Differential expression of two fructokinases in Oryza sativa seedlings grown under aerobic and anaerobic conditions.

Fructokinases (EC 2.7.1.4) may play an important role in carbohydrate metabolism of Oryza sativa L. (rice) seedlings under anoxia. We present here the molecular and biochemical characterizations of two rice fructokinases, namely OsFK1 and OsFK2. The results show that, at both a transcriptional and a transductional level, OsFK1 is preferentially expressed under aerobic conditions, whereas OsFK2 is induced under anoxia. Substrate inhibition was demonstrated for OsFK1, while OsFK2 appears to be largely unaffected by fructose concentrations up to 10 mM. Sugar modulation of anoxia-induced proteins has been proposed, but our results on rice calli treated with or without glucose (10, 30 or 90 mM) for different time indicate that neither OsFK1 nor OsFK2 are sugar-regulated. We propose that OsFK2 plays a major role in fructose phosphorylation under anoxic conditions.