Significance of the plastidial stringent response for plant growth on soil

The regulatory nucleotide, guanosine 3′,5′-bis(pyrophosphate) (ppGpp), originally identified in Escherichia coli, controls transcription, translation and enzyme activities in bacteria and plastids of plant cells. We recently reported that seedlings of the ppGpp over-producing mutant of Arabidopsis thaliana grown on agar-solidified medium showed larger biomass than those of wild type (WT), especially under nutrient-limiting conditions. However, the reproducibility of the phenotype on the soil is unknown. To better understand the impact of the ppGpp accumulation on plant growth on the soil, phenotypes of the Arabidopsis ppGpp-less and ppGpp-accumulating mutants were characterized on nitrogen-rich and nitrogen-limiting soil. We found that although fresh weight of the ppGpp-accumulating mutant was significantly larger than that of WT under nitrogen-limiting conditions, dry weights of the mutants were the same as that of WT, indicating that impact of the ppGpp accumulation on plant biomass relies on growth conditions. These results confirmed that artificial modulation of ppGpp-dependent plastidial stringent response could contribute to plant growth on soil in response to nitrogen availability.