Kutschera U
Department of Plant Biology, Carnegie Institution, 260 Panama Street, Stanford, CA 94305, USA.
Ann Bot. 2008 Apr;101(5):615-21. doi: 10.1093/aob/mcn015. Epub 2008 Feb 7.
The cells of growing plant organs secrete an extracellular fibrous composite (the primary wall) that allows the turgid protoplasts to expand irreversibly via wall-yielding events, which are regulated by processes within the cytoplasm. The role of the epidermis in the control of stem elongation is described with special reference to the outer epidermal wall (OEW), which forms a 'tensile skin'.
The OEW is much thicker and less extensible than the walls of the inner tissues. Moreover, in the OEW the amount of cellulose per unit wall mass is considerably greater than in the inner tissues. Ultrastructural studies have shown that the expanding OEW is composed of a highly ordered internal and a diffuse outer half, with helicoidally organized cellulose microfibrils in the inner (load-bearing) region of this tension-stressed organ wall. The structural and mechanical backbone of the wall consists of helicoids, i.e. layers of parallel, inextensible cellulose microfibrils. These 'plywood laminates' contain crystalline 'cables' orientated in all directions with respect to the axis of elongation (isotropic material). Cessation of cell elongation is accompanied by a loss of order, i.e. the OEW is a dynamic structure. Helicoidally arranged extracellular polymers have also been found in certain bacteria, algae, fungi and animals. In the insect cuticle crystalline cutin nanofibrils form characteristic 'OEW-like' herringbone patterns.
Theoretical considerations, in vitro studies and computer simulations suggest that extracellular biological helicoids form by directed self-assembly of the crystalline biopolymers. This spontaneous generation of complex design 'without an intelligent designer' evolved independently in the protective 'skin' of plants, animals and many other organisms.
正在生长的植物器官的细胞分泌一种细胞外纤维复合材料（初生壁），它允许膨胀的原生质体通过细胞壁屈服事件不可逆地扩张，这些事件受细胞质内过程的调节。特别参考形成“拉伸表皮”的外表皮壁（OEW），描述了表皮在控制茎伸长中的作用。
OEW比内部组织的壁更厚且伸展性更小。此外，在OEW中，每单位壁质量的纤维素含量比内部组织中高得多。超微结构研究表明，正在扩张的OEW由高度有序的内部和弥散的外部半部组成，在这个受拉应力的器官壁的内部（承重）区域有螺旋状排列的纤维素微纤丝。壁的结构和机械骨架由螺旋体组成，即平行、不可伸展的纤维素微纤丝层。这些“胶合板层压板”包含相对于伸长轴在所有方向取向的结晶“缆绳”（各向同性材料）。细胞伸长的停止伴随着有序性的丧失，即OEW是一种动态结构。在某些细菌、藻类、真菌和动物中也发现了螺旋状排列的细胞外聚合物。在昆虫角质层中，结晶角质纳米纤维形成特征性的“类OEW”人字形图案。
理论思考、体外研究和计算机模拟表明，细胞外生物螺旋体由结晶生物聚合物的定向自组装形成。这种“没有智能设计者”的复杂设计的自发产生在植物、动物和许多其他生物体的保护性“表皮”中独立进化。
