The complexities of starch biosynthesis in cereal endosperms

Starch serves not only as an energy source for plants, animals, and humans but also as an environmentally friendly alternative for fossil fuels. Here, we describe recent findings concerning the synthesis of this important molecule in the cereal endosperm. Results from six separate transgenic reports point to the importance of adenosine diphosphate glucose pyrophosphorylase in controlling the amount of starch synthesized. The unexpected cause underlying the contrast in sequence divergence of its two subunits is also described. A major unresolved question concerning the synthesis of starch is the origin of nonrandom or clustered alpha-1,6 branch-points within the major component of starch, amylopectin. Developing evidence that several of the starch biosynthetic enzymes involved in amylopectin synthesis occur in complexes is reviewed. These complexes may provide the specificity for the formation of nonrandom branch-points.