ETHANOLIC FERMENTATION IN TRANSGENIC TOBACCO EXPRESSING ZYMOMONAS-MOBILIS PYRUVATE DECARBOXYLASE

During oxygen limitation in higher plants, energy metabolism switches from respiration to fermentation. As part of this anaerobic response t he expression of genes encoding pyruvate decarboxylase (PDC) and alcoh ol dehydrogenase (ADH) is strongly induced. In addition there-is ample evidence for post-translational regulation. In order to understand th is multi-level regulation of the anaerobic response, we provided tobac co with the constitutive capacity of ethanolic fermentation by express ing a PDC gene derived from the obligate anaerobe Zymomonas mobilis. T he protein accumulated to high levels and was active in an in vitro as say. During the first 2-4 h of anoxia, acetaldehyde accumulated to 10- to 35-fold and ethanol to 8- to 20-fold higher levels than in wild-ty pe. Under normoxic conditions no accumulation of acetaldehyde and etha nol could be measured. Instead, the two products may be immediately re -metabolized in tobacco leaf tissue. We show that aerobic fermentation takes place when the respiratory system is inhibited. Although these conditions enhance ethanolic fermentation under normoxia, they fail to increase ADH transcript levels. These results indicate that anaerobic transcription is triggered not by the metabolic consequences of oxyge n limitation, but directly through an oxygen-sensing system.