Boron

Abstract

Boron (B), a metalloid element, is atomic number 5 of periodic table. Element B and some B compounds have industrial values. B essentiality is proven in some living organisms including plants, but probably in human. At present, the only role of B in plants is demonstrated as the structural maintenance of cell wall. Soil B, as boric acid, is acquired through roots and then distributed around the plant via the passive and active transport pathway. To adapt variations in the environmental B status, the active B transport system is tightly regulated at the molecular level in plants. In agriculture, both deficient and excess levels of soil B impair plant growth, resulting in the reduction of quantity and quality of crops. The major causes of B toxicity in plants contain oxidative stress, metabolism alteration and deoxyribonucleic acid damage. Several plant traits/genes that ensure tolerance to B stress have been isolated.

Key Concepts:

  • Boron has numerous biological functions in almost all living organisms including plants although its biochemical importance is poorly understood.

  • B is widely distributed in the earth, and its main form in soils is boric acid that is available for plants.

  • B availability in soils depends on factors such as soil type and climates, and it is a key determinant for the crop yields in agriculture.

  • Plants uptake and distribute B via two distinct pathways: passive diffusion and active transport mediated by B transport molecules.

  • The molecules working in the active B transport system are tightly regulated dependently to the environmental B conditions to promote B acquisition under limited B supply or to avoid accumulating toxic levels of B in plants.

  • The active B efflux from cells to soils or outside of cells and the protection of cellular mechanism from B attack are key molecular processes for B‐toxicity tolerance in plants.

Keywords: boron availability; boron function; boron transport; boron deficiency; boron toxicity; boron stress‐tolerance

Figure 1.

Regulation of B distribution via B‐transporting molecules in A. thaliana. (a) Active B transport at the roots mediated by a channel, NIP5;1, and an efflux type transporter, BOR1, under the limited B condition. NIP5;1 imports B into epidermal, cortical and endodermal cells. BOR1 exports B from endodermal and pericycle cells to stele apoplast. The casparian band prevents the apoplastic B flow towards stele. Distinct polar localisation of BOR1 and NIP5;1 ensure symplastic B flow at endodermal cells. (b) Active B regulation both in root and shoot cells mediated by an efflux type transporter, BOR4, and a channel, TIP5;1, under the excess B condition. BOR4 exports cytosolic excess B to the outside of the roots at epidermal cells or apoplast at leaf cells. TIP5;1 imports excess B into vacuole to compartmentalise B. These active regulatory mechanisms of B distribution result in the reduction or the avoidance of excess B cytotoxicity. Note that at this moment, the cell specificity of TIP5;1 expression remains unclear.

Figure 2.

B‐containing biomolecules. (a) Predicted chemical structure of 3,4 3,4 (upper) and 1,2 1,2 (lower) bis‐mannitol‐borate based on Hu et al. . (b) Cross‐linking of two RG‐II monomers by formation of a borate ester between the apiosyl residues in the 2‐O‐Me‐Xyl‐containing side chain A (green circle) of each RG‐II domain of pectin based on O'Neill et al. (). This cross‐linking contributes to the structural integrity of cell wall. (c) Chemical structure of quorum‐sensing ligand based on Chen et al. .

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Further Reading

Camacho‐Cristóbal JJ, Rexach J and González‐Fontes A (2008) Boron in plants: deficiency and toxicity. Journal of Integrative Plant Biology 50: 1247–1255.

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Sakamoto, Takuya(May 2012) Boron. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0023742]