Commonwealth Scientific and Industrial Research Organization, Food Futures National Research Flagship, PO Box 93, North Ryde 1670, NSW, Australia.
J Exp Bot. 2010 Mar;61(5):1469-82. doi: 10.1093/jxb/erq011. Epub 2010 Feb 15.
The roles of starch branching enzyme (SBE, EC 2.4.1.18) IIa and SBE IIb in defining the structure of amylose and amylopectin in barley (Hordeum vulgare) endosperm were examined. Barley lines with low expression of SBE IIa or SBE IIb, and with the low expression of both isoforms were generated through RNA-mediated silencing technology. These lines enabled the study of the role of each of these isoforms in determining the amylose content, the distribution of chain lengths, and the frequency of branching in both amylose and amylopectin. In lines where both SBE IIa and SBE IIb expression were reduced by >80%, a high amylose phenotype (>70%) was observed, while a reduction in the expression of either of these isoforms alone had minor impact on amylose content. The structure and properties of the high amylose starch resulting from the concomitant reduction in the expression of both isoforms of SBE II in barley were found to approximate changes seen in amylose extender mutants of maize, which result from lesions eliminating expression of the SBE IIb gene. Amylopectin chain length distribution analysis indicated that both SBE IIa and SBE IIb isoforms play distinct roles in determining the fine structure of amylopectin. A significant reduction in the frequency of branches in amylopectin was noticed only when both SBE IIa and SBE IIb were reduced, whereas there was a significant increase in the branching frequency of amylose when SBE IIb alone was reduced. Functional interactions between SBE isoforms are suggested, and a possible inhibitory role of SBE IIb on other SBE isoforms is discussed.
研究了淀粉分支酶（SBE，EC 2.4.1.18）IIa 和 SBE IIb 在定义大麦（Hordeum vulgare）胚乳直链淀粉和支链淀粉结构中的作用。利用 RNA 介导的沉默技术生成了 SBE IIa 或 SBE IIb 表达量低，两种同工酶表达量均低的大麦品系。这些品系能够研究每种同工酶在确定直链淀粉含量、链长分布以及直链淀粉和支链淀粉分支频率中的作用。在 SBE IIa 和 SBE IIb 表达均降低>80%的品系中，观察到高直链淀粉表型（>70%），而单独降低这两种同工酶中的任何一种的表达对直链淀粉含量的影响较小。同时降低 SBE II 两种同工酶表达导致的大麦高直链淀粉淀粉的结构和性质与玉米直链淀粉延伸突变体中观察到的变化近似，这些突变体是由于消除 SBE IIb 基因表达而导致的。支链淀粉链长分布分析表明，SBE IIa 和 SBE IIb 同工酶在决定支链淀粉精细结构方面发挥着不同的作用。只有当 SBE IIa 和 SBE IIb 都降低时，才会发现支链淀粉分支频率显著降低，而当仅降低 SBE IIb 时，直链淀粉的分支频率显著增加。同工酶之间存在功能相互作用，并且讨论了 SBE IIb 对其他 SBE 同工酶的可能抑制作用。
