Abe Natsuko, Asai Hiroki, Yago Hikari, Oitome Naoko F, Itoh Rumiko, Crofts Naoko, Nakamura Yasunori, Fujita Naoko
Department of Biological Production, Akita Prefectural University, Akita City, Akita 010-0195, Japan.
BMC Plant Biol. 2014 Mar 26;14:80. doi: 10.1186/1471-2229-14-80.
Starch is the most important carbohydrate in plant storage tissues. Multiple isozymes in at least four enzyme classes are involved in starch biosynthesis. Some of these isozymes are thought to interact and form complexes for efficient starch biosynthesis. Of these enzyme classes, starch synthases (SSs) and branching enzymes (BEs) play particularly central roles.
We generated double mutant lines (ss1/be1 and ss1L/be2b) between SSI (the largest component of total soluble SS activity) and BEI or BEIIb (major BEs in developing rice endosperm) to explore the relationships among these isozymes. The seed weight of ss1/be1 was comparable to that of wild type, although most ss1/be2b seeds were sterile and no double recessive plants were obtained. The seed weight of the double recessive mutant line ss1L/be2b, derived from the leaky ss1 mutant (ss1L) and be2b, was higher than that of the single be2b mutant. Analyses of the chain-length distribution of amylopectin in ss1/be1 endosperm revealed additive effects of SSI and BEI on amylopectin structure. Chain-length analysis indicated that the BEIIb deficiency significantly reduced the ratio of short chains in amylopectin of ss1L/be2b. The amylose content of endosperm starch of ss1/be1 and ss1L/be2b was almost the same as that of wild type, whereas the endosperm starch of be2b contained more amylose than did that of wild type. SSI, BEI, and BEIIb deficiency also affected the extent of binding of other isozymes to starch granules.
Analysis of the chain-length distribution in amylopectin of the double mutant lines showed that SSI and BEI or BEIIb primarily function independently, and branching by BEIIb is followed by SSI chain elongation. The increased amylose content in be2b was because of reduced amylopectin biosynthesis; however, the lower SSI activity in this background may have enhanced amylopectin biosynthesis as a result of a correction of imbalance between the branching and elongation found in the single mutant. The fact that a deficiency of SSI, BEI, or BEIIb affected the affinity of other starch biosynthetic isozymes for the starch granule implies that there is a close interaction among SSI, BEI and BEIIb during amylopectin biosynthesis in rice endosperm.
淀粉是植物储存组织中最重要的碳水化合物。至少四类酶中的多种同工酶参与淀粉生物合成。其中一些同工酶被认为会相互作用并形成复合物以实现高效的淀粉生物合成。在这些酶类中，淀粉合酶（SSs）和分支酶（BEs）发挥着尤为核心的作用。
我们构建了SSI（总可溶性SS活性的最大组分）与BEI或BEIIb（发育中的水稻胚乳中的主要BEs）之间的双突变系（ss1/be1和ss1L/be2b），以探究这些同工酶之间的关系。ss1/be1的种子重量与野生型相当，尽管大多数ss1/be2b种子不育且未获得双隐性植株。源自渗漏型ss1突变体（ss1L）和be2b的双隐性突变系ss1L/be2b的种子重量高于单突变体be2b。对ss1/be1胚乳中支链淀粉链长分布的分析揭示了SSI和BEI对支链淀粉结构的累加效应。链长分析表明，BEIIb缺陷显著降低了ss1L/be2b支链淀粉中短链的比例。ss1/be1和ss1L/be2b胚乳淀粉的直链淀粉含量与野生型几乎相同，而be2b的胚乳淀粉比野生型含有更多直链淀粉。SSI、BEI和BEIIb缺陷也影响了其他同工酶与淀粉颗粒的结合程度。
对双突变系支链淀粉链长分布的分析表明，SSI和BEI或BEIIb主要独立发挥作用，BEIIb分支后是SSI链延伸。be2b中直链淀粉含量增加是由于支链淀粉生物合成减少；然而，在此背景下较低的SSI活性可能由于校正了单突变体中发现的分支与延伸之间的失衡而增强了支链淀粉生物合成。SSI、BEI或BEIIb缺陷影响其他淀粉生物合成同工酶对淀粉颗粒亲和力这一事实表明，在水稻胚乳支链淀粉生物合成过程中，SSI、BEI和BEIIb之间存在密切相互作用。
