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

与动物和微生物不同，植物对硼元素(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元素。植物生命的这一独特而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讨。充足的硼在整个植物中都是必不可少的，尤其是在生长组织中；硼被认为是植物细胞扩增（生长）机制的核心。虽然缺硼可以通过施肥来治愈，但在一些地方，土壤中硼过量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，将有助于对植物生长机制的基本理解和作物产量的优化。硼是植物组装正常细胞壁所必需的，这是细胞扩增机制的关键。硼的主要作用是在一种叫做鼠李糖半乳糖醛酸- ii （RGII）的细胞壁果胶的两个分子之间形成一座桥梁。尽管RGII的数量较少，但它显然在植物的生命中起着重要作用，因为它的硼桥对植物生长至关重要。先前的工作已经揭示了由硼原子桥接的RGII的确切原子。硼的第二个作用是维持植物膜的正常功能，例如控制钾等其他重要营养物质的吸收。硼似乎还能帮助细胞膜正确地附着在细胞壁上。然而，我们还不知道植物膜上与硼相互作用的特定分子是什么。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颖的酶活性：迄今为止，还没有已知的酶作用于硼化合物。具体来说，我们将研究出RGII分子在“职业生涯”的哪个阶段，以及在细胞中的哪个位置，它通常会在健康的植物组织中成为硼桥。我们还将发现硼-RGII桥是否是永久性的，或者硼是否可以在以后再循环到不同的RGII分子中。重要的是，我们还不知道“玩家”是什么在硼桥过程中。我们将探讨（带负电的）RGII在桥接时被其他（带正电的）分子“陪伴”的想法；我们将测试用于造桥的硼是由硼酸本身提供的，还是由一些特殊的硼载体提供的，比如水溶性糖相关物质或水不溶性脂质。如上所述，携带硼的脂质原则上可能是“植物膜上与硼相互作用的特定分子”。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，上面列出的想法主要是假设（用于测试），而不是断言为“事实”。有些会被证明是错误的开始，我们不会浪费时间在他们有用的生命之后去追求他们。然而，在探索新的科学途径的过程中，有这样的假设是有价值的。在这个项目中产生的知识的影响将使我们能够在未来操纵作物植物的硼桥和桥断，从而有可能在硼过量或不足的土壤上提高作物产量。这可以通过植物育种或优化使用含硼酸肥料来实现。

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.