Structural Properties And Development Of Starch Granule In High-amylose Rice Endosperm

In rice, amylose content was known as the critical factor to influence not only the eating and cooking properties but also the industrial application. High-amylose starch was a source of resistant starch which had a great benefit on human health. A transgenic rice line (TRS) enriched amylose and resistant starch had been developed by antisense RNA inhibition of starch branching enzymes. In this study, the native starch granules were isolated from mature and developing TRS kernels as well as its wild type Teqing (TQ), their structural properties and development were carefully investigated with morphological and spectroscopical methods. These researches could be helpful to cultivate high-amylose crops and utilize high-amylose starches. The main results were shown as followings:Structural properties of high-amylose TRS starch granules The results of various microscopical observation showed that starch granules were compound starches in TQ and semi-compound starches in TRS. TRS semi-compound starch granules consisted of packed smaller subgranules, some of which located at the periphery of starch granules were fused to each other with adjacent ones and formed a thick band or wall encircling the entire circumference of the granules. The amylose molecular probe APTS staining of starch granules showed that TQ starch granules had a high concentration of amylose in the concentric hilum, whereas TRS starch granules showed a relatively even distribution of amylose with intense amylose in both hilum and band. The resutls of spectroscopical analyses indicated that TQ starch was A-type crystal starch, while TRS starch was C-type crystal starch which is a combination of both A-type and B-type crystal starch. To determine the distribution of A-and B-type allomorphs, TRS starches were hydrolysed by HC1. The morphology of starch granules after various durations of acid hydrolysis was compared by microscopical methods. The results showed that amorphous regions were located at the center part of TRS starch subgranules. During acid hydrolysis, starch was degraded from the interior of the subgranule to the outer surface, while the peripheral part of the subgranules and the surrounding band of the starch granule were highly resistant to acid hydrolysis. The spectroscopic changes after acid hydrolysis showed that the A-type allomorph was hydrolyzed more rapidly than the B-type, and TRS starches changed from a native C-type to a B-type with increasing hydrolysis time. These results showed that, in TRS starch, the A-type allomorph was located around the amorphous region, and was surrounded by the B-type allomorph located in the peripheral region of the subgranules and the surrounding band of the starch granule. Though TQ starches were quickly hydrolysed by a-amylase, TRS starches showed high resistance to a-amylase hydrolysis. The gelatinization and crystalline properties, swelling power, water solubility, morphological and structural changes of starches from TRS and TQ were also carefully investigated during heating. Compared to TQ, TRS starch showed higher gelatinization temperatures, lower gelatinization enthalpy and swelling power. Morphological and structural changes showed that TQ starch drastically swelled after70℃, then gradually disrupted with increasing heating temperature. The surrounding band of TRS starch restrained granule swelling, though the subgranules disrupted to form the cavity. The results of spectroscopic analyses indicated that A-type crystalline of TQ changed to amorphous starch after75℃, while C-type crystalline of TRS gradually changed to B-type crystalline after75℃, then became amorphous starch at95℃.Development of high-amylose TRS starch granules TRS starch subgranules, each with a central hilum, were individually initiated in amyloplast and showed an A-type crystal at the early stage of starch granule development, which was similar to that in TQ. However, with the endosperm development, the contents amylose in TRS endosperm starch increased and the B-type starch crystal was deposited in the periphery of subgranules; then, the adjacent subgranules fused together and finally formed a continuous outer layer band surrounding the entire circumference of the starch granule. Accordingly, a mechanistic model for the formation of semi-compound C-type starch granules is proposed.