Xyloglucans xyloglucan endotransglucosylase (XET) activity and arabinogalactan-protein (AGP)-like molecules: a new inter-relationship

植物中的每个细胞都有一个外细胞壁，可以是可伸展的(允许细胞生长，例如在幼嫩的豆茎中)，也可以是不可伸展的(例如在老叶中)。我们的实验旨在发现植物如何改变它的细胞壁(使它们或多或少具有伸缩性)，从而发现植物如何能够增加和减少细胞生长。这种能力很重要，例如，在强光下发育的幼苗可以长出短而粗的茎和大的叶子，这对光合作用是有效的，而在黑暗中发育的幼苗可以长出又长又细的茎(更有可能到达任何光线)，但只长出很小的叶子。在黑暗中种植大树叶将是浪费的，因为它们将无法进行光合作用。细胞壁还将相邻细胞粘合在一起，因此细胞壁的变化可以决定植物组织是否软化，例如在成熟的果实中。幼嫩植物细胞(那些有可能生长的细胞)的壁是由几种称为多糖的线状结构分子组成的。两个重要的是纤维素和木葡聚糖。我们的实验室一直处于发现和了解称为木葡聚糖内葡糖苷酶(XETs)的酶的作用的前沿。它们催化了一种反应，在该反应中，线状的木葡聚糖分子被切割，然后重新连接。切割/重新连接过程可能是必要的(1)新的木葡聚糖分子在建造过程中结合到壁上，以及(2)现有的细胞壁松散其结构(使细胞生长)。其他实验室的工作突出了存在于植物细胞壁及其周围的另一类有趣的聚合物。这些是阿拉伯半乳糖蛋白(AGPs)，是由碳水化合物和蛋白质组成的复杂分子。它们可以与某些其他碳水化合物结合，但就植物而言，这种结合的任何好处都是未知的。AGPS只占细胞壁重量的一小部分。它们不是真正的结构壁组件，也不是酶；然而，各种实验表明，它们以某种方式增加了壁的拉伸能力，因此对细胞生长至关重要。我们不知道AGP如何影响壁延展性。然而，我们的假设是，AGP通过增加XET催化其切割/重新连接反应的速度来起作用。我们最近发现一些AGPs及其相关分子确实可以促进XET在试管中的作用。我们将努力加强和扩大这一假说的证据。从花椰菜和相关的植物组织开始，我们将提纯并研究出更多关于AGPs的化学结构的细节。我们还将从这些组织中纯化几种不同的XET。然后，我们将测量在每个不同的AGP存在下，每个XET催化其切割/重新连接反应的速度。我们将通过观察Yariv抗原(使AGPs失活的化学物质)是否阻止XET在活的植物组织中发挥作用来进一步检验新的假设。我们还将测试AGP分子是否以及如何与XET和/或木聚糖结合。所获得的知识将为识别调控植物细胞壁特性的基因开辟道路，从而使我们在未来能够从基因或生物技术上控制作物的生长和发展。

Each cell in a plant has an outer cell wall, which may be either stretchable (allowing the cell to grow, e.g. in a young beanstalk) or not (e.g. in an old leaf). Our experiments aim to discover how the plant can change its cell walls (making them more, or less, stretchable) and thus how the plant is able to increase and decrease cell growth. This ability is important so that, for instance, a seedling developing in bright light can produce short stocky stems and large leaves, efficient for photosynthesis, whereas one developing in the dark can grow a long thin stem (with a better chance of reaching any light) but only small leaves. Growing big leaves in the dark would be wasteful because they wouldn't be able to photosynthesise. Cell walls also glue neighbouring cells together, and so changes in the walls can dictate whether or not a plant tissue softens, e.g. in ripening fruit. Walls of young plant cells (those potentially able to grow) are built of several kinds of string-like structural molecules called polysaccharides. Two important ones are cellulose and xyloglucan. Our lab has been at the forefront of discovering, and understanding the role of, enzymes called xyloglucan endotransglucosylases (XETs). These catalyse a reaction in which the string-like xyloglucan molecules are cut and then re-joined. The cutting/re-joining process may be necessary (1) for new xyloglucan molecules to become bonded into the wall while it is being built, and (2) for an existing cell wall to loosen its structure (enabling cell growth). Work in other labs had highlighted another interesting class of polymers present in and around the plant cell wall. These are the arabinogalactan-proteins (AGPs), complex molecules made of carbohydrate and protein. They can bind to certain other carbohydrates, but any benefit of this binding so far as the plant is concerned is unknown. AGPs make up only a small proportion of the weight of the cell wall. They are not true structural wall components and are not enzymes; however, various experiments show that they somehow increase the ability of the wall to stretch and are thus important in enabling cell growth. We do not know how AGPs affect wall stretchability. However, our HYPOTHESIS is that AGPs act by increasing the rate at which XETs catalyse their cutting/re-joining reaction. We recently showed that some AGPs and related molecules can indeed promote XET action in the test tube. We will try to strengthen and extend the evidence for this hypothesis. Starting with cauliflowers and related plant tissues, we will purify and work out more details of the chemical structures of AGPs. We will also purify several different XETs from these tissues. We will then measure how fast each XET can catalyse its cutting/re-joining reaction in the presence of each different AGP. We will further test the new hypothesis by seeing whether Yariv antigens (chemicals that inactivate AGPs) stop XETs working in living plant tissues. We will also test whether, and how, AGP molecules bind to XETs and/or to xyloglucans. The knowledge gained would open the way to identifying genes that regulate the plant cell wall's properties, and thus in the future could give us the ability to control genetically or biotechnologically the growth and development of crops.