Sedoheptulose 1,7-bisphosphatase (SBPase) is an enzyme catalyzing the reaction converting sedoheptulose 1,7-bisphosphate into sedoheptulose 7-phosphate. This enzyme is located in the chloroplast in leaves and stems. Overexpression of SBPase in transgenic plants is provided to improve plant yield by increasing leaf starch biosynthetic ability in particular and sucrose production in general. Deregulated variants of the enzymes are also provided.