Nitrogen acquisition, PEP carboxylase, and cellular pH homeostasis: new views on old paradigms

The classic biochemical pH-stat model of cytosolic pH regulation in plant cells presupposes a pH-dependent biosynthesis and degradation of organic acids, specifically malic acid, in the cytosol. This model has been used to explain the higher tissue accumulation of organic acids in nitrate (NO3-)-grown, relative to ammonium (NH4+)-grown, plants, the result of proposed cytosolic alkalinization by NO3- metabolism, and acidification by NH4+ metabolism. Here, a critical examination of the model shows that its key assumptions are fundamentally problematic, particularly in the context of the effects on cellular pH of nitrogen source differences. Specifically, the model fails to account for proton transport accompanying inorganic nitrogen transport, which, if considered, renders the H+ production of combined transport and assimilation (although not the accumulation) to be equal for NO3- and NH4+ as externally provided N sources. We show that the model's evidentiary basis in total-tissue mineral ion and organic acid analysis is not directly relevant to subcellular (cytosolic) pH homeostasis, while the analysis of the ionic components of the cytosol is relevant to this process. A literature analysis further shows that the assumed greater activity of the enzyme phosphoenolpyruvate (PEP) carboxylase under nitrate nutrition, which is a key characteristic of the biochemical pH-stat model as it applies to nitrogen source, is not borne out in numerous instances. We conclude that this model is not tenable in its current state, and propose an alternative model that reaffirms the anaplerotic role of PEP carboxylase within the context of N nutrition, in the production of carbon skeletons for amino acid synthesis.