Discussion

Rice plants adapt well to submergence. Among graminaceous monocots, rice plants possess extremely well developed aerenchyma tissues in their leaves, as well as in their roots. These air spaces allow rice plants to survive under heavily flooded conditions.13,14 However, severe flooding leads to complete submergence of plants. Under such conditions, oxidative phosphorylation in mitochondria is inhib-ited.15,16 Thus, plants are limited to the production of ATP via the glycolytic pathway.3,17,18 The effects of anaerobiosis on gene expression have been studied extensively. Transcriptional induction under anaerobic conditions has been investigated for enzymes involved in alcoholic fermentation and glycolysis such as alcohol dehydrogenase,9,20 glyceraldehyde phosphate dehydrogenase21 and a-amylase.22

Umeda and Uchimiya23 analyzed the expression levels of genes associated with glycolysis and alcohol fermentation in rice plants under submergence stress. They reported that two types of gene (type I and type II) with respect to the accumulation of mRNA were expressed in response to submergence stress. Transcripts of type I genes, such as the genes for glucosephosphate isomerase, phosphof-ructokinase, glyceraldehydephosphate dehy-drogenase and enolase, reached a maximum level after 24 h of submergence. In contrast, transcripts of type II genes, genes such as those for aldolase and pyruvate kinase, reached a maximum level after 10 h of submergence. This latter pattern coincides with the expression pattern of a gene for ribosomal protein. In the case of adenylate kinase, the accumulation of mRNA was similar to that of type I genes, and reached a maximum at 24 h after the start of submergence. When rice seedlings were only partially submerged, the activity of adenylate kinase was not induced, which is similar to the expression patterns of type I genes. The difference between the times of the maximum enzyme activity (72 h) and mRNA accumulation (24 h) in adenylate kinase may be due to the stabilities of protein or mRNA, or to some other factors.

Treatment with N2 gas was as effective as submersion in inducing adenylate kinase activity, which suggests that oxygen deficiency might be a major factor in the induction of adenylate kinase activity by submergence stress. It seems likely that an oxygen deficit increases the transcription of genes for adenylate kinase and several other proteins associated with the anaerobic production of energy. Adenylate kinase activity was highly induced in coleoptile and endosperm under submergence conditions. In rice plant, the elongation of coleoptile is a well known characteristic of submerged seedlings. In young seedlings, energy supply for growth is dependent on starch breakdown in endosperm. Among several cereal seeds, only rice is able to degrade the starchy endosperm under anaerobic conditions.24 It is interesting that stimulation of adenylate kinase activity was shown in such organs in submerged conditions.

Enzyme assay using the submergence tolerant rice FR13A and submergence intolerant IR42 showed induction in FR13A after 24 h of submergence. In IR42, adenylate ki-nase activity continued to increase steadily to 96 h. Umeda and Uchimiya23 analyzed the mRNA level of glycolysis and alcohol fermentation genes, and considered that the transcript levels of type I genes (glucose phosphate isomerase, phosphofructokinase, glyceraldehyde phosphate dehydrogenase and enolase) may change in FR13A. These results suggest that the expression of adenylate kinase may be coregulated with several glycolysis and alcoholic fermentation genes.

Sodium Chloride Stimulates Adenylate Kinase Level in Seedlings of Salt-Sensitive Rice Varieties

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