Thick glistening cell walls occur in sieve tubes of all major land plant taxa. Historically, these 'nacreous walls' have been considered a diagnostic feature of sieve elements; they represent a conundrum, though, in the context of the widely accepted pressure-flow theory as they severely constrict sieve tubes. We employed the cucurbit Gerrardanthus macrorhizus as a model to study nacreous walls in sieve elements by standard and in situ confocal microscopy and electron microscopy, focusing on changes in functional sieve tubes that occur when prepared for microscopic observation. Over 90% of sieve elements in tissue sections processed for microscopy by standard methods exhibit nacreous walls. Sieve elements in whole, live plants that were actively transporting as shown by phloem-mobile tracers, lacked nacreous walls and exhibited open lumina of circular cross-sections instead, an appropriate structure for Münch-type mass flow of the cell contents. Puncturing of transporting sieve elements with micropipettes triggered the rapid (<1 min) development of nacreous walls that occluded the cell lumen almost completely. We conclude that nacreous walls are preparation artefacts rather than structural features of transporting sieve elements. Nacreous walls in land plants resemble the reversibly swellable walls found in various algae, suggesting that they may function in turgor buffering, the amelioration of osmotic stress, wounding-induced sieve tube occlusion, and possibly local defence responses of the phloem.

中文翻译：

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筛分元件在受伤后会迅速形成“网状壁”，这是常见的伤口反应吗？

在所有主要陆地植物类群的筛管中都出现了厚厚的闪闪发光的细胞壁。从历史上看，这些“珠壁”被认为是筛分元件的诊断特征。但是，在被广泛接受的压力流理论的背景下，它们代表了一个难题，因为它们严重地收缩了筛管。我们采用葫芦瓜（Gerrardanthus macrorhizus）作为模型，通过标准和原位共聚焦显微镜和电子显微镜研究筛子元素中的珠壁，重点是准备进行显微镜观察时发生的功能性筛子管的变化。通过标准方法进行显微镜处理的组织切片中超过90％的筛子元素显示出珠壁。韧皮部移动示踪剂显示，正在迁移的整个活植物中的筛分元素，缺少珍珠质壁，并显示出圆形横截面的开孔腔，这是细胞内容物的Münch型质量流的合适结构。用微量移液管刺穿运输中的筛子元素，触发了珍珠壁的快速发展（<1分钟），该壁几乎完全堵塞了细胞腔。我们得出的结论是，珍珠质壁是准备制品，而不是运输筛网元件的结构特征。陆地植物的珠壁类似于在各种藻类中发现的可逆膨胀壁，表明它们可能在缓冲缓冲，减轻渗透压，伤口引起的筛管阻塞以及韧皮部的局部防御反应中起作用。用微量移液管刺穿运输中的筛子元素，触发了珍珠壁的快速发展（<1分钟），该壁几乎完全堵塞了细胞腔。我们得出的结论是，珍珠质壁是准备制品，而不是运输筛网元件的结构特征。陆地植物的珠壁类似于在各种藻类中发现的可逆膨胀壁，表明它们可能在缓冲缓冲，减轻渗透压，伤口引起的筛管阻塞以及韧皮部的局部防御反应中起作用。用微量移液管刺穿运输中的筛子元素，触发了珍珠壁的快速发展（<1分钟），该壁几乎完全堵塞了细胞腔。我们得出的结论是，珍珠质壁是准备制品，而不是运输筛网元件的结构特征。陆地植物的珠壁类似于在各种藻类中发现的可逆膨胀壁，表明它们可能在缓冲缓冲，减轻渗透压，伤口引起的筛管阻塞以及韧皮部的局部防御反应中起作用。