What Controls the Amount and Structure of Starch in Storage Organs?

The regulation of starch synthesis in the starch-storing organs of higher plants-organs such as tubers, the em- bryos of grain legumes, and the endosperms of cereals, in which starch constitutes 50 to 80% of the dry weight at maturity-is complex and poorly understood. Our igno- rance of this process has been highlighted by the recent upsurge of interest in starch synthesis, fueled largely by the possibility of producing nove1 starches by genetic modifi- cation. Details of the metabolic pathway itself remain the subject of controversy, and remarkably little is known of the factors that determine either the rate of synthesis or the structure of the starch in storage organs. Some widely accepted views about the regulation of starch synthesis are no longer tenable or require revision in the light of recent findings. We shall discuss briefly the nature of the pathway of starch synthesis, then examine critically the current ideas about factors that determine the flux through the pathway and two important aspects of starch structure: the ratio of amylose to amylopectin and the branching of amylopectin.