Starch Biosynthesis and Degradation in Plants

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.

Overexpression of a wheat α‐amylase type 2 impact on starch metabolism and abscisic acid sensitivity during grain germination

“…According to Lloyd and Kötting (2016), the starch degradation pathway to glucose proceeds via concerted and sequential actions of the following enzymes: α‐amylase, Limit dextrinase, β‐amylase, and Maltase producing branched glucans and linear α‐gluco‐oligosaccharides, linear glucans, maltose and glucose respectively. However, the accumulation of α‐gluco‐oligosaccharides when acarbose is used would suggest a more complex role of α‐amylase in this degradation cascade.…”

Recent Advances in Carbon and Nitrogen Metabolism in C3 Plants

“…On the other hand, amylopectin is a highly branched α-1,4- and α-1,6-linked glucose polymer, which occupies 70–80% of starch granules in most plants. Amylopectin is involved in the formation of a semi-crystalline structure with crystalline and amorphous lamella, and amylose likely does not contribute to the starch granules’ structural formation [ 149 ].…”

“…Starch degradation has been intensively investigated in germinating cereal seeds; αamylase, debranching enzyme (R-enzyme), and α-glucosidase are involved in the breakdown process [197,198]. Recently, the degradation of transient starch in the chloroplast was clarified in Arabidopsis [149,199,200]. The starch molecules are phosphorylated and Starch degradation has been intensively investigated in germinating cereal seeds; α-amylase, debranching enzyme (R-enzyme), and α-glucosidase are involved in the breakdown process [197,198].…”

“…The starch molecules are phosphorylated and Starch degradation has been intensively investigated in germinating cereal seeds; α-amylase, debranching enzyme (R-enzyme), and α-glucosidase are involved in the breakdown process [197,198]. Recently, the degradation of transient starch in the chloroplast was clarified in Arabidopsis [149,199,200]. The starch molecules are phosphorylated and dephosphorylated by glucan water dikinase (GWD), phosphoglucan water dikinase (PWD), and phosphoglucan phosphatase (DSP).…”

The Association between Germination Vigour and Biochemical, Physiological, Subcellular Changes in Bambara Groundnut Seeds after Seed Scarification

“…From the current observations, there is greater space for metabolic reactions to occur under greater area; hence germination metabolism occurred greatly on starch granules with large area to speed up the process of germination. Starch catabolism provides simple sugars [46] that are available for use by the growing embryo. This means that larger starch granules produced greater sugar content in seeds for quick energy supply for the embryo to germinate, though the relationship was not strong in the current research (R = 0.456).…”