The present invention relates to a process for increasing the phosphate content of starches of genetically modified plant cells in comparison with starches from corresponding wild-type plant cells by introducing a foreign nucleic acid molecule which codes for a soluble starch synthase II. The present invention furthermore relates to the overexpression of this soluble starch synthase II in the genetically modified plant cells. Furthermore, the present invention relates to rice starch and rice flour with improved quality characteristics, to rice grains comprising this rice starch, and to rice plants on which these rice grains grow.