Characterization of a Maize Sucrose–phosphate Synthase Protein and Its Effect on Carbon Partitioning in Transgenic Rice Plants

Abstract

We obtained transgenic rice (Oryza sativa L. cv. Nipponbare) plants with the gene for maize sucrose-phosphate synthase (EC 2.4.1.14, SPS). Some of the transgenic plants over-expressed maize SPS (over-expressing plants) and some had reduced levels of native SPS protein (co-suppressed plants). There was a positive correlation between the amounts of maize SPS protein and SPS activities. However, apparent Km values for uridine diphosphoglucose (UDPG) were higher in over-expressing plants than in control rice plants. These results suggest that overproduced maize SPS protein was not fully activated. The sucrose contents did not differ significantly between control and over-expressing rice plants, but they were lower in co-suppressed plants than in control plants. The starch contents were negatively and the sucrose/starch ratios were positively correlated with SPS activities. Thus, carbon partitioning in the transgenic rice was changed, even though rice is predominantly a sucrose-former.