Formation of starch in plant cells

Abstract

Starch-rich crops form the basis of our nutrition, but plants have still to yield all their secrets as to how they make this vital substance. Great progress has been made by studying both crop and model systems, and we approach the point of knowing the enzymatic machinery responsible for creating the massive, insoluble starch granules found in plant tissues. Here, we summarize our current understanding of these biosynthetic enzymes, highlighting recent progress in elucidating their specific functions. Yet, in many ways we have only scratched the surface: much uncertainty remains about how these components function together and are controlled. We flag-up recent observations suggesting a significant degree of flexibility during the synthesis of starch and that previously unsuspected non-enzymatic proteins may have a role. We conclude that starch research is not yet a mature subject and that novel experimental and theoretical approaches will be important to advance the field.

Recreating the synthesis of starch granules in yeast

“…Amylopectin biosynthesis involves multiple enzyme activities, often composed of several isoforms with distinct specificities: four classes of starch synthases (SSs) elongate glucan chains using ADPglucose as glucosyl donor, creating α-1,4 glucose linkages; two classes of branching enzymes (BEs) introduce branches in the form of α-1,6 linked chains; and at least one isoamylase-type debranching enzyme hydrolyzes some branches again, probably tailoring the glucan for crystallization (Pfister and Zeeman, 2016; Tetlow and Emes, 2011; Zeeman et al, 2010; Jeon et al, 2010). In contrast, amylose is synthesized by a single class of granule-bound starch synthases (Denyer et al, 2001).…”

Starch Biosynthesis in the Developing Endosperms of Grasses and Cereals

“…Indeed, various mutations (including single nucleotide polymorphisms) in SSIIa are responsible for altered rice starch quality (cooking quality characteristics reflected in variations in gelatinization temperature (see [218])) and are the basis for the distinction between indica and japonica types [219][220][221][222][223][224]. Mutations of SSII in dicot storage tissues, such as pea (Pisum sativum L.) and potato (Solanum tuberosum L.), produce similar starch phenotypes as their counterparts in monocots (see [141]), suggesting that functional specificity of some SS isoforms was established prior to the divergence of the monocots and dicots. Both SSI and SSII are also localized within the starch granules [202,221,225], and have been shown to interact with other enzymes of starch biosynthesis, notably the SBEII class in cereal endosperm, as well as SSIII [226][227][228].…”

“…Details of the operation of these enzymes in storage starch synthesis in the endosperm amyloplasts are discussed below. For details on starch formation and turnover in chloroplasts of source tissues the reader is directed to more general recent reviews of the starch metabolic pathway in plants [141,142].…”

Transitory Starch Metabolism in Guard Cells: Unique Features for a Unique Function

“…It consists of two types of a-1,4-linked glucan polymers-amylose and amylopectin-that differ in chain length and frequency of a-1,6-branches. These polymers adopt complex secondary and tertiary structures that organize into insoluble, semicrystalline granules to store energy in a dense, osmotically inert form (Pfister and Zeeman, 2016).Starch is a vital substance for plants, both for shortand long-term storage of carbohydrates. In heterotrophic organs, such as potato tubers, cassava roots, cereal seed endosperm, and the stems of woody perennials, starch is synthesized in specialized amylopasts from imported sucrose (Suc) and stored over the seasons, or even for many years.…”

“…It consists of two types of a-1,4-linked glucan polymers-amylose and amylopectin-that differ in chain length and frequency of a-1,6-branches. These polymers adopt complex secondary and tertiary structures that organize into insoluble, semicrystalline granules to store energy in a dense, osmotically inert form (Pfister and Zeeman, 2016).…”