Isolation and expression analysis of two tomato ADP-glucose pyrophosphorylase S (large) subunit gene promoters

ADP-glucose pyrophosphorylase (AGPase, EC2.7.7.27) is a key enzyme in starch synthesis. The enzyme is a heterotetramer with two S (large) and two B (small) subunits. In tomato (Lycopersicon esculentum Mill), there are three S subunit genes. The Agp S1 and Agp S3 genes and their promoters were isolated from a tomato genomic library. The Agp S1 promoter region has a TATAA box at −57 bp and a CCAAT box at the −8 bp position. Tomato Agp S1 promoter is active in starch storage tissues, and in organs that are carbohydrate sinks. Its activity is localized in the guard cells and veins of the leaf, in the root cap and root vascular tissues, in the starch sheath cells of the stem and in the ovary and stamens of the flower. It is also active in pollen, seeds and in the inner pericarp wall and placental tissue of developing tomato fruits. No activity was observed in mesophyll cells, or in the ovaries at anthesis. The Agp S3 promoter has a CCAAT box at the −115 bp position and multiple GC rich regions around the CCAAT box, but it does not contain a TATAA box. Tomato Agp S3 promoter is expressed in the mesophyll cells, guard cells and veins of the leaf. In the flower, Agp S3 is strongly expressed in the sepals, ovary and anthers at anthesis. Deletions in the distil 5′ upstream region of Agp S3 indicated that only a 0.5 kb fragment of the Agp S3 promoter is required for complete expression in transgenic plants. Agp S3 contains a minimal promoter with little evidence of functional cis-acting regulatory elements. For AGPase, the different requirements for regulation of enzyme activity between source leaves and sink organs are met in part by the evolution of multiple S subunit genes with very different promoters.