A Review of Starch Biosynthesis in Relation to the Building Block-Backbone Model

Abstract

Starch is a water-insoluble polymer of glucose synthesized as discrete granules inside the stroma of plastids in plant cells. Starch reserves provide a source of carbohydrate for immediate growth and development, and act as long term carbon stores in endosperms and seed tissues for growth of the next generation, making starch of huge agricultural importance. The starch granule has a highly complex hierarchical structure arising from the combined actions of a large array of enzymes as well as physicochemical self-assembly mechanisms. Understanding the precise nature of granule architecture, and how both biological and abiotic factors determine this structure is of both fundamental and practical importance. This review outlines current knowledge of granule architecture and the starch biosynthesis pathway in relation to the building block-backbone model of starch structure. We highlight the gaps in our knowledge in relation to our understanding of the structure and synthesis of starch, and argue that the building block-backbone model takes accurate account of both structural and biochemical data.

Starch Granules in Arabidopsis thaliana Mesophyll and Guard Cells Show Similar Morphology but Differences in Size and Number

“…Previous studies showed that the biochemical processes in mesophyll cells change with leaf age. Thus, the double-mutant dpe2/phs1 displayed an uneven distribution of starch, and the detectable starch was restricted to young leaves [ 3 , 10 ]. Most of the mesophyll cell chloroplasts in young leaves had only a single and nearly perfect spherically shaped starch granule, whereas, in mature leaves, there were no detectable starch granules, and the chloroplasts were mostly disintegrated [ 3 ].…”

“…Transitory starch is a major storer of carbon and energy, and it allows plants to overcome dark phases when photosynthesis is impossible. In recent years, huge progress has occurred toward understanding plastidial starch metabolism in the model plant Arabidopsis thaliana [ 1 , 2 , 3 ]. Generally, starch metabolism consists of two processes: synthesis and degradation.…”

Starch granule initiation in Arabidopsis thaliana chloroplasts

“…In plants, starch appears in the form of granules in the plastids (mainly in chloroplasts of photosynthetic tissues or amyloplasts of storage organs such as tubers, endosperm, or roots) (Ball and Morell, 2003). The size, number, and morphology of these granules vary greatly depending on the species and even the organ analyzed (Tetlow and Bertoft, 2020). Starch granules are composed of two different polymers: amylose and amylopectin.…”

“…Amylopectin is the main component of starch, and has a large number of branches ordered in clusters that confer a semi‐crystalline structure to the granule. In contrast, amylose, the minor component, has a small number of branches and no known structure (Tetlow and Bertoft, 2020). The enzymatic activities involved in the formation of starch are relatively simple: ADP‐glucose pyrophosphorylase (AGPase, EC 2.7.7.27) provides ADP‐glucose, which is one substrate used by starch synthases (SSs, ADP‐Glc:α‐1,4‐glucan α‐4‐glucosyl transferases, EC 2.4.1.21) to elongate the glucan polymer chains.…”

Posttranslational Modification of Waxy to Genetically Improve Starch Quality in Rice Grain

“…During the development of grains, cereals undergo various PTMs of proteins [ 148 ]. Many of the class of enzymes involved in biosynthesis of starch are regulated and coordinated and in a range of ways from gene expression to different posttranslational mechanisms including protein phosphorylation and redox modulation [ 149 ]. Meng et al [ 147 ] also observed that the enzymes involved in starch biosynthesis and regulatory pathways, including starch branching enzymes (OsBEI and OsBEIIb), ADP-glucose pyrophosphorylases (OsAGPS2 and OsAGPL2), sucrose synthase (SUS2 and SUS3), starch debranching enzymes (OsPUL), UDP-glucose pyrophosphorylase (UGP), phosphoglycerate mutase (PGM), and starch phosphorylase (OsPHOL), in developing rice seeds undergo succinylation.…”