some amino acid contents than WT; similar metabolic change is known to be critical for plants to maintain carbon-nitrogen balance in their cells. These results suggest that artificially overproducing ppGpp modulates the organelle functions that play an important role in controlling photosynthetic performance and metabolite balance during nitrogen starvation.