The regulatory PII protein controls arginine biosynthesis in Arabidopsis

In higher plants, PII is a nuclear‐encoded plastid protein which is homologous to bacterial PII signalling proteins known to be involved in the regulation of nitrogen metabolism. A reduced ornithine, citrulline and arginine accumulation was observed in two Arabidopsis PII knock‐out mutants in response to {\mathrm{NH}}_4^{+} resupply after N starvation. This difference could be explained by the regulation of a key enzyme of the arginine biosynthesis pathway, N‐acetyl glutamate kinase (NAGK) by PII. In vitro assays using purified recombinant proteins showed the catalytic activation of Arabidopsis NAGK by PII giving the first evidence of a physiological role of the PII protein in higher plants. Using Arabidopsis transcriptome microarray (CATMA) and RT‐PCR analyses, it was found that none of the genes involved in the arginine biosynthetic or catabolic pathways were differentially expressed in a PII knock‐out mutant background. In conclusion, the observed changes in metabolite levels can be explained by the reduced activation of NAGK by PII.