Plant starch synthesis

J. Preiss    Michigan State University, USA

1.1 Introduction: localization and function of starch in plants

This chapter reviews starch synthesis in higher plants and algae. Since the reactions leading to glycogen synthesis in the cyanobacteria are similar to those observed in the higher plants there will be some referral to studies in those organisms particularly in regulation of cyanobacterial α1,4-glucan synthesis. The enzymology and biochemistry of the various enzymes in the plant, algal and cyanobacterial systems will also be described. In view of the existing information available on the properties of the starch biosynthetic enzymes and the effects of certain mutants on starch structure a pathway of starch synthesis is described which postulates specific functions for the starch synthases and branching enzymes. Finally regulation of starch synthesis at the enzymatic level will be discussed and in relation to this regulation, recent results indicating how starch content has been increased in certain plants will be descibed. A previous chapter1 in the second edition of Starch Chemistry and Technology which reviewed starch biosynthesis discussed the various maize endosperm mutants or mutant combinations, 26 of them, that showed an effect on the quantity or the nature of the starch formed. This information remains of interest and the reader is referred to that review. However, for many of those mutants the biochemical basis for mutation effects on starch quantity or quality was unknown. This review will deal with only the mutants where the biochemical process affected by the mutation has been elucidated to at least some extent. There are some recent reviews on starch biosynthesis2-11 that discuss many of the areas presented in this chapter.

1.1.1 Leaf starch

Starch is deposited in granules in almost all green plants and in various types of plant tissues and organs, e.g., leaves, roots, shoots, fruits, grains, and stems. Illumination of the leaf in bright light causes the formation of starch granules in the chloroplast organelle and was demonstrated in the nineteenth century.12 Disappearance of the starch occurs either by exposure of the leaf to low light or by extended exposure in the dark (24-48 hours). This is readily observed by iodine staining of the tissue13 or by light or electron microscopy.14 Starch accumulates due to carbon fixation during photosynthesis and the starch formed in the light is degraded in the dark to products that are in most cases utilized for sucrose synthesis. Mutants of Arabidopsis thaliana unable to synthesize starch, grow at the same rate as the wild type in a continuous light regime because they are able to synthesize sucrose,15 but their growth rate is drastically reduced if grown in a day-night regime. The reason for this is that the accumulated starch is required for sucrose synthesis at night; the sucrose is transported from the leaf to the sink tissues. Biosynthesis and degradation of starch in the leaf is therefore a dynamic process having diurnal fluctuations in its stored levels.

Starch also plays an important role in the operation of stomatal guard cells, where it is degraded during the day. In the late afternoon or evening while the stomata are open, the starch is resynthesized. Leaf starch is lower in amylose content than what is observed in storage tissues.16 The amylose structure is also of a smaller molecular size.

1.1.2 Starch in storage tissues

In storage organs, fruit or seed, during the development and maturation of the tissue, synthesis of starch occurs. At the time of sprouting or germination of the seed or tuber, or ripening of the fruit, starch degradation in these tissues then occurs and the derived metabolites are used as a source both for carbon and energy. The degradative and biosynthetic processes in the storage tissues may therefore be temporally separated. However, there is some possibility that during each phase of starch metabolism some turnover of the starch molecule occurs.

The main site of starch synthesis and accumulation in the cereals is the endosperm, with starch granules that are located within the amyloplasts. Starch content in potato tuber, maize endosperm, and in roots of yam, cassava and sweet potato ranges between 65 and 90% of the total dry matter. Patterns of starch accumulation during development of the tissue are specific to the species and are related to the unique pattern of differentiation of the organ.

Starch granules in storage tissues can vary in shape, size and composition. The shape and size of the granules depends on the source, but in each tissue there is a range of sizes and shapes. The diameter of the starch granule changes during the development of the reserve tissue. There are also some fine features, characteristic of each species, e.g., the ‘growth rings’, spaced 4–7 μm apart, and the fibrillar organization seen in potato starch, which allows one to identify the botanical source of the starch by microscopic examination.

Two polymers are distinguished in the starch granule. Amylose, which is essentially linear, and amylopectin, highly branched. Amylose is mainly found as linear chains of about 840 to 22,000 units of α-D-glucopyranosyl residues linked by α-(1- > 4) bonds (molecular weight around 136,000 to 3.5 × 106). The number of anhydroglucose units, however, varies quite widely with plant species and stage of development. Some of the amylose molecules are branched to a small extent (α-l- > 6-D glucopyranose; one per 170 to 500 glucosyl units). Amylopectin, in contrast, which usually comprises about 70% of the starch granule, is more highly branched with about 4 to 5% of the glucosidic linkages being α-1- > 6.

Amylopectin molecules are large flattened disks consisting of α-(1,4)-glucan chains joined by frequent α-(1,6)-branch points. Many models of amylopectin structure have been proposed but from these the most satisfactory models, i.e., those that best fit the experimental data available, are those proposed by Robin et al.17, Manners and Matheson18 and by Hizukuri.19 These are known as cluster models. The chemical and physical aspects of the starch granule and its components amylose and amylopectin have been discussed in some recent excellent reviews by Morrison and Karkalis20 and Hizukuri.21

1.2 Starch synthesis: enzyme reactions in plants and algae and glycogen synthesis in cyanobacteria

1.2.1 Enzyme reactions of starch synthesis

The sugar nucleotide utilized for synthesis of the α-1,4 glucosidic linkages in amylose and amylopectin is ADP-glucose and not UDP-glucose. ADP-glucose synthesis is catalyzed by ADP-glucose (synthetase) pyrophosphorylase (reaction 1, E.C. 2.7.7.27; ATP:α-D-glucose-l-phosphate adenylyltransferase).

TP+α‐glucose‐1‐P⇔ADP‐glucose+PPi

DP‐glucose+α‐1,4glucan⇒α−1,4−glucosyl‐α‐1,4glucan+ADP

α‐1,4‐oligosaccharidechain⇒α‐1,4‐α‐1,6branched‐glucanpro‐amylopectinphytoglyvogen

Reaction 2 is catalyzed by starch synthase (E.C. 2.4.1.21; ADP-glucose;1,4-α-D-glucan 4-α-glucosyltransferase). A similar reaction is noted for glycogen synthesis in cyanobacteria and other bacteria (see references 22 and 23 but the reaction is referred to as glycogen synthase (also E.C. 2.4.1.21). Reaction 3 is catalyzed by branching enzyme (E.C. 2.4.1.18; 1,4-α-D-glucan 6-α-(l,4-α-glucano)-transferase). The branch chains in amylopectin are longer (about 20 to 24 glucose units long) and there is less branching in amylopectin (~ 5% of the glucosidic linkages are α-1,6 as seen in glycogen (10-13 glucose units long and 10% of linkages are α-1,6). Thus, the starch branching enzymes may have different properties with respect to size of chain transferred, or placement of branch point, than enzyme that branches glycogen. Alternatively, the interaction of the starch branching enzymes with the starch synthases may be different from the interaction of the bacterial branching enzymes with their respective glycogen synthases. The chain elongating properties of the starch synthases could be different from those observed for the bacterial glycogen synthases and may account for some of the differences observed in the amylopectin structure. The differences in the catalytic properties of the starch synthases and branching enzymes isolated from different plant sources may also account for the differences observed in the various plant starch structures.

Isozymic forms of plant starch synthases (cited in references, 3, 4, 24-28) and branching enzymes (cited in references 3, 4, 24, and in recent literature, 29-34) have been reported. They seem to play different roles in the synthesis of the two polymers of starch, amylose and amylopectin and are products from different genes. In many...