A combined reduction in activity of starch synthases II and III of potato has novel effects on the starch of tubers

A chimeric antisense construct has been used to generate transgenic potatoe s (Solanum tuberosum L.) in which activities of both of the main starch syn thases responsible for amylopectin synthesis in the tuber (SSII and SSIII) are reduced. The properties of starch from tubers of these plants have been compared with those of starches from transgenic plants in which activity o f either SSII or SSIII has been reduced. Starches from the three types of t ransgenic plant are qualitatively different from each other and from the st arch of control plants with unaltered starch synthase activities, with resp ect to granule morphology, the branch lengths of amylopectin, and the gelat inisation behaviour analysed by viscometry. The effects of reducing SSII an d SSIII together cannot be predicted from consideration of the effects of r educing these two isoforms individually. These results indicate that differ ent isoforms of starch synthase make distinct contributions to the synthesi s of amylopectin, and that they act in a synergistic manner, rather than in dependently, during amylopectin synthesis.