Plants, green algae, and cyanobacteria synthesize storage polysaccharides by a similar ADPglucose-based pathway. Plant starch metabolism can be distinguished from that of bacterial glycogen by the presence of multiple forms of enzyme activities for each step of the pathway. This multiplicity does not coincide with any functional redundancy, as each form has seemingly acquired a distinctive and conserved role in starch metabolism. Comparisons of phenotypes generated by debranching enzyme-defective mutants in Escherichia coli and plants suggest that enzymes previously thought to be involved in polysaccharide degradation have been recruited during evolution to serve a particular purpose in starch biosynthesis. Speculations have been made that link this recruitment to the appearance of semicrystalline starch in photosynthetic eukaryotes. Besides the common core pathway, other enzymes of malto-oligosaccharide metabolism are required for normal starch metabolism. However, according to the genetic and physiological system under study, these enzymes may have acquired distinctive roles.

From bacterial glycogen to starch: understanding the biogenesis of the plant starch granule.

Effect of starch granule structure, associated components and processing on nutritive value of cereal starch: A review

Plant starch granules provide the largest percentage of calories in the human diet. Starch consists almost entirely of the Glc homopolymers amylopectin and amylose. Amylopectin is the major contributor to both mass and granule structure. Because of the very basic role that starch plays in our

Recent progress toward understanding biosynthesis of the amylopectin crystal.

Comprehensive reverse genetic resources, which have been key to understanding gene function in diploid model organisms, are missing in many polyploid crops. Young polyploid species such as wheat, which was domesticated less than 10,000 y ago, have high levels of sequence identity among subgenomes that mask the effects of recessive alleles. Such redundancy reduces the probability of selection of favorable mutations during natural or human selection, but also allows wheat to tolerate high densities of induced mutations. Here we exploited this property to sequence and catalog more than 10 million mutations in the protein-coding regions of 2,735 mutant lines of tetraploid and hexaploid wheat. We detected, on average, 2,705 and 5,351 mutations per tetraploid and hexaploid line, respectively, which resulted in 35-40 mutations per kb in each population. With these mutation densities, we identified an average of 23-24 missense and truncation alleles per gene, with at least one truncation or deleterious missense mutation in more than 90% of the captured wheat genes per population. This public collection of mutant seed stocks and sequence data enables rapid identification of mutations in the different copies of the wheat genes, which can be combined to uncover previously hidden variation. Polyploidy is a central phenomenon in plant evolution, and many crop species have undergone recent genome duplication events. Therefore, the general strategy and methods developed herein can benefit other polyploid crops.

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Uncovering hidden variation in polyploid wheat.

The present investigation revealed that the alk and gel(t) genes, which cause the differences between a japonica rice variety Nipponbare and an indica rice variety Kasalath in terms of the disintegration of endosperm starch granules in alkali solution and their gelatinisation in a 4 M urea solution, respectively, cosegregated in backcross inbred lines derived from a cross between the two varieties. The segregation pattern of the profile for amylopectin chain-length, which was distinguished by enrichment in short chains of DP≦11 and depletion in intermediate-size chains of 12≦DP≦24 in japonica as compared with indica, was exactly the same as those of the above physico-chemical properties of starch granules, and the gene was designated as acl(t). Gene-mapping analysis showed that the starch synthase IIa (SSIIa) gene is located at the alk locus on chromosome 6 in the rice genome. These results lead us to the possibility that different alleles of the SSIIa gene are responsible for differences in amylopectin structure between the two varieties, in that SSIIa plays a distinct role in the elongation of short chains within clusters (A+B1 chains) of amylopectin. It is proposed that the activity of SSIIa in japonica rice is reduced in amount or functional capacity relative to the activity of this enzyme in indica rice. This, in turn, would explain why starch from japonica rice has a lower gelatinisation temperature than starch from indica rice and is more susceptible to disintegration in alkali or urea. The evidence for this hypothesis is that the alk(t), gel(t), acl(t) and SSIIa genes all map to the same locus.

Mapping of a gene responsible for the difference in amylopectin structure between japonica-type and indica-type rice varieties.

The starch granules of hexaploid wheat (Triticum aestivum) contain a group of three proteins known as SGP-1 (starch granule protein-1) proteins, which have apparent molecular masses of 100, 108, and 115 kD. The nature and role of these proteins has not been defined previously. We demonstrate that these polypeptides are starch synthases that are present in both the starch granule and the soluble fraction at the early stages of wheat endosperm development, but that are exclusively granule bound at mid and late endosperm development. A partial cDNA clone encoding a fragment of the 100-kD protein was obtained by screening a wheat endosperm cDNA expression library using monoclonal antibodies. Three classes of cDNA were subsequently isolated from a wheat endosperm cDNA library by nucleic acid hybridization and were shown to encode the 100-, 108-, and 115-kD proteins. The cDNA sequences are highly homologous to class II starch synthases and have the highest homology with the maize SSIIa (starch synthase IIa) gene. mRNA for the SGP-1 proteins was detected in the leaf, pre-anthesis florets, and endosperm of wheat and is highly expressed in the leaf and in the grain during the early to mid stages of development. We discuss the roles of the SGP-1 proteins in starch biosynthesis in wheat.

The localization and expression of the class II starch synthases of wheat

Three genes encoding granule-bound starch synthase (wx-TmA, wx-TsB, and wx-TtD) have been isolated from Triticum monococcum (AA), and Triticum speltoides (BB), by the polymerase chain reaction (PCR) approach, and from Triticum tauschii (DD), by screening a genomic DNA library. Multiple sequence alignment indicated that the wx-TmA, wx-TsB, and wx-TtD genes had the same extron and (or) intron structure as the previously reported waxy gene from barley. The lengths of the three wx-TmA, wx-TsB, and wx-TtD genes were 2834 bp, 2826 bp, and 2893 bp, respectively, each covering 31 bp in the untranslated leader and the entire coding region consisting of 11 exons and 10 introns. The three genes had identical lengths of exons, except exon1, and shared over 95% identity with each other within the exon regions. The majority of introns were significantly variable in length and sequence, differing mainly in length (1-57 bp) as a result of insertion and (or) deletion events. The deduced amino acid sequence from these thre...

The genes encoding granule-bound starch synthases at the waxy loci of the A, B, and D progenitors of common wheat.

The granule-bound starch (GBSS I, waxy protein) in Triticum timopheevii (AtAtGG) and T. zhukovskyi (AtAtAzAzGG) and a diagnostic section of the genes encoding GBSS-I from the Wx-TtA and Wx-G loci of T. timopheevii and the Wx-TtA, Wx-G, and Wx-TzA loci of T. zhukovskyi were investigated in this study. The waxy proteins in these two polyploid wheats could not be separated into distinct bands, in contrast to those in the T. turgidum (AABB)  T. aestivum (AABBDD) lineage. Alignment of sequences of the section covering exon4intron4exon5 of the various waxy genes led to the identification of gene-specific sequences in intron 4. The sequences specific to the Wx-TtA and Wx-G genes of T. timopheevii were different from those of the Wx-A1 gene and Wx-B1 genes of T. turgidum and T. aestivum. A surprising observation was that the Wx-TzA of T. zhukovskyi did not match with the Wx-TmA of T. monococcum, a putative donor of the Az genome, but matched unexpectedly and perfectly with the Wx-B1 gene on chromosome 4A, whic...

Liuling Yan

Papers

The localization and expression of the class II starch synthases of wheat

The genes encoding granule-bound starch synthases at the waxy loci of the A, B, and D progenitors of common wheat.

Characterization of waxy proteins and waxy genes of Triticum timopheevii and T. zhukovskyi and implications for evolution of wheat

RESEARCH NOTE A Novel Starch Granule-bound Protein in Endosperm of Wheat