Enhancing sucrose synthase activity results in increased levels of starch and ADP-glucose in maize (Zea mays L.) seed endosperms.

Abstract

Sucrose synthase (SuSy) is a highly regulated cytosolic enzyme that catalyzes the conversion of sucrose and a nucleoside diphosphate into the corresponding nucleoside diphosphate glucose and fructose. In cereal endosperms, it is widely assumed that the stepwise reactions of SuSy, UDPglucose pyrophosphorylase and ADPglucose (ADPG) pyrophosphorylase (AGP) take place in the cytosol to convert sucrose into ADPG necessary for starch biosynthesis, although it has been also suggested that SuSy may participate in the direct conversion of sucrose into ADPG. In this study, the levels of the major primary carbon metabolites, and the activities of starch metabolism related enzymes were assessed in endosperms of transgenic maize plants ectopically expressing StSUS4 , which encodes a potato SuSy isoform. A total of 29 fertile lines transformed with StSUS4 were obtained, 5 of them containing a single copy of the transgene that was still functional after five generations. The number of seeds per ear of the 5 transgenic lines containing a single StSUS4 copy was comparable to that of wild type (WT) control seeds. However, transgenic seeds accumulated 10-15% more starch at the mature stage, and contained a higher amylose/amylopectin balance than WT seeds. Endosperms of developing StSUS4-expressing seeds exhibited a significant increase in SuSy activity, and in starch and ADPG contents when compared with WT endosperms. No significant changes could be detected in the transgenic seeds in soluble sugars content, and in activities of starch metabolism related enzymes when compared to WT seeds. A suggested metabolic model is presented wherein both AGP and SuSy are involved in the production of ADPG linked to starch biosynthesis in maize endosperm cells.