Effect of Anoxia on Energy Charge and Protein Synthesis in Rice Embryo

Abstract

Energy charge, adenine nucleotide levels, and protein synthesis were studied during the transfer of rice seedlngs from air to anoxia. Within minutes, the energy charge value dropped from 0.90 in air to 0.50 in the seed and 0.60 in the coleoptile after the transfer to a nitrogen atmosphere, and then increased to a value of 0.80 during the subsequent hours. The sum of nucleotides also dropped to 60% of the value in air in the seeds and to 30% in the coleoptiles. However, during the anaerobic growth of coleoptiles, a considerable increase in the nucleotide pool occurred.The incorporation of amino acids into proteins was measured at different stages in anoxic treatment. In rice embryo, we observed a considerable protein synthesis correlated with a high value of energy charge under anoxia. The analysis oflabeled proteins by two-dimensional polyacrylamide gel electrophoresis showed a modified pattern of polypeptides synthesized during anoxic treatment. Some of these proteins were intensively labeled and appeared to be induced by anaerobic treatment.Our data indicate that high metabolic activity occurs in rice embryo under anoxia, which can be correlated with a high energy charge value. These phenomena may be part of the mechanisms which permit the adaptation of rice embryos to anaerobiosis.In nonchlorophyllous plant tissues and in animals, it is generally assumed that ATP synthesis is drastically reduced when oxidative phosphorylation is inhibited by a lack of 02. The knowledge of the balance between ATP regeneration and utilization for metabolic work should permit a better understanding of the mechanisms used by these organisms to survive under anoxic conditions.In animal tissues, there is a rapid inhibition of protein synthesis after a few min of anoxia (10,12 Preliminary data obtained in our laboratory indicate that rice embryos or coleoptiles germinated in anoxia or transferred from air to nitrogen have a high energy charge (20, 21) and an active RNA (2) and DNA synthesis (16).The purpose of this paper is to study the effect of anoxia on adenine nucleotide levels, energy charge, and incorporation of labeled amino acids into proteins in rice embryos. The correlation between energy charge and protein synthesis in anoxia is discussed, and anoxic protein patterns sustain the hypothesis of an adaptation of rice embryo to anaerobic conditions. MATERIALS AND METHODSPlant Materials. Rice seeds (Oryza sativa L., var. Cigalon) cultivated by the Station d'Amelioration des Plantes (INRA, Montpellier, France) were mechanically husked and sterilized with commercial (NaOCl (150 g Cl/l), as described previously (16).Germination of Rice Seeds. Seeds were placed in glass flasks with water and shaken at 26 C in darkness. They remained under aerobic conditions for 40 to 48 h until coleoptiles reached 4 to 5 mm.For anoxia, samples were placed in a nitrogen flow (100 ml/ min), as previously described (16). Oxygen content was checked with an 02 analyzer (WOM, Mecanalyse, France) or by gas chromatography. PO2 in N2 was lower than...

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