Department of Animal & Plant Sciences, The University of Sheffield, Sheffield, S10 2TN, UK.
New Phytol. 2019 Mar;221(4):1685-1690. doi: 10.1111/nph.15519. Epub 2018 Nov 24.
Although a requirement for boron is a well-established feature of vascular plants, its designation, for almost a century, as essential is challenged and, instead, the proposal is made that it has never been so as conventionally defined. This is because an alternative interpretation of published evidence negates its compliance with one of the criteria for essentiality, that its effects are direct. The alternative, here postulated, is that boron is, and always has been, potentially toxic, a feature which, for normal growth, development and reproduction, needed to be nullified. This was enabled by exploitation of boron's ability to be chemically bound to compounds with cis-hydroxyl groups. Although particular cell wall carbohydrate polymers, glycoproteins and membrane glycolipids are among candidates for this role, it is here proposed that soluble phenolic metabolites of, or related to, the components of the pathway of lignin biosynthesis, themselves potentially toxic, are primarily used by vascular plants. When metabolic circumstances allow these phenolics to accumulate endogenously in the cytoplasm, their own inherent toxicity is also alleviated, partially at least, by formation of complexes with boron. This chemical reciprocity, enhanced by physical sequestration of the complexes in vacuoles and/or apoplast, thus achieves, in a flexible but indirect manner, a minimization of the inherent toxicities of both boron and relevant phenolics. In these ways, the multifarious outcomes of impairments, natural or experimental, to this interplay are responsible for the lack of consensus to explain the diverse effects observed in the many searches for boron's primary metabolic role, here considered to be nonexistent. In particular, since a toxic element cannot have 'deficiency symptoms', those previously so-called are postulated to be largely due to the expressed toxicity of phenylpropanoids. A principal requirement for the otherwise toxic boron is to nullify, by means of its indirect chemical and physical sequestration, such expression. In these ways, it is therefore neither an essential nor a beneficial element as currently strictly defined.
尽管硼是维管植物的一个既定需求特征，但近一个世纪以来，它作为必需元素的地位受到了挑战，相反，人们提出它从未像传统定义那样被定义为必需元素。这是因为对已发表证据的另一种解释否定了它符合必需性标准之一，即其作用是直接的。在这里提出的替代方案是，硼一直以来都是潜在的有毒物质，对于正常的生长、发育和繁殖，需要消除这种毒性。这是通过利用硼与具有顺式羟基的化合物形成化学结合的能力来实现的。虽然特定的细胞壁碳水化合物聚合物、糖蛋白和膜糖脂是这种作用的候选物之一，但在这里提出的是，木质素生物合成途径的成分的可溶性酚类代谢物或与之相关的代谢物，本身是潜在有毒的，是维管植物主要使用的物质。当代谢条件允许这些酚类物质在细胞质内内源积累时，它们自身的固有毒性也会得到缓解，至少部分缓解，这是通过与硼形成复合物来实现的。这种化学互惠性，通过将复合物物理隔离在液泡和/或质外体中得到增强，从而以灵活但间接的方式，最大限度地减少了硼和相关酚类物质的固有毒性。通过这种方式，这种相互作用的自然或实验干扰的各种结果，导致了缺乏共识来解释在许多寻找硼的主要代谢作用的研究中观察到的多样性效应，这里认为这种作用是不存在的。特别是，由于有毒元素不可能有“缺乏症状”，以前所谓的那些症状被认为主要是由于苯丙烷类的表达毒性。对于否则有毒的硼来说，一个主要的要求是通过其间接的化学和物理隔离来消除这种表达。通过这种方式，因此，它既不是必需元素，也不是有益元素，如当前严格定义的那样。
