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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. [ABSTRACT FROM AUTHOR]

Rice grains can germinate under anoxia, while most other cereals fail to behave similarly. The ability of rice grains to degrade starch, thanks to the successful production of α-amylase even in the absence of oxygen is likely one of the factors allowing rice to germinate anaerobically. In this paper we review the most recent results concerning the physiology and molecular biology related to the expression of α-amylase genes under anoxia. The current view is that expression of sugar-starvation induces isoforms of α-amylase playing a major role during germination under anoxia, while gibberellin induces α-amylase predominating under aerobic conditions. [ABSTRACT FROM AUTHOR]

Production of α-amylase during the germination of rice grains is thought to play an important role for tolerance to anoxia of these cereal grains. Under aerobic conditions α-amylases production is enhanced in response to gibberellins produced by the embryos, but the role of these hormones is less clear under anoxia. In this paper we analysed α-amylase gene expression in a rice mutant (Tan-ginbozu) severely impaired in gibberellin biosynthesis. Expression of α-amylase genes others than the gibberellin-induced Amy1A gene is observed. The expression of the Amy3D gene, which does dot require gibberellins to be induced, is high under anoxia in the Tan-ginbozu mutant suggesting that germination under anoxia can proceed thanks to the activity of the α-amylase isoform encoded by the Amy3D gene. Amy3D gene expression is repressed in the presence of high levels of soluble carbohydrates, indicating that the anaerobic expression of this gene can be triggered by a lower carbohydrate content of rice grains kept under anoxia. Germination under anoxia of Tan-ginbozu grains can proceed even in absence of exogenously-added gibberellic acid. Overall, results indicate that gibberellins are not required for the anaerobic germination of rice grains. [ABSTRACT FROM AUTHOR]