Phosphoenolpyruvate Carboxylase in Arabidopsis Leaves Plays a Crucial Role in Carbon and Nitrogen Metabolism

Phosphoenolpyruvate carboxylase (PEPC) is a crucial enzyme that catalyzes an irreversible primary metabolic reaction in plants. Previous studies have used transgenic plants expressing ectopic PEPC forms with diminished feedback inhibition to examine the role of PEPC in carbon and nitrogen metabolism. To date, the in vivo role of PEPC in carbon and nitrogen metabolism has not been analyzed in plants. In this study, we examined the role of PEPC in plants, demonstrating thatPPC1andPPC2were highly expressed genes encoding PEPC in Arabidopsis (Arabidopsis thaliana) leaves and thatPPC1andPPC2accounted for approximately 93% of total PEPC activity in the leaves. A double mutant,ppc1/ppc2, was constructed that exhibited a severe growth-arrest phenotype. Theppc1/ppc2mutant accumulated more starch and sucrose than wild-type plants when seedlings were grown under normal conditions. Physiological and metabolic analysis revealed that decreased PEPC activity in theppc1/ppc2mutant greatly reduced the synthesis of malate and citrate and severely suppressed ammonium assimilation. Furthermore, nitrate levels in theppc1/ppc2mutant were significantly lower than those in wild-type plants due to the suppression of ammonium assimilation. Interestingly, starch and sucrose accumulation could be prevented and nitrate levels could be maintained by supplying theppc1/ppc2mutant with exogenous malate and glutamate, suggesting that low nitrogen status resulted in the alteration of carbon metabolism and prompted the accumulation of starch and sucrose in theppc1/ppc2mutant. Our results demonstrate that PEPC in leaves plays a crucial role in modulating the balance of carbon and nitrogen metabolism in Arabidopsis.