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内容記述 |
In early developing tomato (Solanum lycopersicum) fruit, starch accumulates at high levels and is used by various primary metabolites, such sugars, in ripening fruits. ADP-glucose pyrophosphorylase (AGPase) is responsible for the first key step of starch biosynthesis. Although it has been reported that AgpL1 and AgpS1 isoforms are mainly expressed in early developing fruit, their regulatory mechanism has not been elucidated. The present study investigated the transcriptional response of AgpL1 and AgpS1 to various metabolizable sugars, nonmetabolizable sugar analogues, hexokinase inhibitors and proline by an experimental system using half-cut fruits. AgpL1 was upregulated in response to sucrose and constituted hexoses such glucose, whereas the AgpS1 gene almost did not exhibit a prominent sugar response. Further analyses revealed that other disaccharides did not show a remarkable effect on both AgpL1 and AgpS1 expressions. These results indicated that there are two distinct regulatory mechanisms, sugar metabolism-dependent and -independent, for the regulation of AGPase gene expression. Interestingly, the ADP treatment, a hexokinase inhibitors, cancelled the sugar response of AgpL1, therefore, it indicated that hexokinase-mediated sugar signalling necessary to the sugar response of AgpL1. Our results suggest that sugar-dependent (AgpL1) including sugar phosphorylation metabolism and sugar-independent (AgpS1) pathways coordinatively regulate starch biosynthesis in immature tomato fruit. |
