Sugar Modulation of a-Amylase Genes under Anoxia

Tolerance to low oxygen availability is likely to be due to the interaction of several factors. Sugar availability is one of the elements required to support anaerobic metabolism. In cereal grains the availability of soluble sugars is limited, while starch is stored in large amounts. Degradation of starch under anoxia is therefore needed to avoid sugar starvation leading to rapid cell death. The striking difference in the ability to produce a-amylase when comparing the anoxia-tolerant rice (Oryza sativa L.) grains with grains of other cereals is not easily explained. Rice is able to respond to gibberellins under anoxia, but the response is too slow to explain the rapid production of a-amylase enzyme. In the present work we demonstrated that a-amylase production during the ®rst 2 d after imbibition is mostly due to the activity of the Ramy3D gene, encoding for the G and H isoforms of a-amylase. The induction of Ramy3D transcription is likely to result from a low sugar content in the grains incubated under anoxia. The ability of rice embryos to sense sugars under anoxia is reported. ã 2003 Annals of Botany Company