Starch Biosynthesis and Degradation in Plants

Abstract

Abstract Starch is the main form in which plants store carbon. Its presence and turnover are important for proper plant growth and productivity. The glucose polymers that constitute the semi‐crystalline starch granule are synthesised by the concerted actions of well‐conserved classes of isoforms of starch synthase and starch‐branching enzyme, via a process that also requires the debranching enzyme isoamylase. The degradation of the granule proceeds via different pathways in different types of starch‐storing tissues. The pathway of starch degradation differs between different plant tissues, but has been elucidated in most detail in leaves. The polymer is first phosphorylated to allow access to the insoluble granule by enzymes that cleave bonds between glucose residues. The main product of this degradation is maltose, which is exported into the cytosol before a series of enzymatic steps convert it to sucrose. Key Concepts Starch is the main carbon store of most plants. It is a glucose polymer that is stored as insoluble granules within plastids. The polymer is composed of two fractions, branched amylopectin and unbranched amylose. It is synthesised by a number of enzymes that control the amount made, as well as the amounts of amylose and amylopectin. Starch is synthesised in leaves during the day and is mobilised at night. The rate of leaf starch degradation at night is closely controlled by mechanisms linked to the circadian clock so that starch is eliminated just before the beginning of day. Leaf starch degradation involves many enzymes that lead to the production of soluble sugars, mainly maltose. Maltose is exported into the cytosol where it is converted to sucrose. Arabidopsis plants in which starch metabolism is impaired grow poorly. Starch synthesised in leaves is often termed transitory starch, while that in storage organs can be termed storage starch. They are different in terms of the structure of the amylose and amylopectin within them.