The making of the architecture of the plant cell wall: How cells exploit geometry

Cell wall deposition is a key process in the formation, growth, and differentiation of plant cells. The most important structural components of the wall are long cellulose microfibrils, which are synthesized by synthases embedded in the plasma membrane. A fundamental question is how the microfibrils become oriented during deposition at the plasma membrane. The current textbook explanation for the orientation mechanism is a guidance system mediated by cortical microtubules. However, too many contraindications are known in secondary cell walls for this to be a universal mechanism, particularly in the case of helicoidal arrangements, which occur in many situations. An additional construction mechanism involves liquid crystalline self-assembly [A. C. Neville (1993) Biology of Fibrous Composites: Development Beyond the Cell Membrane (Cambridge Univ. Press, Cambridge, U.K.)], but the required amount of bulk material that is able to equilibrate thermally is not normally present at any stage of the wall deposition process. Therefore, we have asked whether the complex ordered texture of helicoidal cell walls can be formed in the absence of direct cellular guidance mechanisms. We propose that they can be formed by a mechanism that is based on geometrical considerations. It explains the genesis of the complicated helicoidal texture and shows that the cell has intrinsic, versatile tools for creating a variety of textures. A compelling feature of the model is that local rules generate global order, a typical phenomenon of life.

细胞壁沉积是植物细胞形成、生长与分化过程中的关键过程。细胞壁最关键的结构组分为长链纤维素微纤丝（cellulose microfibrils），其由嵌入质膜的纤维素合酶催化合成。当前存在一个核心科学问题：微纤丝在质膜处沉积时，其取向是如何被调控的。目前教科书中对该取向机制的主流解释为皮层微管（cortical microtubules）介导的引导系统。然而，次生细胞壁中已发现大量与之矛盾的实验证据，表明该机制并非普适性机制，尤其是在多种场景中普遍存在的螺旋状排列结构的形成过程中。另有研究提出一种基于液晶自组装（liquid crystalline self-assembly）的替代构建机制[A. C. Neville (1993) Biology of Fibrous Composites: Development Beyond the Cell Membrane (Cambridge Univ. Press, Cambridge, U.K.)]，但在细胞壁沉积的任一阶段，通常都不存在足以实现热平衡的足量本体材料。因此，本研究探讨了：在缺乏直接细胞引导机制的前提下，螺旋状细胞壁的复杂有序纹理能否形成。我们提出，此类结构可通过基于几何原理的机制形成。该机制可阐释复杂螺旋纹理的起源，并表明植物细胞拥有内在且多功能的调控工具，可生成多样化的细胞壁纹理。该模型的一个显著特征在于，局部规则可生成全局有序性——这是生命系统的典型现象。