Control of Starch Synthesis in Cereals: Metabolite Analysis of Transgenic Rice Expressing an Up-Regulated Cytoplasmic ADP-Glucose Pyrophosphorylase in Developing Seeds
Control of Starch Synthesis in Cereals: Metabolite Analysis of Transgenic Rice Expressing an Up-Regulated Cytoplasmic ADP-Glucose Pyrophosphorylase in Developing Seeds
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DOI：
10.1093/pcp/pcp021
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发表时间：
2009-03-01
影响因子：
4.9
通讯作者：
Okita, Thomas W.
中科院分区：
文献类型：
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作者：
Nagai, Yasuko S.;Sakulsingharoj, Chotipa;Okita, Thomas W.
我们以前已经证明，在水稻胚乳中表达的细胞质定位的ADP葡萄糖焦磷酸化酶（AGP 13）突变体基因，导致增强淀粉合成，反过来，更高的种子重量。在这项研究中，主要的初级碳代谢物的水平进行了评估，野生型和四个转基因CS 8水稻株系表达3- 6倍高AGRINGS活性。与AGG活性的增加一致，所有四个转基因CS 8系显示出升高的ADP葡萄糖（ADPglc）水平，尽管如通过种子重量测量的，该代谢物的增加程度远高于淀粉的增加程度。令人惊讶的是，其他几种关键中间体的水平也发生了显著变化。葡萄糖1-磷酸（Glc 1-P），一种AGG反应的底物，以及UDP葡萄糖和Glc 6-P也在转基因株系中升高到与野生型对照相比相同的相对程度。代谢物比率的分析表明野生型和转基因株系之间没有显著差异，表明从蔗糖代谢到ADPglc形成的反应接近平衡。此外，葡萄糖和果糖水平也升高，在三个转基因株系，表现出最大的差异，代谢产物和种子重量超过野生型，这表明转化酶的诱导。总体而言，结果表明，AGPase催化的反应在转基因品系中不再是限制性的，并且对进入淀粉的碳通量的限制在ADPglc形成的下游，导致ADPglc形成上游的前体的升高。
We had previously demonstrated that expression of a cytoplasmic-localized ADPglucose pyrophosphorylase (AGPase) mutant gene from Escherichia coli in rice endosperm resulted in enhanced starch synthesis and, in turn, higher seed weights. In this study, the levels of the major primary carbon metabolites were assessed in wild type and four transgenic CS8 rice lines expressing 3- to 6-fold higher AGPase activity. Consistent with the increase in AGPase activity, all four transgenic CS8 lines showed elevated levels of ADPglucose (ADPglc) although the extent of increases in this metabolite was much higher than the extent of increases in starch as measured by seed weight. Surprisingly, the levels of several other key intermediates were significantly altered. Glucose 1-phosphate (Glc 1-P), a substrate of the AGPase reaction, as well as UDPglucose and Glc 6-P were also elevated to the same relative extent in the transgenic lines compared with the wild-type control. Analysis of metabolite ratios showed no significant differences between the wild type and transgenic lines, indicating that the reactions leading from sucrose metabolism to ADPglc formation were in near equilibrium. Moreover, glucose and fructose levels were also elevated in three transgenic lines that showed the largest differences in metabolites and seed weight over the wild type, suggesting the induction of invertase. Overall, the results indicate that the AGPase-catalyzed reaction is no longer limiting in the transgenic lines, and constraints on carbon flux into starch are downstream of ADPglc formation, resulting in an elevation of precursors upstream of ADPglc formation.