Miura Satoko, Narita Maiko, Crofts Naoko, Itoh Yuki, Hosaka Yuko, Oitome Naoko F, Abe Misato, Takahashi Rika, Fujita Naoko
Department of Biological Production, Akita Prefectural University, Akita, 010-0195, Japan.
Rice (N Y). 2022 Jun 4;15(1):28. doi: 10.1186/s12284-022-00573-5.
Resistant starch (RS) is beneficial for human health. Loss of starch branching enzyme IIb (BEIIb) increases the proportion of amylopectin long chains, which greatly elevates the RS content. Although high RS content cereals are desired, an increase in RS content is often accompanied by a decrease in seed weight. To further increase the RS content, genes encoding active-type starch synthase (SS) IIa, which elongates amylopectin branches, and high expression-type granule-bound SSI (GBSSI), which synthesizes amylose, were introduced into the be2b mutant rice. This attempt increased the RS content, but further improvement of agricultural traits was required because of a mixture of indica and japonica rice phonotype, such as different grain sizes, flowering times, and seed shattering traits. In the present study, the high RS lines were backcrossed with an elite rice cultivar, and the starch properties of the resultant high-yielding RS lines were analyzed.
The seed weight of high RS lines was greatly improved after backcrossing, increasing up to 190% compared with the seed weight before backcrossing. Amylopectin structure, gelatinization temperature, and RS content of high RS lines showed almost no change after backcrossing. High RS lines contained longer amylopectin branch chains than the wild type, and lines with active-type SSIIa contained a higher proportion of long amylopectin chains compared with the lines with less active-SSIIa, and thus showed higher gelatinization temperature. Although the RS content of rice varied with the cooking method, those of high RS lines remained high after backcrossing. The RS contents of cooked rice of high RS lines were high (27-35%), whereas that of the elite parental rice was considerably low (< 0.7%). The RS contents of lines with active-type SSIIa and high-level GBSSI expression in be2b or be2b ss3a background were higher than those of lines with less-active SSIIa.
The present study revealed that backcrossing high RS rice lines with elite rice cultivars could increase the seed weight, without compromising the RS content. It is likely that backcrossing introduced loci enhancing seed length and width as well as loci promoting early flowering for ensuring an optimum temperature during RS biosynthesis.
抗性淀粉（RS）对人体健康有益。淀粉分支酶IIb（BEIIb）缺失会增加支链淀粉长链的比例，从而大幅提高RS含量。尽管人们期望获得高RS含量的谷物，但RS含量增加往往伴随着种子重量的下降。为了进一步提高RS含量，将编码可延长支链淀粉分支的活性型淀粉合酶（SS）IIa以及合成直链淀粉的高表达型颗粒结合型SSI（GBSSI）的基因导入be2b突变水稻中。这一尝试提高了RS含量，但由于籼稻和粳稻表型的混合，如不同的粒型、开花时间和种子散落特性，还需要进一步改善农艺性状。在本研究中，将高RS品系与优良水稻品种进行回交，并分析了所得高产RS品系的淀粉特性。
回交后，高RS品系的种子重量得到了极大改善，与回交前相比增加了190%。高RS品系的支链淀粉结构、糊化温度和RS含量在回交后几乎没有变化。高RS品系的支链淀粉分支链比野生型更长，与活性较低的SSIIa品系相比，具有活性型SSIIa的品系含有更高比例的支链淀粉长链，因此糊化温度更高。尽管水稻的RS含量会因烹饪方法而异，但高RS品系在回交后的RS含量仍然很高。高RS品系的熟米饭RS含量很高（27 - 35%），而优良亲本水稻的RS含量则相当低（<0.7%）。在be2b或be2b ss3a背景下，具有活性型SSIIa和高水平GBSSI表达的品系的RS含量高于活性较低的SSIIa品系。
本研究表明，将高RS水稻品系与优良水稻品种回交可以增加种子重量，同时不影响RS含量。回交可能引入了增强种子长度和宽度的基因座以及促进早花的基因座，以确保RS生物合成过程中的最佳温度。
