Boron in plant cell walls

Boron is an essential element for higher plants, yet the primary functions remain unclear. In intact tissues of higher plants, this element occurs as both water soluble and water insoluble forms. In this review, the intracellular localisation of B and possible function of B in cell walls of higher plants are discussed. The majority of the water soluble B seems to be localised in the apoplastic region as boric acid. The water insoluble B is associated with rhamnogalacturonan II (RG-II) and the complex is ubiquitous in higher plants. In the Brassicaceae, Apiaceae, Chenopodiaceae, Asteraceae, Amaryllidaceae, and Liliaceae, nearly all the cell wall B is associated with RG-II, while in the Cucurbitaceae, only half of the cell wall B is in this complex. In duckweed, a different type of B-polysaccharide complex has been identified. Analysis of the structure of the B-RG-II complex reveals that the complex is composed of boric acid and two chains of monomeric RG-II. Boric acid does not merely bind to sugars but crosslinks two chains of pectic polysaccharide at the RG-II region through borate-diester bonding, thus forming a network of pectic polysaccharides in cell walls. The B-RG-II complex is reconstituted in vitro only by mixing monomeric RG-II and boric acid at pH 4.0. In the in vitro reconstitution, germanic acid can substitute for boric acid to some extent. The RG-II epitope, which cross reacts with the antibody toward the B-RG-II complex, is detected in walls of every cell in radish roots. The epitope is also detected in growing pollen tube cell walls, which are claimed to require B. Whilst it is now clear that boric acid links some cell wall components, it is not yet clear whether there is a structural requirement for B in cell wall function.