Cereal Chem. 76(5):629-637Structures and properties of starches isolated from different botanical sources were investigated. Apparent and absolute amylose contents of starches were determined by measuring the iodine affinity of defatted whole starch and of fractionated and purified amylopectin. Branch chainlength distributions of amylopectins were analyzed quantitatively using a high-performance anion-exchange chromatography system equipped with a postcolumn enzyme reactor and a pulsed amperometric detector. Thermal and pasting properties were measured using differential scanning calorimetry and a rapid viscoanalyzer, respectively. Absolute amylose contents of most of the starches studied were lower than their apparent amylose contents. This difference correlated with the number of very long branch chains of amylopectin. Studies of amylopectin structures showed that each starch had a distinct branch chain-length distribution profile. Average degrees of polymerization (dp) of amylopectin branch chain length ranged from 18.8 for waxy rice to 30.7 for high-amylose maize VII. Compared with X-ray A-type starches, B-type starches had longer chains. A shoulder of dp 18-21 (chain length of 6.3-7.4 nm) was found in many starches; the chain length of 6.3-7.4 nm was in the proximity of the length of the amylopectin crystalline region. Starches with short average amylopectin branch chain lengths (e.g., waxy rice and sweet rice starch), with large proportions of short branch chains (dp 11-16) relative to the shoulder of dp 18-21 (e.g., wheat and barley starch), and with high starch phosphate monoester content (e.g., potato starch) displayed low gelatinization temperatures. Amylose contents and amylopectin branch chain-length distributions predominantly affected the pasting properties of starch. MATERIALS AND METHODS MaterialsChinese taro, mung bean, waxy rice, sweet rice, green leaf canna, lotus root, water chestnut, and cattail millet starches were isolated in the laboratory as reported by Lim et al (1994). Waxy amaranth I was isolated by the alkaline method (Myers and Fox 1994). Waxy amaranth II and normal glacier barley starch were isolated by a combination of mild alkaline and protease treatments (Radosavljevic et al 1998). Green banana starch was provided by A. R. Bonilla, University of Costa Rica. Waxy maize, amyloseextender (ae) waxy and dull (du) waxy maize starches were gifts of Cerestar, USA (Hammond, IN); potato and wheat starches were purchased from Sigma Chemical Co. (St. Louis, MO);

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;This is the first report on regulation of the isoamylase1 gene to modify the structure of amylopectin and properties of starch by using antisense technology in plants. The reduction of isoamylase1 protein by about 94% in rice endosperm changed amylopectin into a water-insoluble modified amylopectin and a water-soluble polyglucan (WSP). As compared with wild-type amylopectin, the modified amylopectin had more short chains with a degree of polymerization of 5-12, while their molecular sizes were similar. The WSP, which structurally resembled the phytoglycogen in isoamylase-deficient sugary-1 mutants, accounted for about 16% of the total a-polyglucans in antisense endosperm, and it was distributed throughout the whole endosperm unlike in sugary-1 mutant. The reduction of isoamylase activity markedly lowered the gelatinization temperature from 54 to 43°C and the viscosity, and modified X-ray diffraction pattern and the granule morphology of the starch. The activity of pullulanase, the other type of starch debranching enzyme, in the antisense endosperm was similar to that in wild-type, whereas it is deficient in sugary-1 mutants. These results indicate that the isoamylase1 is essential for amylopectin biosynthesis in rice endosperm, and that alteration of the isoamylase activity is an effective means to modify the physicochemical properties and granular structure of starch.

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Function of the maize (Zea mays) gene sugary1 (su1) is required for normal starch biosynthesis in endosperm. Homozygous su1-mutant endosperms accumulate a highly branched polysaccharide, phytoglycogen, at the expense of the normal branched component of starch, amylopectin. These data suggest that both branched polysaccharides share a common precursor, and that the product of the su1 gene, designated SU1, participates in kernel starch biosynthesis. SU1 is similar in sequence to ␣-(136) glucan hydrolases (starch-debranching enzymes [DBEs]). Specific antibodies were produced and used to demonstrate that SU1 is a 79-kD protein that accumulates in endosperm coincident with the time of starch biosynthesis. Nearly full-length SU1 was expressed in Escherichia coli and purified to apparent homogeneity. Two biochemical assays confirmed that SU1 hydrolyzes ␣-(136) linkages in branched polysaccharides. Determination of the specific activity of SU1 toward various substrates enabled its classification as an isoamylase. Previous studies had shown, however, that su1-mutant endosperms are deficient in a different type of DBE, a pullulanase (or R enzyme). Immunoblot analyses revealed that both SU1 and a protein detected by antibodies specific for the rice (Oryza sativa) R enzyme are missing from su1-mutant kernels. These data support the hypothesis that DBEs are directly involved in starch biosynthesis.Starch is a reserve carbohydrate that accumulates in the storage organs of many higher plants. This storage compound consists of a mixture of two Glc homopolymers, amylopectin and amylose, in which linear chains are formed via ␣-(13 4) glucosyl linkages and branches are introduced by ␣-(13 6) glucosyl linkages. Starch synthesis in maize (Zea mays) occurs within the amyloplasts of endosperm cells during kernel development via the concerted actions of ADP-Glc pyrophosphorylase, starch synthases, and starch-branching enzymes (for reviews, see Preiss, 1991;Hannah et al., 1993;Martin and Smith, 1995;Nelson and Pan, 1995; Preiss and Sivak, 1996; Smith et al., 1996). In addition, selective removal of branch linkages by DBEs is proposed to play an essential role in the final determination of amylopectin structure (Ball et al., 1996).Physical and chemical analyses of granular starch have led to a widely accepted model for amylopectin structure called the "cluster model," in which amorphous and crystalline regions alternate with a defined periodicity (for reviews, see French, 1984;Manners, 1989;Jenkins et al., 1993). Within amylopectin the crystalline component is composed of parallel arrays of linear chains packed tightly in double helices. Branch linkages, which account for approximately 5% of the glucosyl linkages in amylopectin, are located at the root of each cluster in the amorphous region. This periodic clustering of branches allows for the alignment of the intervening linear chains and their dense packing into crystalline regions, thus providing an efficient mechanism for nutrient storage. Undoubtedly, the enzymatic processes requir...

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Starches from eight soft wheat samples (two parent lines and six offspring) were isolated; relationships between their structures and properties were examined. Branch chain‐length distributions of amylopectins were determined by using high‐performance anion exchange chromatography equipped with an amyloglucosidase reactor and a pulsed amperometric detector (HPAEC‐ENZ‐PAD). Results showed that the average chain length of the eight samples varied at DP 25.6–26.9. Starch samples of lines 02, 60, 63, 95, and 114 consisted of amylopectins with more long chains (DP ≥ 37) and longer average chain length (DP 26.2–26.9) than that of other samples. These starch samples of longer branch chain length displayed higher gelatinization temperatures (55.3–56.5°C) than that of other samples (54.4–54.9°C) and higher peak viscosity (110–131 RVU) and lower pasting temperature (86.3–87.6°C) than others (83–100 RVU and 88.2–88.9°C, respectively). The Mw of amylopectins, determined by using high‐performance size exclusion chromatography equipped with multiangle laser‐light scattering and refractive index detectors (HPSEC‐MALLS‐RI), were similar for all samples (6.17 × 108 to 6.97 × 108). There were no significant differences in amylose and phosphorus contents between samples. These results indicated that physical properties of wheat starch were affected by the branch‐chain length of amylopectin.

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