Growing 'Up': Mechano-chemical aspects of anisotropic cell growth

人类已经变得非常擅长建造东西；桥梁、集装箱、房屋、摩天大楼、礼堂、汽车，这个名单是无止境的。我们可以制定一个计划，并操纵材料，以获得最理想的形状，并将这些形状组装成一个物体。然而，这个过程与自然形成形状的过程有着根本的不同，因为自然是生长事物的。一个细胞可以增殖，它的子细胞会以不同的方式改变，它们随后的生长速度和形状也会改变，直到形成一个理想的物体。除此之外，这些形状可以随着时间的推移而改变，对环境和其他线索做出反应，这是最终的智能材料。我们才刚刚开始了解植物生长的物理过程，也就是说，产生不同形状的植物材料发生了什么变化，这些变化是如何调节的？为了回答这些问题，我们将描述一个涉及形状简单变化的系统-一个特殊器官的延伸，下胚轴，负责在发芽后将茎尖从土壤中抬起。当种子种在土壤中时，幼小的植物必须将自己伸到阳光和空气中开始光合作用。这种扩展以一种看似简单的方式发生：在幼嫩植物的茎尖和根尖之间有一个特殊的器官-下胚轴。下胚轴有固定数量的细胞，它们开始时大致是立方体，但最终长度增加到200倍以上，变成非常细长的矩形或长方体。这种类型的生长被称为各向异性，其中一个方向的膨胀超过另一个方向；这里的长度增量大于宽度增量。下胚轴各向异性生长的结果是茎尖向上伸出土壤，非常迅速。该系统的另一个优点是它对光和重力的动态响应。我们对植物细胞生长方向的控制了解多少？植物细胞基本上是一个充满压力的盒子，其中内部的压力为生长提供了非方向性的力，而壁上的局部变化提供了生长方向的信息，从而提供了形状的变化。传统的各向异性细胞膨胀的观点，如在下胚轴中，认为每个细胞的细胞壁上都有一个特殊的组织，即定向的纤维素纤维，它控制着细胞膨胀的方向。这些纤维可以被认为是绕在桶上的箍，限制了宽度的扩张，但能够散开以允许长度的增加。然而，最近的研究指出了一个更加动态和复杂的图景。在下胚轴细胞中，纤维素的方向比以前认为的更动态——这些不仅仅是桶上的静态环。事实上，壁内真正的限宽取向只在各向异性膨胀开始后短暂地观察到，即使在那时，也不是在细胞表面周围，而只是在细胞的内面。此外，细胞壁的另一部分，纤维素纤维嵌入的果胶凝胶，对调节植物的生长非常重要。在这个项目中，我们将描述与各向异性细胞生长相关的细胞壁特性的生物物理和化学变化，并测试这些变化是由植物激素生长素协调的想法。

Human beings have become very skilled at building things; bridges, containers, houses, skyscrapers, auditoriums, cars, the list is endless. We can draw up a plan and manipulate materials to get most desired shapes and assemble those shapes into an object. However, this process is fundamentally different from how nature makes shapes, because nature grows things. A single cell can be multiplied, and its daughters altered in different ways, their subsequent growth rates and shapes changed, and those of their daughters, until a desired object is formed. Beyond even that, these shapes can be changed over time, responding to their environment and other cues, the ultimate smart-materials. We are only just beginning to understand the physical process of growth in plants, that is to say: what are the changes in the plant material that yield different shapes, and how are these changes regulated? To answer these questions, we will characterize a system involving a simple change in shape- the extension of a special organ, the hypocotyl, responsible for lifting the shoot tip out of the soil following germination.When a seed is planted in the soil, the young plant must extend itself out into the light and air to begin photosynthesis. This extension happens in a deceptively simple way: between the shoot tip and the root tip of a young plant there is a special organ- the hypocotyl. The hypocotyl has a fixed number of cells which begin as roughly cubed shapes, but who eventually increase in length over 200x, becoming very elongated rectangles, or cuboids. This type of growth is called anisotropic, where one direction of expansion exceeds the other; here length increase is greater than width. The result of anisotropic growth in the hypocotyl is the extension of the shoot tip up and out of the soil, very rapidly. An added bonus to the system is its dynamic response to light and gravity.What do we know about the control of cell growth direction in plants? A plant cell is basically a pressure filled box, where the pressure inside provides a non-directional force for growth and local changes in the wall provides the information about the direction of growth and hence any change in shape. The traditional view of anisotropic cell expansion, such as in the hypocotyl, is that each cell has a special organization in the wall, oriented cellulose fibers, that controls the direction of expansion. These fibers can be thought of as hoops around a barrel, limiting width expansion but capable of spreading apart to allow length increases. However, recent research points to a much more dynamic and complex picture. Within hypocotyl cells, cellulose orientation is more dynamic than previously thought- these are not just static hoops on barrels. In fact, true width-limiting orientations within the wall are only observed transiently after anisotropic expansion has begun, and even then not all the way around the cell surface, but only on their inner faces. Furthermore, another part of the cell wall, the pectin gel in which cellulose fibers are imbedded, is very important for regulating growth in plants.In this project we will characterize the biophysical and chemical changes in cell wall properties associated with anisotropic cell growth, and test the idea that these changes are coordinated by the plant hormone, auxin.