Zeeman Samuel C, Tiessen Axel, Pilling Emma, Kato K Lisa, Donald Athene M, Smith Alison M
John Innes Centre, Colney Lane, Norwich NR4 7UH, United Kingdom.
Plant Physiol. 2002 Jun;129(2):516-29. doi: 10.1104/pp.003756.
The aim of this work was to characterize starch synthesis, composition, and granule structure in Arabidopsis leaves. First, the potential role of starch-degrading enzymes during starch accumulation was investigated. To discover whether simultaneous synthesis and degradation of starch occurred during net accumulation, starch was labeled by supplying (14)CO(2) to intact, photosynthesizing plants. Release of this label from starch was monitored during a chase period in air, using different light intensities to vary the net rate of starch synthesis. No release of label was detected unless there was net degradation of starch during the chase. Similar experiments were performed on a mutant line (dbe1) that accumulates the soluble polysaccharide, phytoglycogen. Label was not released from phytoglycogen during the chase indicating that, even when in a soluble form, glucan is not appreciably degraded during accumulation. Second, the effect on starch composition of growth conditions and mutations causing starch accumulation was studied. An increase in starch content correlated with an increased amylose content of the starch and with an increase in the ratio of granule-bound starch synthase to soluble starch synthase activity. Third, the structural organization and morphology of Arabidopsis starch granules was studied. The starch granules were birefringent, indicating a radial organization of the polymers, and x-ray scatter analyses revealed that granules contained alternating crystalline and amorphous lamellae with a periodicity of 9 nm. Granules from the wild type and the high-starch mutant sex1 were flattened and discoid, whereas those of the high-starch mutant sex4 were larger and more rounded. These larger granules contained "growth rings" with a periodicity of 200 to 300 nm. We conclude that leaf starch is synthesized without appreciable turnover and comprises similar polymers and contains similar levels of molecular organization to storage starches, making Arabidopsis an excellent model system for studying granule biosynthesis.
这项工作的目的是对拟南芥叶片中的淀粉合成、组成和颗粒结构进行表征。首先，研究了淀粉降解酶在淀粉积累过程中的潜在作用。为了探究在净积累过程中淀粉是否同时进行合成和降解，通过向完整的光合植物供应(14)CO(2)来标记淀粉。在空气追踪期内，利用不同光照强度改变淀粉合成的净速率，监测该标记从淀粉中的释放情况。除非在追踪期内淀粉有净降解，否则未检测到标记的释放。在积累可溶性多糖植物糖原的突变株系(dbe1)上进行了类似实验。在追踪期内植物糖原未释放标记，这表明即使处于可溶形式，葡聚糖在积累过程中也不会明显降解。其次，研究了生长条件和导致淀粉积累的突变对淀粉组成的影响。淀粉含量的增加与淀粉直链淀粉含量的增加以及颗粒结合淀粉合酶与可溶性淀粉合酶活性比值的增加相关。第三，研究了拟南芥淀粉颗粒的结构组织和形态。淀粉颗粒具有双折射性，表明聚合物呈径向排列，X射线散射分析显示颗粒含有周期性为9 nm的交替结晶和无定形薄片。野生型和高淀粉突变体sex1的颗粒扁平呈盘状，而高淀粉突变体sex4的颗粒更大且更圆。这些较大的颗粒含有周期为200至300 nm的“生长环”。我们得出结论，叶片淀粉在合成过程中没有明显的周转，其包含的聚合物与储存淀粉相似，分子组织水平也相似，这使得拟南芥成为研究颗粒生物合成的优秀模型系统。
