Boron: the essential element for vascular plants that never was

Abstract

Summary 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.

Elucidating the Possible Involvement of Maize Aquaporins in the Plant Boron Transport and Homeostasis Mediated by Rhizophagus irregularis under Drought Stress Conditions

“…Recently, it has been also suggested that B is neither a beneficial nor an essential element for plant growth. Indeed, it was hypothesized that it is maintained in a homeostatic balance within the plant, thanks to the natural selection of constitutive biochemical mechanisms [41].…”

Boron toxicity in higher plants: an update

“…Besides the various examples illustrating the essential role of B in plants, a recent paper by Lewis (2019) argued against the essentiality of B because, from the author's point of view, an alternative interpretation of published evidence negates its compliance with one of the criteria for essentiality: the direct metabolic effect. Lewis (2019) proposed that B is, and always has been, potentially toxic for plants, and this feature needs to be nullified for normal growth, development and reproduction.…”

Boron Sensitivity and Storage in Brassica Napus and Its Potential for Clubroot Management

“…Previous studies indicated that the boron speciation within plant tissues observed in the current study likely consisted of unchanged boric acid, B complex binding with soluble phenypropanoids in the cytoplasm (B associated with carbonate) for detoxi cation of excess B, and B complex binding with insoluble carbohydrate polymers such as pectin on primary cell wall (tetrahedral B(OH) 4 ) and sucrose in phloem [46,47,41]. The predominant peak at 194 eV in sensitive accessions without added B may indicate the presence of free boric acid (trigonal B(OH) 3 ), which would lower pH in the cytoplasm.…”