Starch, as the major nutritional component of our staple crops and a feedstock for industry, is a vital plant product. It is composed of glucose polymers that form massive semi-crystalline granules. Its precise structure and composition determine its functionality and thus applications; however, there is no versatile model system allowing the relationships between the biosynthetic apparatus, glucan structure and properties to be explored. Here, we expressed the core Arabidopsis starch-biosynthesis pathway in Saccharomyces cerevisiae purged of its endogenous glycogen-metabolic enzymes. Systematic variation of the set of biosynthetic enzymes illustrated how each affects glucan structure and solubility. Expression of the complete set resulted in dense, insoluble granules with a starch-like semi-crystalline organization, demonstrating that this system indeed simulates starch biosynthesis. Thus, the yeast system has the potential to accelerate starch research and help create a holistic understanding of starch granule biosynthesis, providing a basis for the targeted biotechnological improvement of crops.DOI: http://dx.doi.org/10.7554/eLife.15552.001

The major component of starch is the branched glucan amylopectin, the branching pattern of which is one of the key factors determining its ability to form semicrystalline starch granules. Here, we investigated the functions of different branching enzyme (BE) types by expressing proteins from maize (Zea mays BE2a), potato (Solanum tuberosum BE1), and Escherichia coli (glycogen BE [EcGLGB]) in Arabidopsis (Arabidopsis thaliana) mutant plants that are deficient in their endogenous BEs and therefore, cannot make starch. The expression of each of these three BE types restored starch biosynthesis to differing degrees. Full complementation was achieved using the class II BE ZmBE2a, which is most similar to the two endogenous Arabidopsis isoforms. Expression of the class I BE from potato, StBE1, resulted in partial complementation and high amylose starch. Expression of the glycogen BE EcGLGB restored only minimal amounts of starch production, which had unusual chain length distribution, branch point distribution, and granule morphology. Nevertheless, each type of BE together with the starch synthases and debranching enyzmes were able to create crystallization-competent amylopectin polymers. These data add to the knowledge of how the properties of the BE influence the final composition of starch and fine structure of amylopectin.Starch is composed of two glucan polymers: amylopectin and amylose. Amylopectin constitutes around 80% of the mass of most starches and is a large, branched polymer with a tree-like architecture. The positioning and frequency of branch points together with the distribution of chain lengths are thought to be critical factors allowing amylopectin to adopt a semicrystalline state. Within amylopectin molecules, clusters of unbranched chain segments align, and adjacent chains form double helices. These pack into crystalline lamellae that alternate with amorphous regions containing the branch points. Longer chain segments span from one cluster to the next .Amylopectin is synthesized by three enzyme activities. First, starch synthases (SSs) transfer the glucosyl part of ADP-Glc to the nonreducing end of existing glucan chains, forming new a-1,4 glucosidic bonds. Second, branching enzymes (BEs) cleave part of an a-1,4-linked chain and through an inter-or intramolecular transfer reaction, reattach it, creating a-1,6-branch points. This reaction creates additional nonreducing ends on which SSs can act. Third, debranching enzymes (DBEs) hydrolyze some of these branches, tailoring the structure of the polymer to promote its crystallization.

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