Starch Biosynthesis and Degradation in Plants


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.

Keywords: amylase; maltose; starch‐branching enzyme; starch granule; starch synthase

Figure 1. The structure of the starch polymers and the starch granule. Top left: representations of the structures of amylose and amylopectin. The chains in the amylose molecule are 1000 or more glucosyl residues in length. The short chains within the clusters of the amylopectin molecule are typically 12–20 glucosyl residues in length. Top right: adjacent chains within the clusters of the amylopectin molecule form double helices, and these associate together to form crystalline lamellae. The regions between the clusters that contain the branch points do not crystallise, giving rise to alternating crystalline and amorphous lamellae with a periodicity of 9 nm. The layers of the sandwich are parallel with the surface of the granule; in other words, the lamellae form concentric shells within the granule matrix. Bottom: scanning electron micrographs of the inner face of a starch granule from a potato tuber, cracked open and treated with a starch‐degrading enzyme to reveal the growth rings. Each ring consists of tens of the 9 nm repeats shown above. The bar represents 5 µm; the picture on the right is a closer image of part of the picture on the left.
Figure 2. The actions of ADPglucose pyrophosphorylase, starch synthase and starch‐branching enzyme. Starch synthase catalyses the addition of the glucosyl moiety of ADPglucose on to the nonreducing end of a chain via an α1,4 linkage. Starch‐branching enzyme cleaves sections of chains from the nonreducing end and adds them to the side of the same or an adjacent chain via an α1,6 linkage.
Figure 3. The pathway of starch degradation in the endosperm of a germinating cereal seed. The starch granule is attacked by the endoamylase α‐amylase, which releases soluble linear and branched glucans. These are acted on by the debranching enzyme limit dextrinase and the exoamylase β‐amylase to produce maltose. Maltose is then hydrolysed to glucose by an α‐glucosidase (maltase). The glucose is taken up into the growing embryo.
Figure 4. The pathway of starch degradation in an leaf at night. Leaf starch degradation is initiated by phosphorylation of amylopectin via glucan, water dikinases (GWD and PWD). Dephosphorylation of the phosphoglucans by glucan phosphatases (SEX4 and LSF2) presumably occurs concurrently. Debranching of the starch polymers at the granule surface is mainly via isoamylase 3, and linear glucans are metabolised via β‐amylase to yield maltose as the main product, with maltotriose as a more minor product. Possibly, α‐amylase can release branched glucans from the granule surface, which are then debranched via the debranching enzymes isoamylase 3 and limit dextrinase. However, this is at most a minor pathway as indicated by the dashed arrows. Maltose is exported from the chloroplast to the cytosol via the maltose transporter MEX1, and then metabolised via the cytosolic disproportionating enzyme (D‐enzyme; DPE2). DPE2 releases one of the glucosyl moieties of maltose as free glucose and transfers the other to a cytosolic heteroglycan, from which it is released via glucan phosphorylase as hexose phosphate. The maltotriose product of β‐amylase is converted via chloroplastic D‐enzyme (DPE1) to maltopentaose and free glucose. The maltopentaose is a substrate for the further action of β‐amylase, and the glucose is assumed to be transported to the cytosol via a glucose transporter. For convenience, maltotriose and maltopentaose in this figure are represented under the generic term ‘linear glucans’. Hexose phosphates produced in the cytosol from free glucose and the deglucosylation of the heteroglycan are converted to sucrose for export to the nonphotosynthetic parts of the plant.


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Further Reading

Arias MC, Pelletier S, Hilliou F, et al. (2014) From dusk till dawn: the Arabidopsis thaliana sugar starving responsive network. Frontiers in Plant Science 5: 482.

Ballicora MA, Iglesias AA and Preiss J (2004) ADP‐glucose pyrophosphorylase: a regulatory enzyme for plant starch synthesis. Photosynthesis Research 79: 1–24.

Blennow A and Engelsen SB (2010) Helix‐breaking news: fighting crystalline starch energy deposits in the cell. Trends in Plant Science 15: 236–240.

Comparot‐Moss S and Denyer K (2009) The evolution of the starch biosynthetic pathway in cereals and other grasses. Journal of Experimental Botany 60: 2481–2492.

Kötting O, Kossmann J, Zeeman SC, et al. (2010) Regulation of starch metabolism: the age of enlightenment? Current Opinion in Plant Biology 13: 321–329.

Li C and Gilbert RG (2016) Progress in controlling starch‐structure by modifying starch‐branching enzymes. Planta 243: 13–22.

Silver D, Kötting O and Moorhead GBG (2014) Phosphoglucan phosphatase function sheds light on starch degradation. Trends in Plant Science 19: 471–478.

Stitt M and Zeeman SC (2012) Starch turnover: pathways, regulation and role in growth. Current Opinion in Plant Biology 15: 282–292.

Tuncel A and Okita TW (2013) Improving starch yield in cereals by over‐expression of ADPglucose pyrophosphorylase: expectations and unanticipated outcomes. Plant Science 211: 52–60.

Zeeman SC, Kossmann J and Smith AM (2010) Starch: its metabolism, evolution, and biotechnological modification in plants. Annual Review of Plant Biology 61: 209–234.

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Lloyd, James R, and Kötting, Oliver(Jul 2016) Starch Biosynthesis and Degradation in Plants. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0020124.pub2]