SYNOPSIS It Takes Two to Tango: A New Partner in Amylose Synthesis

During the daytime, plants capture the energetic proceeds of photosynthesis by building up a cache of starch granules. At night, plants can draw on their starch stores to fuel their metabol-ic needs. The most abundant component of starch, amylopectin, is a polymer composed of many short, highly branched chains of glucose molecules. The less abundant component of starch, amylose, has a simpler structure comprised of a single, unbranched chain of glucose molecules. The structural characteristics of amylopectin and amylose lend them to very different uses. For instance, amylopectin is easily digested by humans and is therefore a major source of die-tary glucose. It also has several industrial uses because its highly branched nature makes it an excellent thickener and texturant. By contrast, amylose can’t be digested well by people because its long glucose chains tend to repack together after cooking. Curiously, despite the obvious im-portance of starches, the mechanisms of starch synthesis are still not fully understood. In this issue of PLOS Biology, David Seung, Samuel Zeeman, and colleagues describe a protein with a newly identified role in the synthesis of amylose. Scientists have identified four chain-elongating and two branching enzymes that work to