The anaerobic response

When plants experience anoxic conditions there is a shift in carbohydrate metabolism from an oxidative to a fermentative pathway (Fig. 1). In the absence of oxygen, ATP is generated not by the Krebs cycle but by alcoholic fermentation, i.e. glycolysis and ethanol synthesis.

As well as the change in carbohydrate metabolism there is a change in the pattern of polypeptide synthesis under anoxia (Sachs et al., 1980; Bailey-Serres et al., 1988). Synthesis of polypeptides normally present under aerobic conditions stops and synthesis of a number of specific polypeptides – the anaerobic polypeptides (ANPs) – commences. In maize there are about 20 ANPs which have been identified chiefly as enzymes associated with the flow of carbon into and through glycolysis and through alcoholic fermentation; in particular UDP-sucrose synthetase, pyruvate decarboxylase and alcohol dehydrogenase (ADH) are induced approximately 10-fold (Lazlo & St Lawrence, 1983; Springer et al., 1986). Glucose phosphate isomerase (Kelley & Freeling, 1984a), one of the isozymes of glyceraldehyde 3-phosphate dehydrogenase, and cytoplasmic aldolase (Kelley & Freeling, 1984b) have also been shown to be induced to a lesser degree. The levels of two enzymes which are thought to be responsible for regulating the glycolytic pathway, phosphofructokinase and pyruvate kinase, do not change significantly during anaerobiosis (Bailey-Serres et al., 1988).

It is assumed that the enzymes of glycolysis are induced by anaerobiosis to allow a greater flux of carbohydrate through the pathway because only 2 molecules of ATP are produced per molecule of glucose under anaerobic conditions whereas 36 molecules of ATP are produced under oxidative conditions.