Starch synthases (SS) are responsible for elongating the alpha-1,4 glucan chains of starch. A doubled haploid population was generated by crossing a line of wheat, which lacks functional ssIIa genes on each genome (abd), and an Australian wheat cultivar, Sunco, with wild type ssIIa alleles on each genome (ABD). Evidence has been presented previously indicating that the SGP-1 (starch granule protein-1) proteins present in the starch granule in wheat are products of the ssIIa genes. Analysis of 100 progeny lines demonstrated co-segregation of the ssIIa alleles from the three genomes with the SGP-1 proteins, providing further evidence that the SGP-1 proteins are the products of the ssIIa genes. From the progeny lines, 40 doubled haploid lines representing the eight possible genotypes for SSIIa (ABD, aBD, AbD, ABd, abD, aBd, Abd, abd) were characterized for their grain weight, protein content, total starch content and starch properties. For some properties (chain length distribution, pasting properties, swelling power, and gelatinization properties), a progressive change was observed across the four classes of genotypes (wild type, single nulls, double nulls and triple nulls). However, for other grain properties (seed weight and protein content) and starch properties (total starch content, granule morphology and crystallinity, granule size distribution, amylose content, amylose-lipid dissociation properties), a statistically significant change only occurred for the triple nulls, indicating that all three genes had to be missing or inactive for a change to occur. These results illustrate the importance of SSIIa in controlling grain and starch properties and the importance of amylopectin fine structure in controlling starch granule properties in wheat.

To examine the role of isoamylase1 (ISA1) in amylopectin biosynthesis in plants, a genomic DNA fragment from Aegilops tauschii was introduced into the ISA1-deficient rice (Oryza sativa) sugary-1 mutant line EM914, in which endosperm starch is completely replaced by phytoglycogen. A. tauschii is the D genome donor of wheat (Triticum aestivum), and the introduced fragment effectively included the gene for ISA1 for wheat (TaISA1) that was encoded on the D genome. In TaISA1-expressing rice endosperm, phytoglycogen synthesis was substantially replaced by starch synthesis, leaving only residual levels of phytoglycogen. The levels of residual phytoglycogen present were inversely proportional to the expression level of the TaISA1 protein, although the level of pullulanase that had been reduced in EM914 was restored to the same level as that in the wild type. Small but significant differences were found in the amylopectin chain-length distribution, gelatinization temperatures, and A-type x-ray diffraction patterns of the starches from lines expressing TaISA1 when compared with wild-type rice starch, although in the first two parameters, the effect was proportional to the expression level of TaISA. The impact of expression levels of ISA1 on starch structure and properties provides support for the view that ISA1 is directly involved in the synthesis of amylopectin.Amylopectin is generally the major constituent of starch, accounting for about 65% to 85% of storage starch. The remainder is amylose, which is essentially linear. Amylopectin has a defined structure composed of tandem linked clusters (approximately 9-10 nm each in length), where linear a-1,4-glucan chains are regularly branched via a-1,6-glucosidic linkages, whereas the glycogens of bacteria and animals have a more randomly branched structure (Thompson, 2000). The distinct structure of amylopectin (referred to as a tandem-cluster structure) contributes to the crystalline organization of the starch granule (Gallant et al., 1997). Variation of the cluster fine structure is considered to cause variations in starch functional properties between species (e.g. maize [Zea mays] starch versus potato [Solanum tuberosum] starch), tissues (e.g. storage starch versus assimilatory starch), and genetic backgrounds (e.g. japonica rice [Oryza sativa] starch versus indica rice starch). However, genetic engineering could remove such species-specific limitations of starch functional properties by modifying the fine structure of amylopectin in a variety of ways.According to our current understanding, the structure of amylopectin is determined by four classes of enzymes: ADP-Glc pyrophosphorylase (AGPase), soluble starch synthase (SS), starch-branching enzyme (BE), and starch-debranching enzyme (DBE; Van den Koornhuyse et al., 1996;Smith et al., 1997;Kossmann and Lloyd, 2000;Myers et al., 2000;Nakamura, 2002;Ball and Morell, 2003;James et al., 2003). A current focus of research in starch biosynthesis is to evaluate the metabolic functions of individual enzymes involved in amylopecti...

The role of starch branching enzyme-I (SBE-I) in determining starch structure in the endosperm has been investigated. Null mutations of SBE-I at the A, B and D genomes of wheat were identified in Australian wheat varieties by immunoblotting. By combining individual null mutations at the B and D genomes through hybridisation, a double-null mutant wheat, which lacks the B and D isoforms of SBE-I, was developed. Wheat mutants lacking all the three isoforms of SBE-I were generated from a doubled haploid progeny of a cross between the BD double-null mutant line and a Chinese Spring (CS) deletion line lacking the A genome isoform. Comparison of starch from this mutant wheat to that from wild type revealed no substantial alteration in any of the structural or functional properties analysed. Further analysis of this triple-null mutant line revealed the presence of another residual peak of SBE-I activity, referred to as SBE-Ir, in wheat endosperm representing < 3% of the activity of SBE-I in wild type endosperm.

SummaryManipulation of SSI expression in wheat using RNAi technology leads to the production of wheat grains with novel starch structure and properties.

This paper presents a small-scale test for measuring starch quality and demonstrates its ability to predict end-product quality. This 40 mg swelling test measures the swelling power of gelatinised starch by determining the mass of the final product after a defined gelatinisation process. It is compared with the larger scale swelling power and swelling volume tests and with other starch gel analyses methods (differential scanning calorimetry, hot stage microscopy and Rapid Visco Analyser). Suitability of the 40 mg swelling test as a small-scale early generation breeding test for measuring starch quality is evaluated. This new method is also evaluated as being suitable to provide an indication of the end-product quality of Japanese white salted noodles.

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The increased incidence in many countries in lifestyle diseases such as colorectal cancer, cardiovascular disease, and diabetes has led to an enhanced interest in disease-prevention measures that can be delivered to target populations through diet. Resistant starch (RS) is emerging as an important dietary component that has the potential to reduce the incidence of bowel health disorders. However, the range of crop species that can serve as effective sources of RS is limited. In this paper the state of knowledge of the starch biosynthesis pathway is reviewed and opportunities to manipulate crop genetics in order to generate additional sources of RS are discussed. The need for a “whole of chain” approach to delivery of RS to the consumer is highlighted because of the impact that different food-processing technologies can have in maintaining, enhancing, or destroying the RS potential of a raw material or food.

Flours from engineered wheat grain overexpressing a single a-amylase type 3 (TaAMY3) were compared to flour from germinated grains to study the impact of elevated levels of a-amylase on small-scale noodle quality. The results showed that a-amylase alone significantly reduced cooking time and increased cooking loss but had no detrimental effect on flour color, noodle color, or firmness. Noodles produced from sprouted grains were significantly detrimentally affected for all the above quality characteristics, in particularly after 24 and 48 h imbibition. A strong correlation was observed between the decrease in unextractable polymeric protein (UPP) levels, and noodle firmness. These findings raise important questions as to the supposition that a-amylase is the source of the adverse effects of pre-harvest sprouting (PHS) and late maturity ]a-amylase (LMA) on Asian white-salted noodles and highlights the usefulness of applying biotechnology in elucidating how flour components affect end product quality.