Temporally extended gene expression of the ADP-Glc pyrophosphorylase large subunit ( AgpL1 ) leads to increased enzyme activity in developing tomato fruit

Tomato plants (Solanum lycopersicum) harboring the allele for the AGPase large subunit (AgpL1) derived from the wild species Solanum habrochaites (AgpL1 H ) are characterized by higher AGPase activity and increased starch content in the immature fruit, as well as higher soluble solids in the mature fruit following the breakdown of the transient starch, as compared to fruits from plants harboring the cultivated tomato allele (AgpL1 E ). Comparisons of AGPase subunit gene expression and protein levels during fruit development indicate that the increase in AGPase activity correlates with a prolonged expression of the AgpL1 gene in the AgpL1 H high starch line, leading to an extended presence of the L1 protein. The S1 (small subunit) protein also remained for an extended period of fruit development in the AgpL1 H fruit, linked to the presence of the L1 protein. There were no discernible differences between the kinetic characteristics of the partially purified AGPase-L1E and AGPase-L1H enzymes. The results indicate that the increased activity of AGPase in the AgpL1 H tomatoes is due to the extended expression of the regulatory L1 and to the subsequent stability of the heterotetramer in the presence of the L1 protein, implying a role for the large subunit not only in the allosteric control of AGPase activity but also in the stability of the AGPase L1–S1 heterotetramer. The introgression line of S. lycopersicum containing the wild species AgpL1 H allele is a novel example of transgressive heterosis in which the hybrid multimeric enzyme shows higher activity due to a modulated temporal expression of one of the subunits.