Non-autotrophic CO2Fixation during Shoot Formation in Tobacco Callus

Abstract

Non-autotrophic carbon fixation has been studied during growth of tobacco callus cultured in dark under shoot-forming (SF) and non-shoot-forming (NSF) conditions. The enzymes involved in malate metabolism—phosphoenolpyruvate carboxylase, malic dehydrogenase, glutamic-oxalacetic transaminase, and malic enzyme—increased sharply during the first 4 d of culture particularly in SF tissue. The activities of the enzymes studied were considerably greater in SF than in NSF tissue. There was a dramatic increase in malate content in SF tissue during the first 4 d of culture. Subsequently malate was rapidly depleted during the time of organogenesis. In NSF tissue there was a continuous build-up of malate content throughout the culture period. We suggest that malate derived from dark fixation of CO₂ plays differing roles in NSF (callus) and SF tissues. In the former, malate acts primarily as an osmotic solute regulating, at least in part, cell expansion between successive cell divisions. In shoot-forming tissue, on the other hand, malate preferentially provides NADPH for reductive biosynthesis.