title: Metabolic regulation of pH in plant cells: role of cytoplasmic pH in defense reaction and secondary metabolism.
abstract: A new biochemical pH-stat hypothesis that revised the classic hypothesis is presented to understand the metabolic regulation of intracellular pH in plant cells. Alternative pathway glycolysis, alternative pathway respiration and malate-derived lactic and alcoholic fermentation (alternative pathway fermentation), all unique to plants, are integrated into a regulatory mechanism of pH in the cytoplasm. Its uniqueness to plant kingdom is discussed from the evolutionary viewpoint: it is suggested that when the ancestors of extant terrestrial plants expanded their habitat from oceans to freshwater, they abandoned a "sodium system" and adopted a "proton system" for nutrient uptake. Validity of the new hypothesis is examined with available data on a secondary active transport, anoxia and other experimental evidence. The hypothesis predicts that biotic and abiotic stress-induced cytoplasmic acidification triggers synthesis of phytoalexins and other secondary metabolites. Possible roles of cyanide-resistant alternative pathway respiration in the secondary metabolite production, metabolic switching between primary and secondary metabolisms, and defense reactions are proposed.
authors.affiliation: Department of Plant Physiology, National Institute of Agrobiological Resources, Tsukuba, Ibaraki, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Sakano
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000502:physiology; D019069:Cell Respiration / Q000502:physiology; D003593:Cytoplasm / Q000201:enzymology / Q000378:metabolism*; D006706:Homeostasis; D006863:Hydrogen-Ion Concentration; D008291:Malate Dehydrogenase / Q000378:metabolism; D010710:Phosphates / Q000378:metabolism; D010730:Phosphoenolpyruvate Carboxylase / Q000378:metabolism; D010944:Plants / Q000378:metabolism* / Q000382:microbiology / Q000469:parasitology; D011188:Potassium / Q000378:metabolism; D011522:Protons; D015398:Signal Transduction / Q000502:physiology; D012964:Sodium / Q000378:metabolism
chemical_list: D010710:Phosphates; D011522:Protons; D012964:Sodium; D008291:Malate Dehydrogenase; D010730:Phosphoenolpyruvate Carboxylase; D011188:Potassium