Effect Of Inhibition Of Starch Branching Enzyme On Starch Properties In Rice Endosperm

High-amylose cereal starches are attracting considerable attention because of their potential health benefits and special industry applications. The inhibition of starch branching enzyme can significantly increase the amylose content. The effect of inhibition of starch branching enzyme on starch properties is widely investigated in mature cereal kernel. However, the effects on the developing starch and the endosperm starch in different regions of kernel are unclear. In this study, three rice cultivars Guang-ling-xiang-nuo (GLXN), Wu-xiang 9915 (WX) and Te-qing (TQ) with different amylose content and their transgenic rice lines with inhibition of starch branching enzyme I and Ⅱb (SBEI/IIb) were used as plant materials. The properties of starches from different developing stages and different regions of kernel were investigated. The digestion properties of heterogeneous starch granules from high-amylose rice transgenic line derived from TQ with inhibition of SBEI/IIb were further analyzed. We also compared the structural and functional properties of high-amylose starch from rice transgenic line and commercial maize. The study would provide important information for cultivation of high-amylose cereal crops, and was useful for application of high-amylose cereal crop starch. The main results were shown as following.1. Effect of inhibition of starch branching enzyme on properties of rice developing starches. Starch was isolated from endosperm at 4,7,10,15,20 and 25 days after flowering and mature stage. Its iodine absorbance properties, molecular weight distribution, crystalline structure, and enzymatic hydrolysis properties were investigated. The results showed that the developing starch from rice cultivar GLXN, WX and TQ had A-type crystallinity. During development, the molecular structure did not vary for the starch of GLXN, but the amylose content gradually increased, and the short branch-chain content and branching degree of amylopectin decreased from 4 to 20 days after flowering for WX and TQ. The starch properties of transgenic rice lines with inhibition of SBEI/IIb were similar to those of their wild type rice cultivars at 4 days after flowering. But, with the development, the B-type crystallinity gradually accumulated, the long branch-chain content of amylopectin increased, and the branching degree of amylopectin decreased for transgenic rice lines. The amylose content gradually increased during endosperm development of transgenic rice lines derived from WX and TQ. The effect of inhibition of SBEI/IIb on properties of developing starch was the most conspicuous for the rice transgenic line derived from TQ.2. Effect of inhibition of starch branching enzyme on properties of starches from different regions of rice endosperm. The starch was isolated from different regions of rice endosperm at 15,20,25 days after flowering and mature stage. Its molecular weight distribution, amylopectin fine structure and crystalline structure were investigated. The results showed that the molecular weight distribution, amylopectin fine structure and crystalline structure were similar in different regions of endosperm from 15 days after flowering to mature stage for rice cultivars GLXN, WX and TQ. For transgenic rice lines with inhibition of SBEI/IIb, the short branch-chain content and branching degree of amylopectin decreased, the long branch-chain content and average branch-chain length of amylopectin increased, and the amylose and B-type crystallinity increased from the interior region to the outer region of kernels. The above variation of molecular and crystalline structure appeared before 15 days after flowering, and was very conspicuous for the rice transgenic line derived from TQ.3. Digestion properties of heterogeneous starch granules from high-amylose rice. The polygonal, aggregate, elongated, and hollow starch granules were separated from mature kernels of high-amylose rice transgenic line derived from TQ with inhibition of SBEI/IIb. Their in vitro digestion properties were investigated. The results showed that the heterogeneous starch granules had significantly different digestion properties. The polygonal and aggregate starches had monophasic digestograms and the elongated and hollow starches had biphasic digestograms from 0 to 8 h of digestion, but only the digestions of polygonal and aggregate starches were well fitted by first-order kinetics. The elongated and hollow starches were rapidly degraded in first 20 min of digestion, but had high resistance to degradation after 2 h of digestion. The polygonal and aggregate starches had side-by-side digestions of amylose and amylopectin as well as crystalline and amorphous components, but amylose and amorphous component of elongated and hollow starches were rapidly degraded in first 20 min of digestion. The B-type allomorph in aggregate, elongated, and hollow C-type starches contributed to the resistance to digestion.4. Comparison of molecular structure and functional properties of high-amylose starches from rice transgenic line and commercial maize. The molecular structure and functional properties of starch were investigated and compared in high-amylose transgenic rice, high-amylose commercial maize, normal rice, and normal maize. The results showed that normal rice and maize starches had A-type crystallinity, and exhibited the similar molecular weight distribution of starch and the branch-chain length distribution of amylopectin. High-amylose rice and maize starch had C-type and CA-type crystallinity, respectively. The high-amylose rice starch had significantly lower short branch-chain content and branching degree of amylopectin, higher amylose content, higher amylopectin long branch-chain content, and longer average branch-chain length of amylopectin than the high-amylose maize starch. The high-amylose rice and maize starches had significantly lower gelatinization enthalpy, viscosity and swelling power, and higher gelatinization temperature, pasting peak time and pasting temperature than normal rice and maize starches. The different structure of high-amylose rice and maize starches resulted in the different thermal properties, swelling power and pasting properties. The high-amylose rice and maize starches had significantly higher resistance to hydrolysis of acid and amylase than normal rice and maize starches. The gelatinized and retrograded starches from high-amylose rice had significantly lower slowly digestible starch and higher resistant starch contents than high-amylose maize starch.