Nocturnal phosphorylation of phosphoenolpyruvate carboxylase in the leaves of hygrophytic C3 monocots

Phosphoenolpyruvate carboxylase (PEPC) undergoes activity regulation through reversible phosphorylation. The day/night phosphorylation of leaf PEPC in 27 C3 plant species was analyzed by immunoblotting. PEPC was phosphorylated in the daytime in 12 species, whereas it was phosphorylated at night in three species, rice, Monochoria vaginalis, and Sagittaria trifolia, all of which are hygrophytic monocots. Immunoblot analysis of isolated chloroplasts of M. vaginalis identified a PEPC protein inside the chloroplast in addition to cytosolic isozyme(s) as previously shown in genus Oryza. Using transgenic rice overexpressing the maize PEPC in the cytosol, we confirmed that the cytosolic PEPC underwent the nocturnal phosphorylation. These results suggest the interrelationship between the presence of chloroplastic PEPC and the nocturnal phosphorylation of cytosolic isozyme(s).