Revision of Biochemical pH-Stat: Involvement of Alternative Pathway Metabolisms

A new hypothesis is proposed to explain the metabolic regulation of intracellular pH (especially of the cytoplasm) in plants and replace the classic biochemical pH-stat [Davies (1973) Symp. Soc. Exp. Biol. 27: 513]. Alternative route glycolysis [Plaxton (1996) Annu. Rev. Plant Physiol. Plant Mol. Biol. 47: 185], alternative pathway respiration [Day et al. (1996) Plant Physiol. 110: 1], and malate-de-rived lactic and alcoholic fermentation (alternative pathway fermentation), all unique to plants, are organized into a novel mechanism that functions in toto as biochemical pH-stat. Its uniqueness to plants is examined from the evolutionary viewpoint that plants adopted the proton as the ion with which to create the electrochemical potential difference across the plasma membrane to energize uphill transports (“proton system”). The biochemical pH-stat is assumed to have evolved as a security mechanism for the “proton system” so that it could cope with the potential acidification of the cytoplasm under environmental stress.