Regulation of starch biosynthesis in response to a fluctuating environment.

Starch is the major storage carbohydrate in higher plants, with many important functions. In photosynthesizing leaves, starch accumulates during the day and is remobilized at night to support continued respiration, Suc export, and growth in the dark (Geiger and Servaites, 1994). In this context, starch has been identified as a major integrator in the regulation of plant growth to cope with continual changes in carbon availability (Sulpice et al., 2009). Its importance is demonstrated by the phenotype of starch-deficient mutants, which grow poorly or even die in short-day conditions (Caspar et al., 1986). In heterotrophic storage organs such as potato (Solanum tuberosum) tubers or developing seeds, starch serves as a longer term carbon store, which is remobilized later in development to support phases of reproductive growth. Since Suc supply to these tissues is fluctuating, regulatory mechanisms are required to stimulate starch synthesis when carbon availability increases (Geigenberger et al., 2004). In addition to its central role in plant physiology, starch is also of great economical importance (Zeeman et al., 2010). It is the secondmost abundant biopolymer on earth, after cellulose, and the most important carbohydrate used for food and feed purposes. Therefore, it represents the major resource of our diet and feedstock for many industrial applications, including bioethanol production (Smith, 2008). Understanding starch biosynthesis in plants could pave the way to new strategies to improve crop yield via the use of reverse genetics or marker-assisted breeding (Geigenberger and Fernie, 2006). Starch metabolism has been the subject of intense research in the past, leading to a sound knowledge of the pathways and the enzymes involved. However, there are many open questions concerning the signals and mechanisms regulating starch metabolism. This review summarizes the current understanding of the regulation of the pathway of starch synthesis in higher plants. The first part will focus on the structure of the pathway, which is an important prerequisite for understanding its regulation. The second part is primarily concerned with signals and mechanisms regulating starch synthesis in response to fluctuations in the carbon and energy status of the plant. The pathway of starch remobilization has recently been reviewed extensively in the literature (Kötting et al., 2010; Zeeman et al., 2010) and will not be covered in any detail here.