Isolation and characterisation of the ADP-glucose pyrophosphorylase small subunit gene (AgpS1) promoter in tomato (Solanum lycopersicum L.)

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ADP-glucose pyrophosphorylase (AGPase) is a key regulatory enzyme in starch biosynthesis. In this research, 2,885 bp of the predicted promoter sequence for the AgpS1 gene encoding the AGPase small subunit was isolated from tomato. Sequence analyses revealed a number of known cis-elements related to responses to salt and dehydration stress and sugar repression; predicted TATA boxes are located at −88 to −94 bp and −114 to −120 bp. The spatial expression pattern and tissue/organ specificity of AgpS1 were analysed in during development using promoter-GUS transgenic tomato plants. Based on GUS staining, the obtained sequence was proven to be the functional promoter and directed broad expression in both sink and source tissues/organs, including seedling, stem, flower, fruit stalk, fruit and root. In source leaf and early developing fruit, GUS staining was observed in all tissues, except for epidermal tissue. In contrast, GUS staining tended to be confined to vascular tissues in seedling, stem, fruit stalk and ripening fruit. In particular, a patchy staining pattern was observed in the phloem of the stem and fruit stalk, suggesting that AgpS1 is expressed in the phloem companion cells in those organs. These results also suggest that AGPase mainly functions in the vascular tissue of those organs.