Chloroplast nucleoids are large, compact nucleoprotein structures containing multiple copies of the plastid genome. Studies on structural and quantitative changes of plastid DNA (ptDNA) during leaf development are scarce and have produced controversial data. We have systematically investigated nucleoid dynamics and ptDNA quantities in the mesophyll of Arabidopsis, tobacco, sugar beet, and maize from the early post-meristematic stage until necrosis. DNA of individual nucleoids was quantified by DAPI-based supersensitive epifluorescence microscopy. Nucleoids occurred in scattered, stacked, or ring-shaped arrangements and in recurring patterns during leaf development that was remarkably similar between the species studied. Nucleoids per organelle varied from a few in meristematic plastids to >30 in mature chloroplasts (corresponding to about 20-750 nucleoids per cell). Nucleoid ploidies ranged from haploid to >20-fold even within individual organelles, with average values between 2.6-fold and 6.7-fold and little changes during leaf development. DNA quantities per organelle increased gradually from about a dozen plastome copies in tiny plastids of apex cells to 70-130 copies in chloroplasts of about 7 μm diameter in mature mesophyll tissue, and from about 80 plastome copies in meristematic cells to 2600-3300 copies in mature diploid mesophyll cells without conspicuous decline during leaf development. Pulsed-field electrophoresis, restriction of high-molecular-weight DNA from chloroplasts and gerontoplasts, and CsCl equilibrium centrifugation of single-stranded and double-stranded ptDNA revealed no noticeable fragmentation of the organelle DNA during leaf development, implying that plastid genomes in mesophyll tissues are remarkably stable until senescence.

中文翻译：

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在被子植物叶片发育过程中，叶绿体类核苷酸在倍性，数量和结构上都是高度动态的。

叶绿体核苷酸是大的，紧凑的核蛋白结构，包含质体基因组的多个副本。关于叶片发育过程中质体DNA（ptDNA）的结构和数量变化的研究很少，并且产生了有争议的数据。我们已经系统地研究了拟南芥，烟草，甜菜和玉米的叶肉从拟虫后早期到坏死之前的叶肉动力学和ptDNA数量。通过基于DAPI的超灵敏落射荧光显微镜对单个核苷的DNA进行定量。在叶片发育过程中，核素以散布，堆叠或环形排列的形式出现，并且以重复出现的方式出现，在所研究的物种之间非常相似。每个细胞器的核素从分生质体中的几个到> 成熟的叶绿体中有30个（每个细胞对应约20-750个核苷）。甚至在单个细胞器中，核样倍体的范围从单倍体到> 20倍不等，平均值在2.6倍至6.7倍之间，并且在叶片发育过程中变化很小。每个细胞器的DNA数量逐渐从顶点细小质体中的大约十二个质体组拷贝增加到成熟的叶肉组织中直径约7μm的叶绿体中的70-130拷贝，并从分生组织中的约80个质体组拷贝增加到2600-3300拷贝中。成熟的二倍体叶肉细胞，在叶片发育过程中没有明显的下降。脉冲场电泳，限制了叶绿体和Gerontoplasts中的高分子量DNA，