Alcoholic Fermentation and Activity of Some Enzymes in Rice Roots Under Anaerobiosis

Abstract

Rice was initially grown in aerated solutions. When tillering commenced, anaerobiosis was imposed by flushing nitrogen gas through nutrient solutions for up to 6 days ('adapted plants'); plants which remained in aerated solutions are referred to as 'non-adapted plants'. Under anaerobiosis, CO2 evolution and ethanol production by excised apical root tissues were five times greater for adapted than for non-adapted plants. For excised tissues from adapted plants, carbon lost as CO2 and ethanol approximately equalled that of aerobic CO2 evolution, i.e. CO2(N2) / CO2(air) = 0.30. This ratio was only 0.06 for apical tissue from non-adapted plants. Ethanol was not produced under aerobic conditions, even by tissue from adapted plants. Anaerobiosis of the root environment caused rapid increases in activities of alcohol dehydrogenase (ADH) and pyruvate decarboxylase (PDC). Cycloheximide largely inhibited the increases in activities of these enzymes under anaerobiosis. Elevated activities of ADH and PDC persisted for several days after transfer of adapted plants to aerated solutions. Despite these high activities of PDC and ADH, ethanol was not produced in root tissue from adapted plants under aerobic conditions, which indicates that either PDC or ADH is under fine control. Experiments in vitro suggest that this control is afforded at least partly by activation of PDC by NADH and inhibition by NAD+. It is suggested that NADH can replace thiamin pyrophosphate in promoting PDC activity via effects on conformation of the enzyme.