Acidification and alkalinization of culture medium by Catharanthus roseus cells - Is anoxic production of lactate a cause of cytoplasmic acidification?

Abstract

Catharanthus roseus (L.) G. Don cells acidified Murashige-Skoog medium rapidly. Upon transfer to fresh medium, the medium pH (initially5.3) dropped below 4 within 2 d. This acidification was reversed under hypoxic conditions. The cells induced a similar acidification in a simple medium consisting of CaCl2, KCl, and glucose: medium pH dropped below 4 within 6 h. The acidification was accompanied by an influx of K+ at a H+(efflux)/K+ ratio of ca. 0.6 as well as by an expansion of endogenous organic acid pool, in which malic and citric acids were the major components. Anoxia reversed all these processes: the direction of both K+ and H+ fluxes reversed with a H+/K+ ratio of 1.70. Anoxia induced a cytoplasmic acidification from pH 7.6 (aerobic) to 7.4 as measured by 31P-NMR, accompanied by a rapid, long-lasting lactate accumulation at expense of malic and citric acids. Evidence suggested that accumulation of lactic acid was not a cause of cytoplasmic acidification under anoxia, but a result of pH regulation by the biochemical pH-stat [Davies (1973) Symp. Soc. Exp. Biol. 27: 513]. The anoxic acidification of the cytoplasm was ascribed to the influx of H+ from the medium.