Boron Function in Plant Cell Walls

Abstract

Warington (1923) conducted convincing experiments that B is an essential element for higher plants. Seventy-eight years have passed, however the mode of action of B on plant metabolism is still unknown. A close association of B with cell wall pectic polysaccharides has been reported by several authors (Smith, 1944; Yamanouchi, 1971; Yamauchi et al., 1986; Hu and Brown, 1994). Matoh et al. (1993) first isolated a particular B-polysaccharide complex from cell walls and the polysaccharide moiety of the complex was then identified as a pectic polysaccharide, rhamnogalacturonan II (RG-II) (Kobayashi et al., 1995, Kobayashi et al., 1996). RG-II, first isolated as a component of pectic polysaccharides from the primary cell walls of cultured sycamore cells by Darvill et al. (1978), has the richest diversity of sugars and linkage structures known (Fig. 1). The backbone of RG-II is composed of at least seven a-1,4-linked galacturonic acid residues (Thomas et al., 1989) and a variety of oligosaccharide side chains are attached to the backbone. The side chain sugar residues are characterized by rare sugars such as aceric acid, apiose, 2-keto-3-deoxysugars, O-methylfucose, and O-methyl xylose. As RG-II is one region of a long chain of pectic polysaccharides and the isolated B complexes are comprised of two chains of RG-II cross-linked with boric acid, two pectic chains are cross-linked in the RG-II region with boric acid to form a supramolecular network.