Metabolic basis of the borate cross-linking of rhamnogalacturonan-II a plant cell wall polysaccharide

与动物和微生物不同，植物对元素硼(B)有明确的需求，它们从土壤中天然存在的可溶性硼酸中获得硼(B)。植物生命的这一独特的和重要的农业特征在生物化学上知之甚少，这里将从几个新的角度进行探索。充足的硼在整个植物中是必不可少的，尤其是在生长组织中；硼被认为是植物细胞扩张(生长)机制的核心。虽然缺硼可以通过化肥来治愈，但在一些地方，土壤中过量的硼是一个严重而棘手的农业问题。了解植物为什么需要硼，以及为什么过量的硼是有毒的，这将有助于从根本上理解植物生长机制和优化作物产量。硼是植物组装正常细胞壁所必需的，而正常细胞壁是细胞扩张机制的关键。硼的主要作用是在一种叫做鼠李半乳糖原糖-II(RGII)的次要细胞壁果胶的两个分子之间形成桥梁。虽然数量很少，但rGII显然在植物的生活中起着重要的作用，因为它的硼桥对植物的生长是必不可少的。以前的工作已经揭示了rGII的确切原子，这些原子由硼原子桥联。硼的第二个作用是植物细胞膜的正常功能，例如控制其他重要营养物质的吸收，如钾。硼似乎也有助于细胞膜正确地附着在细胞壁上。然而，我们还不知道为了这些目的而与硼相互作用的植物膜的特定分子是什么。尽管我们对植物细胞壁中硼-rGII桥的(静态)化学结构有详细的了解，但我们对桥接过程是如何发生的却一无所知：何时何地在细胞中，以及是否由酶催化。该项目将探索(动态)桥接过程，可能导致发现高度新奇的酶活性：到目前为止，还没有已知的作用于硼化合物的酶。具体地说，我们将计算出rGII分子在职业生涯中的哪个阶段，以及在细胞中的哪里，它通常在健康的植物组织中成为硼桥联的。我们还将发现硼-rGII桥是否是永久性的，或者硼是否可以稍后再循环到不同的rGII分子。重要的是，我们还不知道在硼桥接过程中有哪些‘玩家’。我们将探索(带负电荷的)rGII在桥联时刻被其他(带正电荷的)分子“陪伴”的想法；我们将测试用于架桥的硼是由硼酸本身还是由某些特殊的硼载体贡献的，如与水可溶的糖相关的物质或水不溶的脂质。如上所述，原则上，含硼的脂类可以是“植物膜上与硼相互作用的特殊分子”。我们将探索这些可能性，在必要时开发新的方法。在这个阶段，上面概述的想法主要是假设(用于测试)，而不是断言为“事实”。有些将被证明是错误的开始，我们不会浪费时间去追求它们，而不是它们的有用寿命。然而，在探索新的科学途径时，牢记这样的假设是有价值的。该项目中产生的知识的影响将使我们能够在未来操纵作物中的硼桥接和桥接，从而潜在地提高含硼过多或不足的土壤上的作物产量。这可以通过植物育种或优化使用含硼化肥来实现。

Unlike animals and microbes, plants have a clear requirement for the element boron (B), which they obtain from soluble boric acid naturally present in soil. This unique and agriculturally important feature of plant life is poorly understood biochemically, and will be explored here from several novel perspectives. Adequate boron is essential throughout the plant, but particularly in growing tissues; boron is thought to be central to the mechanism of plant cell expansion (growth). Although boron deficiency can be cured by fertilisers, excess boron in the soil is a serious and intractable agricultural problem in some places. Understanding why plants require boron, and why excess boron is toxic, will facilitate progress towards both a fundamental understanding of the mechanism of plant growth and the optimisation of crop yields. Boron is necessary for the plant to assemble normal cell walls, which are key to the mechanism of cell expansion. The principal role of the boron is to form a bridge between two molecules of a minor type of cell-wall pectin called rhamnogalacturonan-II (RGII). Although minor quantitatively, RGII evidently serves a major purpose in the life of the plant since its boron bridges are essential for plant growth. Previous work has revealed the exact atoms of RGII which are bridged by the boron atom. A second role of boron is in the proper functioning of the plant's membranes, for example controlling the uptake of other important nutrients such as potassium. Boron also seems to help the cell membrane to remain correctly attached to the cell wall. However, we don't yet know what is the particular molecule of the plant membrane that interacts with boron for these purposes. Despite our detailed knowledge of the (static) chemical structure of boron-RGII bridges in the plant cell wall, we know nothing about how the bridging process occurs: when and where in the cell, and whether catalysed by enzymes. This project will explore the (dynamic) bridging process, potentially leading to the discovery of highly novel enzyme activities: to date, there are no known enzymes that act on boron compounds. Specifically, we will work out at what stage in the 'career' of an RGII molecule, and where in the cell, it normally becomes boron-bridged in healthy plant tissues. We will also discover whether boron-RGII bridges are permanent or if the boron can later be re-cycled to different RGII molecules. Importantly, we do not yet know what the 'players' are in the boron-bridging process. We will explore the idea that the (negatively charged) RGII is 'chaperoned' by some other (positively charged) molecule at the moment of bridging; and we will test whether the boron used for making the bridge is donated by boric acid itself or by some special boron carrier such as a water-soluble sugar-related substance or a water-insoluble lipid. A boron-carrying lipid could in principle be the 'particular molecule of the plant membrane that interacts with boron', mentioned above. We will explore these possibilities, developing new methodologies where necessary. At this stage, the ideas outlined above are mainly hypotheses (for testing), not asserted as 'facts'. Some will turn out to be false starts and we will not waste time pursuing them beyond their useful lives. However, it is valuable to have such hypotheses in mind during the exploration of new scientific avenues. The impact of the knowledge generated in this project would allow us in the future to manipulate boron bridging and bridge severance in crop plants, thus potentially enhancing crop production on soils with excess or insufficient boron. This could be done either by plant breeding or by optimised use of borate-containing fertilisers.