Starch biosynthesis.

Starch is the most significant form of carbon reserve in plants in terms of the amount made, the universality of its distribution among different plant species, and its commercial importance. It consists of different glucose polymers arranged into a threedimensional, semicrystalline structure-the starch granule. The biosynthesis of starch involves not only the production of the composite glucans but also their arrangement into an organized form within the starch granule. The formation of the starch granule can be viewed as a simple model for the formation of ordered three-dimensional polysaccharide structures in plants. Understanding the biochemical basis for the assembly of the granule could provide a conceptual basis for understanding other higher order biosynthetic systems such as cellulose biosynthesis (see Delmer and Amor, 1995, this issue). For example, one emerging concept is that structure within the granule itself may determine or influence the way in which starch polymers are synthesized. Starch is synthesized in leaves during the day from photosynthetically fixed carbon and is mobilized at night. It is also synthesized transiently in other organs, such as meristems and root cap cells, but its major site of accumulation is in storage organs, including seeds, fruits, tubers, and storage roots. Almost all structural studies have used starch from storage organs because it is readily available and commercially important; we therefore focus on starch biosynthesis in storage organs. However, where aspects of transient biosynthesis are clearly different from long-term reserve synthesis, reference is made to biosynthesis in nonstorage tissues. Starch is synthesized in plastids, which in storage organs committed primarily to starch production are called amyloplasts. These develop directly from proplastids and have little interna1 lamellar structure. Starch may also be synthesized in plastids that have other specialized functions, such as chloroplasts (photosynthetic carbon fixation), plastids of oilseed (fatty acid biosynthesis), and chromoplasts of roots such as carrot (carotenoid biosynthesis). In some cases, for example, in the storage cotyledons of some legumes, amyloplasts in storage organs develop from chloroplasts. These amyloplasts may maintain considerable amounts of stacked lamellar material from the thylakoids and, in cells receiving sufficient light, may undertake some photosynthetic carbon fixation for use in starch