The emphasis of this review is on starch structure and its biosynthesis. Improvements in understanding have been brought about during the last decade through the development of new physicochemical and biological techniques, leading to real scientific progress. All this literature needs to be kept inside the general literature about biopolymers, despite some confusing results or discrepancies arising from the biological variability of starch. However, a coherent picture of starch over all the different structural levels can be presented, in order to obtain some generalizations about its structure. In this review we will focus first on our present understanding of the structures of amylose and amylopectin and their organization within the granule, and we will then give insights on the biosynthetic mechanisms explaining the biogenesis of starch in plants.
far the major compound. It is of very high molecular Unite ´Mixte de Recherche NЊ111 du Centre National weight (10 7 -10 9 Da) and harbors 5% of ␣-1,6 branches de la Recherche Scientifique (reviewed by Manners, 1989). Amylose is often referred Universite ´des Sciences et Technologies de Lille to as a smaller linear molecule (molecular weight of 10 5 -59655 Villeneuve D'Ascq Cedex 10 6 Da) with very few ␣-1,6 branches (less than 1%). Its France association with amylopectin inside the granule remains † Exseed Genetics to be determined. Amylopectin is sufficient to generate 1568 Food Science Building full size granules both in wild-type starch from photosyn-Iowa State University thesizing cells and in mutant starches devoid of amy-Ames, Iowa 50011-1061 lose. No mutants lacking selectively amylopectin have ‡ Department of Biochemistry and Biophysics ever been described in plants suggesting that an under-Iowa State University standing of amylopectin biosynthesis will be sufficient to Ames, Iowa 50011 explain the major features of starch granule biogenesis.
Three dimensional models of crystalline zones and amorphous branching zones of starch granules are reviewed. In crystallites of both A and B starch, double helices are found in pairs, and all chains are packed in parallel arrays. The pairing of double helices is identical in both polymorphs and corresponds to the interaction between double helices that has the lowest energy. The differences between A and B starch arise from water content and the manner in which these pairs are packed in the respective crystals. A transition from B starch to the A form can be accomplished by rearrangement of the pairs of double helices. The 1–6 linked amylopectin branch points occur in amorphous regions, but actually promote the formation of ordered double helices.
Analysis of barley shrunken grain mutants has identified lines with a novel high amylose starch phenotype. The causal mutation is located at the sex6 locus on chromosome 7H, suggesting the starch synthase IIa (ssIIa) gene as a candidate gene altered by the mutation. Consistent with this hypothesis, no evidence of SSIIa protein expression in either the starch granule or soluble fractions of the endosperm was found. Sequences of the starch synthase IIa gene, ssIIa, from three independent sex6 lines showed the presence of a stop codon preventing translation of the ssIIa transcript in each line. Perfect segregation of the starch phenotype with the presence of stop codons in the ssIIa gene was obtained, providing strong evidence for the lesion in the ssIIa gene being the causal mutation for the sex6 phenotype. The loss of SSIIa activity in barley leads to novel and informative phenotypes. First, a decrease in amylopectin synthesis to less than 20% of the wild-type levels indicates that SSIIa accounts for the majority of the amylopectin polymer elongation activity in barley. Secondly, in contrast to high amylose starches resulting from branching enzyme downregulation, the sex6 starches have a shortened amylopectin chain length distribution and a reduced gelatinisation temperature. Thirdly, the mutation leads to pleiotropic effects on other enzymes of the starch biosynthesis pathway, abolishing the binding of SSI, branching enzyme IIa and branching enzyme IIb to the starch granules of sex6 mutants, while not significantly altering their expression levels in the soluble fraction.
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