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首页> 外文期刊>Protoplasma: An International Journal of Cell Biology >Symplasmic transport and phloem loading in gymnosperm leaves
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Symplasmic transport and phloem loading in gymnosperm leaves

机译：裸子植物叶片的同质运输和韧皮部负载

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Despite more than 130 years of research, phloem loading is far from being understood in gymnosperms. In part this is due to the special architecture of their leaves. They differ from angiosperm leaves among others by having a transfusion tissue between bundle sheath and the axial vascular elements. This article reviews the somewhat inaccessible and/or neglected literature and identifies the key points for pre-phloem transport and loading of photoassimilates. The pre-phloem pathway of assimilates is structurally characterized by a high number of plasmodesmata between all cell types starting in the mesophyll and continuing via bundle sheath, transfusion parenchyma, Strasburger cells up to the sieve elements. Occurrence of median cavities and branching indicates that primary plasmodesmata get secondarily modified and multiplied during expansion growth. Only functional tests can elucidate whether this symplasmic pathway is indeed continuous for assimilates, and if phloem loading in gymnosperms is comparable with the symplasmic loading mode in many angiosperm trees. In contrast to angiosperms, the bundle sheath has properties of an endodermis and is equipped with Casparian strips or other wall modifications that form a domain border for any apoplasmic transport. It constitutes a key point of control for nutrient transport, where the opposing flow of mineral nutrients and photoassimilates has to be accommodated in each single cell, bringing to mind the principle of a revolving door. The review lists a number of experiments needed to elucidate the mode of phloem loading in gymnosperms.
机译：尽管进行了130多年的研究，韧皮部的负载远没有被裸子植物所理解。部分原因是其叶子的特殊结构。它们与被子植物的叶子不同，它们在束鞘和轴向血管元件之间具有输血组织。本文回顾了一些难以获得和/或被忽略的文献，并指出了韧皮部前转运和光同化物装载的关键点。同化物的韧皮部前部途径在结构上的特点是，所有类型的细胞之间都有大量的胞浆线虫，这些细胞从叶肉开始，一直到束鞘，输血薄壁组织，Strasburger细胞直至筛子元素。中位数腔和分支的出现表明，原虫在扩展生长过程中被二次修饰并繁殖。只有功能测试才能阐明同化途径对于同化物是否确实是连续的，以及裸子植物中韧皮部的负载是否与许多被子植物树中的同质负载模式相当。与被子植物相比，束鞘具有内胚层的特性，并配备有里海带或其他壁修饰，可形成任何质疏运输的域边界。它构成了控制养分运输的关键点，必须在每个单元格中容纳矿物质养分和光同化物的逆向流动，牢记旋转门的原理。该评论列出了阐明裸子植物韧皮部加载方式所需的大量实验。

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